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==============================
LLVM Language Reference Manual
==============================
.. contents::
:local:
:depth: 4
Abstract
========
This document is a reference manual for the LLVM assembly language. LLVM
is a Static Single Assignment (SSA) based representation that provides
type safety, low-level operations, flexibility, and the capability of
representing 'all' high-level languages cleanly. It is the common code
representation used throughout all phases of the LLVM compilation
strategy.
Introduction
============
The LLVM code representation is designed to be used in three different
forms: as an in-memory compiler IR, as an on-disk bitcode representation
(suitable for fast loading by a Just-In-Time compiler), and as a human
readable assembly language representation. This allows LLVM to provide a
powerful intermediate representation for efficient compiler
transformations and analysis, while providing a natural means to debug
and visualize the transformations. The three different forms of LLVM are
all equivalent. This document describes the human readable
representation and notation.
The LLVM representation aims to be light-weight and low-level while
being expressive, typed, and extensible at the same time. It aims to be
a "universal IR" of sorts, by being at a low enough level that
high-level ideas may be cleanly mapped to it (similar to how
microprocessors are "universal IR's", allowing many source languages to
be mapped to them). By providing type information, LLVM can be used as
the target of optimizations: for example, through pointer analysis, it
can be proven that a C automatic variable is never accessed outside of
the current function, allowing it to be promoted to a simple SSA value
instead of a memory location.
.. _wellformed:
Well-Formedness
---------------
It is important to note that this document describes 'well formed' LLVM
assembly language. There is a difference between what the parser accepts
and what is considered 'well formed'. For example, the following
instruction is syntactically okay, but not well formed:
.. code-block:: llvm
%x = add i32 1, %x
because the definition of ``%x`` does not dominate all of its uses. The
LLVM infrastructure provides a verification pass that may be used to
verify that an LLVM module is well formed. This pass is automatically
run by the parser after parsing input assembly and by the optimizer
before it outputs bitcode. The violations pointed out by the verifier
pass indicate bugs in transformation passes or input to the parser.
.. _identifiers:
Identifiers
===========
LLVM identifiers come in two basic types: global and local. Global
identifiers (functions, global variables) begin with the ``'@'``
character. Local identifiers (register names, types) begin with the
``'%'`` character. Additionally, there are three different formats for
identifiers, for different purposes:
#. Named values are represented as a string of characters with their
prefix. For example, ``%foo``, ``@DivisionByZero``,
``%a.really.long.identifier``. The actual regular expression used is
'``[%@][-a-zA-Z$._][-a-zA-Z$._0-9]*``'. Identifiers that require other
characters in their names can be surrounded with quotes. Special
characters may be escaped using ``"\xx"`` where ``xx`` is the ASCII
code for the character in hexadecimal. In this way, any character can
be used in a name value, even quotes themselves. The ``"\01"`` prefix
can be used on global variables to suppress mangling.
#. Unnamed values are represented as an unsigned numeric value with
their prefix. For example, ``%12``, ``@2``, ``%44``.
#. Constants, which are described in the section Constants_ below.
LLVM requires that values start with a prefix for two reasons: Compilers
don't need to worry about name clashes with reserved words, and the set
of reserved words may be expanded in the future without penalty.
Additionally, unnamed identifiers allow a compiler to quickly come up
with a temporary variable without having to avoid symbol table
conflicts.
Reserved words in LLVM are very similar to reserved words in other
languages. There are keywords for different opcodes ('``add``',
'``bitcast``', '``ret``', etc...), for primitive type names ('``void``',
'``i32``', etc...), and others. These reserved words cannot conflict
with variable names, because none of them start with a prefix character
(``'%'`` or ``'@'``).
Here is an example of LLVM code to multiply the integer variable
'``%X``' by 8:
The easy way:
.. code-block:: llvm
%result = mul i32 %X, 8
After strength reduction:
.. code-block:: llvm
%result = shl i32 %X, 3
And the hard way:
.. code-block:: llvm
%0 = add i32 %X, %X ; yields i32:%0
%1 = add i32 %0, %0 ; yields i32:%1
%result = add i32 %1, %1
This last way of multiplying ``%X`` by 8 illustrates several important
lexical features of LLVM:
#. Comments are delimited with a '``;``' and go until the end of line.
#. Unnamed temporaries are created when the result of a computation is
not assigned to a named value.
#. Unnamed temporaries are numbered sequentially (using a per-function
incrementing counter, starting with 0). Note that basic blocks and unnamed
function parameters are included in this numbering. For example, if the
entry basic block is not given a label name and all function parameters are
named, then it will get number 0.
It also shows a convention that we follow in this document. When
demonstrating instructions, we will follow an instruction with a comment
that defines the type and name of value produced.
High Level Structure
====================
Module Structure
----------------
LLVM programs are composed of ``Module``'s, each of which is a
translation unit of the input programs. Each module consists of
functions, global variables, and symbol table entries. Modules may be
combined together with the LLVM linker, which merges function (and
global variable) definitions, resolves forward declarations, and merges
symbol table entries. Here is an example of the "hello world" module:
.. code-block:: llvm
; Declare the string constant as a global constant.
@.str = private unnamed_addr constant [13 x i8] c"hello world\0A\00"
; External declaration of the puts function
declare i32 @puts(i8* nocapture) nounwind
; Definition of main function
define i32 @main() { ; i32()*
; Convert [13 x i8]* to i8*...
%cast210 = getelementptr [13 x i8], [13 x i8]* @.str, i64 0, i64 0
; Call puts function to write out the string to stdout.
call i32 @puts(i8* %cast210)
ret i32 0
}
; Named metadata
!0 = !{i32 42, null, !"string"}
!foo = !{!0}
This example is made up of a :ref:`global variable <globalvars>` named
"``.str``", an external declaration of the "``puts``" function, a
:ref:`function definition <functionstructure>` for "``main``" and
:ref:`named metadata <namedmetadatastructure>` "``foo``".
In general, a module is made up of a list of global values (where both
functions and global variables are global values). Global values are
represented by a pointer to a memory location (in this case, a pointer
to an array of char, and a pointer to a function), and have one of the
following :ref:`linkage types <linkage>`.
.. _linkage:
Linkage Types
-------------
All Global Variables and Functions have one of the following types of
linkage:
``private``
Global values with "``private``" linkage are only directly
accessible by objects in the current module. In particular, linking
code into a module with a private global value may cause the
private to be renamed as necessary to avoid collisions. Because the
symbol is private to the module, all references can be updated. This
doesn't show up in any symbol table in the object file.
``internal``
Similar to private, but the value shows as a local symbol
(``STB_LOCAL`` in the case of ELF) in the object file. This
corresponds to the notion of the '``static``' keyword in C.
``available_externally``
Globals with "``available_externally``" linkage are never emitted into
the object file corresponding to the LLVM module. From the linker's
perspective, an ``available_externally`` global is equivalent to
an external declaration. They exist to allow inlining and other
optimizations to take place given knowledge of the definition of the
global, which is known to be somewhere outside the module. Globals
with ``available_externally`` linkage are allowed to be discarded at
will, and allow inlining and other optimizations. This linkage type is
only allowed on definitions, not declarations.
``linkonce``
Globals with "``linkonce``" linkage are merged with other globals of
the same name when linkage occurs. This can be used to implement
some forms of inline functions, templates, or other code which must
be generated in each translation unit that uses it, but where the
body may be overridden with a more definitive definition later.
Unreferenced ``linkonce`` globals are allowed to be discarded. Note
that ``linkonce`` linkage does not actually allow the optimizer to
inline the body of this function into callers because it doesn't
know if this definition of the function is the definitive definition
within the program or whether it will be overridden by a stronger
definition. To enable inlining and other optimizations, use
"``linkonce_odr``" linkage.
``weak``
"``weak``" linkage has the same merging semantics as ``linkonce``
linkage, except that unreferenced globals with ``weak`` linkage may
not be discarded. This is used for globals that are declared "weak"
in C source code.
``common``
"``common``" linkage is most similar to "``weak``" linkage, but they
are used for tentative definitions in C, such as "``int X;``" at
global scope. Symbols with "``common``" linkage are merged in the
same way as ``weak symbols``, and they may not be deleted if
unreferenced. ``common`` symbols may not have an explicit section,
must have a zero initializer, and may not be marked
':ref:`constant <globalvars>`'. Functions and aliases may not have
common linkage.
.. _linkage_appending:
``appending``
"``appending``" linkage may only be applied to global variables of
pointer to array type. When two global variables with appending
linkage are linked together, the two global arrays are appended
together. This is the LLVM, typesafe, equivalent of having the
system linker append together "sections" with identical names when
.o files are linked.
Unfortunately this doesn't correspond to any feature in .o files, so it
can only be used for variables like ``llvm.global_ctors`` which llvm
interprets specially.
``extern_weak``
The semantics of this linkage follow the ELF object file model: the
symbol is weak until linked, if not linked, the symbol becomes null
instead of being an undefined reference.
``linkonce_odr``, ``weak_odr``
Some languages allow differing globals to be merged, such as two
functions with different semantics. Other languages, such as
``C++``, ensure that only equivalent globals are ever merged (the
"one definition rule" --- "ODR"). Such languages can use the
``linkonce_odr`` and ``weak_odr`` linkage types to indicate that the
global will only be merged with equivalent globals. These linkage
types are otherwise the same as their non-``odr`` versions.
``external``
If none of the above identifiers are used, the global is externally
visible, meaning that it participates in linkage and can be used to
resolve external symbol references.
It is illegal for a function *declaration* to have any linkage type
other than ``external`` or ``extern_weak``.
.. _callingconv:
Calling Conventions
-------------------
LLVM :ref:`functions <functionstructure>`, :ref:`calls <i_call>` and
:ref:`invokes <i_invoke>` can all have an optional calling convention
specified for the call. The calling convention of any pair of dynamic
caller/callee must match, or the behavior of the program is undefined.
The following calling conventions are supported by LLVM, and more may be
added in the future:
"``ccc``" - The C calling convention
This calling convention (the default if no other calling convention
is specified) matches the target C calling conventions. This calling
convention supports varargs function calls and tolerates some
mismatch in the declared prototype and implemented declaration of
the function (as does normal C).
"``fastcc``" - The fast calling convention
This calling convention attempts to make calls as fast as possible
(e.g. by passing things in registers). This calling convention
allows the target to use whatever tricks it wants to produce fast
code for the target, without having to conform to an externally
specified ABI (Application Binary Interface). `Tail calls can only
be optimized when this, the GHC or the HiPE convention is
used. <CodeGenerator.html#id80>`_ This calling convention does not
support varargs and requires the prototype of all callees to exactly
match the prototype of the function definition.
"``coldcc``" - The cold calling convention
This calling convention attempts to make code in the caller as
efficient as possible under the assumption that the call is not
commonly executed. As such, these calls often preserve all registers
so that the call does not break any live ranges in the caller side.
This calling convention does not support varargs and requires the
prototype of all callees to exactly match the prototype of the
function definition. Furthermore the inliner doesn't consider such function
calls for inlining.
"``cc 10``" - GHC convention
This calling convention has been implemented specifically for use by
the `Glasgow Haskell Compiler (GHC) <http://www.haskell.org/ghc>`_.
It passes everything in registers, going to extremes to achieve this
by disabling callee save registers. This calling convention should
not be used lightly but only for specific situations such as an
alternative to the *register pinning* performance technique often
used when implementing functional programming languages. At the
moment only X86 supports this convention and it has the following
limitations:
- On *X86-32* only supports up to 4 bit type parameters. No
floating point types are supported.
- On *X86-64* only supports up to 10 bit type parameters and 6
floating point parameters.
This calling convention supports `tail call
optimization <CodeGenerator.html#id80>`_ but requires both the
caller and callee are using it.
"``cc 11``" - The HiPE calling convention
This calling convention has been implemented specifically for use by
the `High-Performance Erlang
(HiPE) <http://www.it.uu.se/research/group/hipe/>`_ compiler, *the*
native code compiler of the `Ericsson's Open Source Erlang/OTP
system <http://www.erlang.org/download.shtml>`_. It uses more
registers for argument passing than the ordinary C calling
convention and defines no callee-saved registers. The calling
convention properly supports `tail call
optimization <CodeGenerator.html#id80>`_ but requires that both the
caller and the callee use it. It uses a *register pinning*
mechanism, similar to GHC's convention, for keeping frequently
accessed runtime components pinned to specific hardware registers.
At the moment only X86 supports this convention (both 32 and 64
bit).
"``webkit_jscc``" - WebKit's JavaScript calling convention
This calling convention has been implemented for `WebKit FTL JIT
<https://trac.webkit.org/wiki/FTLJIT>`_. It passes arguments on the
stack right to left (as cdecl does), and returns a value in the
platform's customary return register.
"``anyregcc``" - Dynamic calling convention for code patching
This is a special convention that supports patching an arbitrary code
sequence in place of a call site. This convention forces the call
arguments into registers but allows them to be dynamically
allocated. This can currently only be used with calls to
llvm.experimental.patchpoint because only this intrinsic records
the location of its arguments in a side table. See :doc:`StackMaps`.
"``preserve_mostcc``" - The `PreserveMost` calling convention
This calling convention attempts to make the code in the caller as
unintrusive as possible. This convention behaves identically to the `C`
calling convention on how arguments and return values are passed, but it
uses a different set of caller/callee-saved registers. This alleviates the
burden of saving and recovering a large register set before and after the
call in the caller. If the arguments are passed in callee-saved registers,
then they will be preserved by the callee across the call. This doesn't
apply for values returned in callee-saved registers.
- On X86-64 the callee preserves all general purpose registers, except for
R11. R11 can be used as a scratch register. Floating-point registers
(XMMs/YMMs) are not preserved and need to be saved by the caller.
The idea behind this convention is to support calls to runtime functions
that have a hot path and a cold path. The hot path is usually a small piece
of code that doesn't use many registers. The cold path might need to call out to
another function and therefore only needs to preserve the caller-saved
registers, which haven't already been saved by the caller. The
`PreserveMost` calling convention is very similar to the `cold` calling
convention in terms of caller/callee-saved registers, but they are used for
different types of function calls. `coldcc` is for function calls that are
rarely executed, whereas `preserve_mostcc` function calls are intended to be
on the hot path and definitely executed a lot. Furthermore `preserve_mostcc`
doesn't prevent the inliner from inlining the function call.
This calling convention will be used by a future version of the ObjectiveC
runtime and should therefore still be considered experimental at this time.
Although this convention was created to optimize certain runtime calls to
the ObjectiveC runtime, it is not limited to this runtime and might be used
by other runtimes in the future too. The current implementation only
supports X86-64, but the intention is to support more architectures in the
future.
"``preserve_allcc``" - The `PreserveAll` calling convention
This calling convention attempts to make the code in the caller even less
intrusive than the `PreserveMost` calling convention. This calling
convention also behaves identical to the `C` calling convention on how
arguments and return values are passed, but it uses a different set of
caller/callee-saved registers. This removes the burden of saving and
recovering a large register set before and after the call in the caller. If
the arguments are passed in callee-saved registers, then they will be
preserved by the callee across the call. This doesn't apply for values
returned in callee-saved registers.
- On X86-64 the callee preserves all general purpose registers, except for
R11. R11 can be used as a scratch register. Furthermore it also preserves
all floating-point registers (XMMs/YMMs).
The idea behind this convention is to support calls to runtime functions
that don't need to call out to any other functions.
This calling convention, like the `PreserveMost` calling convention, will be
used by a future version of the ObjectiveC runtime and should be considered
experimental at this time.
"``cxx_fast_tlscc``" - The `CXX_FAST_TLS` calling convention for access functions
Clang generates an access function to access C++-style TLS. The access
function generally has an entry block, an exit block and an initialization
block that is run at the first time. The entry and exit blocks can access
a few TLS IR variables, each access will be lowered to a platform-specific
sequence.
This calling convention aims to minimize overhead in the caller by
preserving as many registers as possible (all the registers that are
perserved on the fast path, composed of the entry and exit blocks).
This calling convention behaves identical to the `C` calling convention on
how arguments and return values are passed, but it uses a different set of
caller/callee-saved registers.
Given that each platform has its own lowering sequence, hence its own set
of preserved registers, we can't use the existing `PreserveMost`.
- On X86-64 the callee preserves all general purpose registers, except for
RDI and RAX.
"``swiftcc``" - This calling convention is used for Swift language.
- On X86-64 RCX and R8 are available for additional integer returns, and
XMM2 and XMM3 are available for additional FP/vector returns.
- On iOS platforms, we use AAPCS-VFP calling convention.
"``cc <n>``" - Numbered convention
Any calling convention may be specified by number, allowing
target-specific calling conventions to be used. Target specific
calling conventions start at 64.
More calling conventions can be added/defined on an as-needed basis, to
support Pascal conventions or any other well-known target-independent
convention.
.. _visibilitystyles:
Visibility Styles
-----------------
All Global Variables and Functions have one of the following visibility
styles:
"``default``" - Default style
On targets that use the ELF object file format, default visibility
means that the declaration is visible to other modules and, in
shared libraries, means that the declared entity may be overridden.
On Darwin, default visibility means that the declaration is visible
to other modules. Default visibility corresponds to "external
linkage" in the language.
"``hidden``" - Hidden style
Two declarations of an object with hidden visibility refer to the
same object if they are in the same shared object. Usually, hidden
visibility indicates that the symbol will not be placed into the
dynamic symbol table, so no other module (executable or shared
library) can reference it directly.
"``protected``" - Protected style
On ELF, protected visibility indicates that the symbol will be
placed in the dynamic symbol table, but that references within the
defining module will bind to the local symbol. That is, the symbol
cannot be overridden by another module.
A symbol with ``internal`` or ``private`` linkage must have ``default``
visibility.
.. _dllstorageclass:
DLL Storage Classes
-------------------
All Global Variables, Functions and Aliases can have one of the following
DLL storage class:
``dllimport``
"``dllimport``" causes the compiler to reference a function or variable via
a global pointer to a pointer that is set up by the DLL exporting the
symbol. On Microsoft Windows targets, the pointer name is formed by
combining ``__imp_`` and the function or variable name.
``dllexport``
"``dllexport``" causes the compiler to provide a global pointer to a pointer
in a DLL, so that it can be referenced with the ``dllimport`` attribute. On
Microsoft Windows targets, the pointer name is formed by combining
``__imp_`` and the function or variable name. Since this storage class
exists for defining a dll interface, the compiler, assembler and linker know
it is externally referenced and must refrain from deleting the symbol.
.. _tls_model:
Thread Local Storage Models
---------------------------
A variable may be defined as ``thread_local``, which means that it will
not be shared by threads (each thread will have a separated copy of the
variable). Not all targets support thread-local variables. Optionally, a
TLS model may be specified:
``localdynamic``
For variables that are only used within the current shared library.
``initialexec``
For variables in modules that will not be loaded dynamically.
``localexec``
For variables defined in the executable and only used within it.
If no explicit model is given, the "general dynamic" model is used.
The models correspond to the ELF TLS models; see `ELF Handling For
Thread-Local Storage <http://people.redhat.com/drepper/tls.pdf>`_ for
more information on under which circumstances the different models may
be used. The target may choose a different TLS model if the specified
model is not supported, or if a better choice of model can be made.
A model can also be specified in an alias, but then it only governs how
the alias is accessed. It will not have any effect in the aliasee.
For platforms without linker support of ELF TLS model, the -femulated-tls
flag can be used to generate GCC compatible emulated TLS code.
.. _runtime_preemption_model:
Runtime Preemption Specifiers
-----------------------------
Global variables, functions and aliases may have an optional runtime preemption
specifier. If a preemption specifier isn't given explicitly, then a
symbol is assumed to be ``dso_preemptable``.
``dso_preemptable``
Indicates that the function or variable may be replaced by a symbol from
outside the linkage unit at runtime.
``dso_local``
The compiler may assume that a function or variable marked as ``dso_local``
- will resolve to a symbol within the same linkage unit. Direct access will
+ will resolve to a symbol within the same linkage unit. Direct access will
be generated even if the definition is not within this compilation unit.
.. _namedtypes:
Structure Types
---------------
LLVM IR allows you to specify both "identified" and "literal" :ref:`structure
types <t_struct>`. Literal types are uniqued structurally, but identified types
are never uniqued. An :ref:`opaque structural type <t_opaque>` can also be used
to forward declare a type that is not yet available.
An example of an identified structure specification is:
.. code-block:: llvm
%mytype = type { %mytype*, i32 }
Prior to the LLVM 3.0 release, identified types were structurally uniqued. Only
literal types are uniqued in recent versions of LLVM.
.. _nointptrtype:
Non-Integral Pointer Type
-------------------------
Note: non-integral pointer types are a work in progress, and they should be
considered experimental at this time.
LLVM IR optionally allows the frontend to denote pointers in certain address
spaces as "non-integral" via the :ref:`datalayout string<langref_datalayout>`.
Non-integral pointer types represent pointers that have an *unspecified* bitwise
representation; that is, the integral representation may be target dependent or
unstable (not backed by a fixed integer).
``inttoptr`` instructions converting integers to non-integral pointer types are
ill-typed, and so are ``ptrtoint`` instructions converting values of
non-integral pointer types to integers. Vector versions of said instructions
are ill-typed as well.
.. _globalvars:
Global Variables
----------------
Global variables define regions of memory allocated at compilation time
instead of run-time.
Global variable definitions must be initialized.
Global variables in other translation units can also be declared, in which
case they don't have an initializer.
Either global variable definitions or declarations may have an explicit section
-to be placed in and may have an optional explicit alignment specified. If there
-is a mismatch between the explicit or inferred section information for the
-variable declaration and its definition the resulting behavior is undefined.
+to be placed in and may have an optional explicit alignment specified. If there
+is a mismatch between the explicit or inferred section information for the
+variable declaration and its definition the resulting behavior is undefined.
A variable may be defined as a global ``constant``, which indicates that
the contents of the variable will **never** be modified (enabling better
optimization, allowing the global data to be placed in the read-only
section of an executable, etc). Note that variables that need runtime
initialization cannot be marked ``constant`` as there is a store to the
variable.
LLVM explicitly allows *declarations* of global variables to be marked
constant, even if the final definition of the global is not. This
capability can be used to enable slightly better optimization of the
program, but requires the language definition to guarantee that
optimizations based on the 'constantness' are valid for the translation
units that do not include the definition.
As SSA values, global variables define pointer values that are in scope
(i.e. they dominate) all basic blocks in the program. Global variables
always define a pointer to their "content" type because they describe a
region of memory, and all memory objects in LLVM are accessed through
pointers.
Global variables can be marked with ``unnamed_addr`` which indicates
that the address is not significant, only the content. Constants marked
like this can be merged with other constants if they have the same
initializer. Note that a constant with significant address *can* be
merged with a ``unnamed_addr`` constant, the result being a constant
whose address is significant.
If the ``local_unnamed_addr`` attribute is given, the address is known to
not be significant within the module.
A global variable may be declared to reside in a target-specific
numbered address space. For targets that support them, address spaces
may affect how optimizations are performed and/or what target
instructions are used to access the variable. The default address space
is zero. The address space qualifier must precede any other attributes.
LLVM allows an explicit section to be specified for globals. If the
target supports it, it will emit globals to the section specified.
Additionally, the global can placed in a comdat if the target has the necessary
support.
-External declarations may have an explicit section specified. Section
-information is retained in LLVM IR for targets that make use of this
-information. Attaching section information to an external declaration is an
-assertion that its definition is located in the specified section. If the
-definition is located in a different section, the behavior is undefined.
+External declarations may have an explicit section specified. Section
+information is retained in LLVM IR for targets that make use of this
+information. Attaching section information to an external declaration is an
+assertion that its definition is located in the specified section. If the
+definition is located in a different section, the behavior is undefined.
By default, global initializers are optimized by assuming that global
variables defined within the module are not modified from their
initial values before the start of the global initializer. This is
true even for variables potentially accessible from outside the
module, including those with external linkage or appearing in
``@llvm.used`` or dllexported variables. This assumption may be suppressed
by marking the variable with ``externally_initialized``.
An explicit alignment may be specified for a global, which must be a
power of 2. If not present, or if the alignment is set to zero, the
alignment of the global is set by the target to whatever it feels
convenient. If an explicit alignment is specified, the global is forced
to have exactly that alignment. Targets and optimizers are not allowed
to over-align the global if the global has an assigned section. In this
case, the extra alignment could be observable: for example, code could
assume that the globals are densely packed in their section and try to
iterate over them as an array, alignment padding would break this
iteration. The maximum alignment is ``1 << 29``.
Globals can also have a :ref:`DLL storage class <dllstorageclass>`,
an optional :ref:`runtime preemption specifier <runtime_preemption_model>`,
an optional :ref:`global attributes <glattrs>` and
an optional list of attached :ref:`metadata <metadata>`.
Variables and aliases can have a
:ref:`Thread Local Storage Model <tls_model>`.
Syntax::
@<GlobalVarName> = [Linkage] [PreemptionSpecifier] [Visibility]
[DLLStorageClass] [ThreadLocal]
[(unnamed_addr|local_unnamed_addr)] [AddrSpace]
[ExternallyInitialized]
<global | constant> <Type> [<InitializerConstant>]
[, section "name"] [, comdat [($name)]]
[, align <Alignment>] (, !name !N)*
For example, the following defines a global in a numbered address space
with an initializer, section, and alignment:
.. code-block:: llvm
@G = addrspace(5) constant float 1.0, section "foo", align 4
The following example just declares a global variable
.. code-block:: llvm
@G = external global i32
The following example defines a thread-local global with the
``initialexec`` TLS model:
.. code-block:: llvm
@G = thread_local(initialexec) global i32 0, align 4
.. _functionstructure:
Functions
---------
LLVM function definitions consist of the "``define``" keyword, an
optional :ref:`linkage type <linkage>`, an optional :ref:`runtime preemption
specifier <runtime_preemption_model>`, an optional :ref:`visibility
style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
an optional :ref:`calling convention <callingconv>`,
an optional ``unnamed_addr`` attribute, a return type, an optional
:ref:`parameter attribute <paramattrs>` for the return type, a function
name, a (possibly empty) argument list (each with optional :ref:`parameter
attributes <paramattrs>`), optional :ref:`function attributes <fnattrs>`,
an optional section, an optional alignment,
an optional :ref:`comdat <langref_comdats>`,
an optional :ref:`garbage collector name <gc>`, an optional :ref:`prefix <prefixdata>`,
an optional :ref:`prologue <prologuedata>`,
an optional :ref:`personality <personalityfn>`,
an optional list of attached :ref:`metadata <metadata>`,
an opening curly brace, a list of basic blocks, and a closing curly brace.
LLVM function declarations consist of the "``declare``" keyword, an
optional :ref:`linkage type <linkage>`, an optional :ref:`visibility style
<visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`, an
optional :ref:`calling convention <callingconv>`, an optional ``unnamed_addr``
or ``local_unnamed_addr`` attribute, a return type, an optional :ref:`parameter
attribute <paramattrs>` for the return type, a function name, a possibly
empty list of arguments, an optional alignment, an optional :ref:`garbage
collector name <gc>`, an optional :ref:`prefix <prefixdata>`, and an optional
:ref:`prologue <prologuedata>`.
A function definition contains a list of basic blocks, forming the CFG (Control
Flow Graph) for the function. Each basic block may optionally start with a label
(giving the basic block a symbol table entry), contains a list of instructions,
and ends with a :ref:`terminator <terminators>` instruction (such as a branch or
function return). If an explicit label is not provided, a block is assigned an
implicit numbered label, using the next value from the same counter as used for
unnamed temporaries (:ref:`see above<identifiers>`). For example, if a function
entry block does not have an explicit label, it will be assigned label "%0",
then the first unnamed temporary in that block will be "%1", etc.
The first basic block in a function is special in two ways: it is
immediately executed on entrance to the function, and it is not allowed
to have predecessor basic blocks (i.e. there can not be any branches to
the entry block of a function). Because the block can have no
predecessors, it also cannot have any :ref:`PHI nodes <i_phi>`.
LLVM allows an explicit section to be specified for functions. If the
target supports it, it will emit functions to the section specified.
Additionally, the function can be placed in a COMDAT.
An explicit alignment may be specified for a function. If not present,
or if the alignment is set to zero, the alignment of the function is set
by the target to whatever it feels convenient. If an explicit alignment
is specified, the function is forced to have at least that much
alignment. All alignments must be a power of 2.
If the ``unnamed_addr`` attribute is given, the address is known to not
be significant and two identical functions can be merged.
If the ``local_unnamed_addr`` attribute is given, the address is known to
not be significant within the module.
Syntax::
define [linkage] [PreemptionSpecifier] [visibility] [DLLStorageClass]
[cconv] [ret attrs]
<ResultType> @<FunctionName> ([argument list])
[(unnamed_addr|local_unnamed_addr)] [fn Attrs] [section "name"]
[comdat [($name)]] [align N] [gc] [prefix Constant]
[prologue Constant] [personality Constant] (!name !N)* { ... }
The argument list is a comma separated sequence of arguments where each
argument is of the following form:
Syntax::
<type> [parameter Attrs] [name]
.. _langref_aliases:
Aliases
-------
Aliases, unlike function or variables, don't create any new data. They
are just a new symbol and metadata for an existing position.
Aliases have a name and an aliasee that is either a global value or a
constant expression.
Aliases may have an optional :ref:`linkage type <linkage>`, an optional
:ref:`runtime preemption specifier <runtime_preemption_model>`, an optional
:ref:`visibility style <visibility>`, an optional :ref:`DLL storage class
<dllstorageclass>` and an optional :ref:`tls model <tls_model>`.
Syntax::
@<Name> = [Linkage] [PreemptionSpecifier] [Visibility] [DLLStorageClass] [ThreadLocal] [(unnamed_addr|local_unnamed_addr)] alias <AliaseeTy>, <AliaseeTy>* @<Aliasee>
The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
might not correctly handle dropping a weak symbol that is aliased.
Aliases that are not ``unnamed_addr`` are guaranteed to have the same address as
the aliasee expression. ``unnamed_addr`` ones are only guaranteed to point
to the same content.
If the ``local_unnamed_addr`` attribute is given, the address is known to
not be significant within the module.
Since aliases are only a second name, some restrictions apply, of which
some can only be checked when producing an object file:
* The expression defining the aliasee must be computable at assembly
time. Since it is just a name, no relocations can be used.
* No alias in the expression can be weak as the possibility of the
intermediate alias being overridden cannot be represented in an
object file.
* No global value in the expression can be a declaration, since that
would require a relocation, which is not possible.
.. _langref_ifunc:
IFuncs
-------
IFuncs, like as aliases, don't create any new data or func. They are just a new
symbol that dynamic linker resolves at runtime by calling a resolver function.
IFuncs have a name and a resolver that is a function called by dynamic linker
that returns address of another function associated with the name.
IFunc may have an optional :ref:`linkage type <linkage>` and an optional
:ref:`visibility style <visibility>`.
Syntax::
@<Name> = [Linkage] [Visibility] ifunc <IFuncTy>, <ResolverTy>* @<Resolver>
.. _langref_comdats:
Comdats
-------
Comdat IR provides access to COFF and ELF object file COMDAT functionality.
Comdats have a name which represents the COMDAT key. All global objects that
specify this key will only end up in the final object file if the linker chooses
that key over some other key. Aliases are placed in the same COMDAT that their
aliasee computes to, if any.
Comdats have a selection kind to provide input on how the linker should
choose between keys in two different object files.
Syntax::
$<Name> = comdat SelectionKind
The selection kind must be one of the following:
``any``
The linker may choose any COMDAT key, the choice is arbitrary.
``exactmatch``
The linker may choose any COMDAT key but the sections must contain the
same data.
``largest``
The linker will choose the section containing the largest COMDAT key.
``noduplicates``
The linker requires that only section with this COMDAT key exist.
``samesize``
The linker may choose any COMDAT key but the sections must contain the
same amount of data.
Note that the Mach-O platform doesn't support COMDATs and ELF only supports
``any`` as a selection kind.
Here is an example of a COMDAT group where a function will only be selected if
the COMDAT key's section is the largest:
.. code-block:: text
$foo = comdat largest
@foo = global i32 2, comdat($foo)
define void @bar() comdat($foo) {
ret void
}
As a syntactic sugar the ``$name`` can be omitted if the name is the same as
the global name:
.. code-block:: text
$foo = comdat any
@foo = global i32 2, comdat
In a COFF object file, this will create a COMDAT section with selection kind
``IMAGE_COMDAT_SELECT_LARGEST`` containing the contents of the ``@foo`` symbol
and another COMDAT section with selection kind
``IMAGE_COMDAT_SELECT_ASSOCIATIVE`` which is associated with the first COMDAT
section and contains the contents of the ``@bar`` symbol.
There are some restrictions on the properties of the global object.
It, or an alias to it, must have the same name as the COMDAT group when
targeting COFF.
The contents and size of this object may be used during link-time to determine
which COMDAT groups get selected depending on the selection kind.
Because the name of the object must match the name of the COMDAT group, the
linkage of the global object must not be local; local symbols can get renamed
if a collision occurs in the symbol table.
The combined use of COMDATS and section attributes may yield surprising results.
For example:
.. code-block:: text
$foo = comdat any
$bar = comdat any
@g1 = global i32 42, section "sec", comdat($foo)
@g2 = global i32 42, section "sec", comdat($bar)
From the object file perspective, this requires the creation of two sections
with the same name. This is necessary because both globals belong to different
COMDAT groups and COMDATs, at the object file level, are represented by
sections.
Note that certain IR constructs like global variables and functions may
create COMDATs in the object file in addition to any which are specified using
COMDAT IR. This arises when the code generator is configured to emit globals
in individual sections (e.g. when `-data-sections` or `-function-sections`
is supplied to `llc`).
.. _namedmetadatastructure:
Named Metadata
--------------
Named metadata is a collection of metadata. :ref:`Metadata
nodes <metadata>` (but not metadata strings) are the only valid
operands for a named metadata.
#. Named metadata are represented as a string of characters with the
metadata prefix. The rules for metadata names are the same as for
identifiers, but quoted names are not allowed. ``"\xx"`` type escapes
are still valid, which allows any character to be part of a name.
Syntax::
; Some unnamed metadata nodes, which are referenced by the named metadata.
!0 = !{!"zero"}
!1 = !{!"one"}
!2 = !{!"two"}
; A named metadata.
!name = !{!0, !1, !2}
.. _paramattrs:
Parameter Attributes
--------------------
The return type and each parameter of a function type may have a set of
*parameter attributes* associated with them. Parameter attributes are
used to communicate additional information about the result or
parameters of a function. Parameter attributes are considered to be part
of the function, not of the function type, so functions with different
parameter attributes can have the same function type.
Parameter attributes are simple keywords that follow the type specified.
If multiple parameter attributes are needed, they are space separated.
For example:
.. code-block:: llvm
declare i32 @printf(i8* noalias nocapture, ...)
declare i32 @atoi(i8 zeroext)
declare signext i8 @returns_signed_char()
Note that any attributes for the function result (``nounwind``,
``readonly``) come immediately after the argument list.
Currently, only the following parameter attributes are defined:
``zeroext``
This indicates to the code generator that the parameter or return
value should be zero-extended to the extent required by the target's
ABI by the caller (for a parameter) or the callee (for a return value).
``signext``
This indicates to the code generator that the parameter or return
value should be sign-extended to the extent required by the target's
ABI (which is usually 32-bits) by the caller (for a parameter) or
the callee (for a return value).
``inreg``
This indicates that this parameter or return value should be treated
in a special target-dependent fashion while emitting code for
a function call or return (usually, by putting it in a register as
opposed to memory, though some targets use it to distinguish between
two different kinds of registers). Use of this attribute is
target-specific.
``byval``
This indicates that the pointer parameter should really be passed by
value to the function. The attribute implies that a hidden copy of
the pointee is made between the caller and the callee, so the callee
is unable to modify the value in the caller. This attribute is only
valid on LLVM pointer arguments. It is generally used to pass
structs and arrays by value, but is also valid on pointers to
scalars. The copy is considered to belong to the caller not the
callee (for example, ``readonly`` functions should not write to
``byval`` parameters). This is not a valid attribute for return
values.
The byval attribute also supports specifying an alignment with the
align attribute. It indicates the alignment of the stack slot to
form and the known alignment of the pointer specified to the call
site. If the alignment is not specified, then the code generator
makes a target-specific assumption.
.. _attr_inalloca:
``inalloca``
The ``inalloca`` argument attribute allows the caller to take the
address of outgoing stack arguments. An ``inalloca`` argument must
be a pointer to stack memory produced by an ``alloca`` instruction.
The alloca, or argument allocation, must also be tagged with the
inalloca keyword. Only the last argument may have the ``inalloca``
attribute, and that argument is guaranteed to be passed in memory.
An argument allocation may be used by a call at most once because
the call may deallocate it. The ``inalloca`` attribute cannot be
used in conjunction with other attributes that affect argument
storage, like ``inreg``, ``nest``, ``sret``, or ``byval``. The
``inalloca`` attribute also disables LLVM's implicit lowering of
large aggregate return values, which means that frontend authors
must lower them with ``sret`` pointers.
When the call site is reached, the argument allocation must have
been the most recent stack allocation that is still live, or the
results are undefined. It is possible to allocate additional stack
space after an argument allocation and before its call site, but it
must be cleared off with :ref:`llvm.stackrestore
<int_stackrestore>`.
See :doc:`InAlloca` for more information on how to use this
attribute.
``sret``
This indicates that the pointer parameter specifies the address of a
structure that is the return value of the function in the source
program. This pointer must be guaranteed by the caller to be valid:
loads and stores to the structure may be assumed by the callee not
to trap and to be properly aligned. This is not a valid attribute
for return values.
``align <n>``
This indicates that the pointer value may be assumed by the optimizer to
have the specified alignment.
Note that this attribute has additional semantics when combined with the
``byval`` attribute.
.. _noalias:
``noalias``
This indicates that objects accessed via pointer values
:ref:`based <pointeraliasing>` on the argument or return value are not also
accessed, during the execution of the function, via pointer values not
*based* on the argument or return value. The attribute on a return value
also has additional semantics described below. The caller shares the
responsibility with the callee for ensuring that these requirements are met.
For further details, please see the discussion of the NoAlias response in
:ref:`alias analysis <Must, May, or No>`.
Note that this definition of ``noalias`` is intentionally similar
to the definition of ``restrict`` in C99 for function arguments.
For function return values, C99's ``restrict`` is not meaningful,
while LLVM's ``noalias`` is. Furthermore, the semantics of the ``noalias``
attribute on return values are stronger than the semantics of the attribute
when used on function arguments. On function return values, the ``noalias``
attribute indicates that the function acts like a system memory allocation
function, returning a pointer to allocated storage disjoint from the
storage for any other object accessible to the caller.
``nocapture``
This indicates that the callee does not make any copies of the
pointer that outlive the callee itself. This is not a valid
attribute for return values. Addresses used in volatile operations
are considered to be captured.
.. _nest:
``nest``
This indicates that the pointer parameter can be excised using the
:ref:`trampoline intrinsics <int_trampoline>`. This is not a valid
attribute for return values and can only be applied to one parameter.
``returned``
This indicates that the function always returns the argument as its return
value. This is a hint to the optimizer and code generator used when
generating the caller, allowing value propagation, tail call optimization,
and omission of register saves and restores in some cases; it is not
checked or enforced when generating the callee. The parameter and the
function return type must be valid operands for the
:ref:`bitcast instruction <i_bitcast>`. This is not a valid attribute for
return values and can only be applied to one parameter.
``nonnull``
This indicates that the parameter or return pointer is not null. This
attribute may only be applied to pointer typed parameters. This is not
checked or enforced by LLVM, the caller must ensure that the pointer
passed in is non-null, or the callee must ensure that the returned pointer
is non-null.
``dereferenceable(<n>)``
This indicates that the parameter or return pointer is dereferenceable. This
attribute may only be applied to pointer typed parameters. A pointer that
is dereferenceable can be loaded from speculatively without a risk of
trapping. The number of bytes known to be dereferenceable must be provided
in parentheses. It is legal for the number of bytes to be less than the
size of the pointee type. The ``nonnull`` attribute does not imply
dereferenceability (consider a pointer to one element past the end of an
array), however ``dereferenceable(<n>)`` does imply ``nonnull`` in
``addrspace(0)`` (which is the default address space).
``dereferenceable_or_null(<n>)``
This indicates that the parameter or return value isn't both
non-null and non-dereferenceable (up to ``<n>`` bytes) at the same
time. All non-null pointers tagged with
``dereferenceable_or_null(<n>)`` are ``dereferenceable(<n>)``.
For address space 0 ``dereferenceable_or_null(<n>)`` implies that
a pointer is exactly one of ``dereferenceable(<n>)`` or ``null``,
and in other address spaces ``dereferenceable_or_null(<n>)``
implies that a pointer is at least one of ``dereferenceable(<n>)``
or ``null`` (i.e. it may be both ``null`` and
``dereferenceable(<n>)``). This attribute may only be applied to
pointer typed parameters.
``swiftself``
This indicates that the parameter is the self/context parameter. This is not
a valid attribute for return values and can only be applied to one
parameter.
``swifterror``
This attribute is motivated to model and optimize Swift error handling. It
can be applied to a parameter with pointer to pointer type or a
pointer-sized alloca. At the call site, the actual argument that corresponds
to a ``swifterror`` parameter has to come from a ``swifterror`` alloca or
the ``swifterror`` parameter of the caller. A ``swifterror`` value (either
the parameter or the alloca) can only be loaded and stored from, or used as
a ``swifterror`` argument. This is not a valid attribute for return values
and can only be applied to one parameter.
These constraints allow the calling convention to optimize access to
``swifterror`` variables by associating them with a specific register at
call boundaries rather than placing them in memory. Since this does change
the calling convention, a function which uses the ``swifterror`` attribute
on a parameter is not ABI-compatible with one which does not.
These constraints also allow LLVM to assume that a ``swifterror`` argument
does not alias any other memory visible within a function and that a
``swifterror`` alloca passed as an argument does not escape.
.. _gc:
Garbage Collector Strategy Names
--------------------------------
Each function may specify a garbage collector strategy name, which is simply a
string:
.. code-block:: llvm
define void @f() gc "name" { ... }
The supported values of *name* includes those :ref:`built in to LLVM
<builtin-gc-strategies>` and any provided by loaded plugins. Specifying a GC
strategy will cause the compiler to alter its output in order to support the
named garbage collection algorithm. Note that LLVM itself does not contain a
garbage collector, this functionality is restricted to generating machine code
which can interoperate with a collector provided externally.
.. _prefixdata:
Prefix Data
-----------
Prefix data is data associated with a function which the code
generator will emit immediately before the function's entrypoint.
The purpose of this feature is to allow frontends to associate
language-specific runtime metadata with specific functions and make it
available through the function pointer while still allowing the
function pointer to be called.
To access the data for a given function, a program may bitcast the
function pointer to a pointer to the constant's type and dereference
index -1. This implies that the IR symbol points just past the end of
the prefix data. For instance, take the example of a function annotated
with a single ``i32``,
.. code-block:: llvm
define void @f() prefix i32 123 { ... }
The prefix data can be referenced as,
.. code-block:: llvm
%0 = bitcast void* () @f to i32*
%a = getelementptr inbounds i32, i32* %0, i32 -1
%b = load i32, i32* %a
Prefix data is laid out as if it were an initializer for a global variable
of the prefix data's type. The function will be placed such that the
beginning of the prefix data is aligned. This means that if the size
of the prefix data is not a multiple of the alignment size, the
function's entrypoint will not be aligned. If alignment of the
function's entrypoint is desired, padding must be added to the prefix
data.
A function may have prefix data but no body. This has similar semantics
to the ``available_externally`` linkage in that the data may be used by the
optimizers but will not be emitted in the object file.
.. _prologuedata:
Prologue Data
-------------
The ``prologue`` attribute allows arbitrary code (encoded as bytes) to
be inserted prior to the function body. This can be used for enabling
function hot-patching and instrumentation.
To maintain the semantics of ordinary function calls, the prologue data must
have a particular format. Specifically, it must begin with a sequence of
bytes which decode to a sequence of machine instructions, valid for the
module's target, which transfer control to the point immediately succeeding
the prologue data, without performing any other visible action. This allows
the inliner and other passes to reason about the semantics of the function
definition without needing to reason about the prologue data. Obviously this
makes the format of the prologue data highly target dependent.
A trivial example of valid prologue data for the x86 architecture is ``i8 144``,
which encodes the ``nop`` instruction:
.. code-block:: text
define void @f() prologue i8 144 { ... }
Generally prologue data can be formed by encoding a relative branch instruction
which skips the metadata, as in this example of valid prologue data for the
x86_64 architecture, where the first two bytes encode ``jmp .+10``:
.. code-block:: text
%0 = type <{ i8, i8, i8* }>
define void @f() prologue %0 <{ i8 235, i8 8, i8* @md}> { ... }
A function may have prologue data but no body. This has similar semantics
to the ``available_externally`` linkage in that the data may be used by the
optimizers but will not be emitted in the object file.
.. _personalityfn:
Personality Function
--------------------
The ``personality`` attribute permits functions to specify what function
to use for exception handling.
.. _attrgrp:
Attribute Groups
----------------
Attribute groups are groups of attributes that are referenced by objects within
the IR. They are important for keeping ``.ll`` files readable, because a lot of
functions will use the same set of attributes. In the degenerative case of a
``.ll`` file that corresponds to a single ``.c`` file, the single attribute
group will capture the important command line flags used to build that file.
An attribute group is a module-level object. To use an attribute group, an
object references the attribute group's ID (e.g. ``#37``). An object may refer
to more than one attribute group. In that situation, the attributes from the
different groups are merged.
Here is an example of attribute groups for a function that should always be
inlined, has a stack alignment of 4, and which shouldn't use SSE instructions:
.. code-block:: llvm
; Target-independent attributes:
attributes #0 = { alwaysinline alignstack=4 }
; Target-dependent attributes:
attributes #1 = { "no-sse" }
; Function @f has attributes: alwaysinline, alignstack=4, and "no-sse".
define void @f() #0 #1 { ... }
.. _fnattrs:
Function Attributes
-------------------
Function attributes are set to communicate additional information about
a function. Function attributes are considered to be part of the
function, not of the function type, so functions with different function
attributes can have the same function type.
Function attributes are simple keywords that follow the type specified.
If multiple attributes are needed, they are space separated. For
example:
.. code-block:: llvm
define void @f() noinline { ... }
define void @f() alwaysinline { ... }
define void @f() alwaysinline optsize { ... }
define void @f() optsize { ... }
``alignstack(<n>)``
This attribute indicates that, when emitting the prologue and
epilogue, the backend should forcibly align the stack pointer.
Specify the desired alignment, which must be a power of two, in
parentheses.
``allocsize(<EltSizeParam>[, <NumEltsParam>])``
This attribute indicates that the annotated function will always return at
least a given number of bytes (or null). Its arguments are zero-indexed
parameter numbers; if one argument is provided, then it's assumed that at
least ``CallSite.Args[EltSizeParam]`` bytes will be available at the
returned pointer. If two are provided, then it's assumed that
``CallSite.Args[EltSizeParam] * CallSite.Args[NumEltsParam]`` bytes are
available. The referenced parameters must be integer types. No assumptions
are made about the contents of the returned block of memory.
``alwaysinline``
This attribute indicates that the inliner should attempt to inline
this function into callers whenever possible, ignoring any active
inlining size threshold for this caller.
``builtin``
This indicates that the callee function at a call site should be
recognized as a built-in function, even though the function's declaration
uses the ``nobuiltin`` attribute. This is only valid at call sites for
direct calls to functions that are declared with the ``nobuiltin``
attribute.
``cold``
This attribute indicates that this function is rarely called. When
computing edge weights, basic blocks post-dominated by a cold
function call are also considered to be cold; and, thus, given low
weight.
``convergent``
In some parallel execution models, there exist operations that cannot be
made control-dependent on any additional values. We call such operations
``convergent``, and mark them with this attribute.
The ``convergent`` attribute may appear on functions or call/invoke
instructions. When it appears on a function, it indicates that calls to
this function should not be made control-dependent on additional values.
For example, the intrinsic ``llvm.nvvm.barrier0`` is ``convergent``, so
calls to this intrinsic cannot be made control-dependent on additional
values.
When it appears on a call/invoke, the ``convergent`` attribute indicates
that we should treat the call as though we're calling a convergent
function. This is particularly useful on indirect calls; without this we
may treat such calls as though the target is non-convergent.
The optimizer may remove the ``convergent`` attribute on functions when it
can prove that the function does not execute any convergent operations.
Similarly, the optimizer may remove ``convergent`` on calls/invokes when it
can prove that the call/invoke cannot call a convergent function.
``inaccessiblememonly``
This attribute indicates that the function may only access memory that
is not accessible by the module being compiled. This is a weaker form
of ``readnone``.
``inaccessiblemem_or_argmemonly``
This attribute indicates that the function may only access memory that is
either not accessible by the module being compiled, or is pointed to
by its pointer arguments. This is a weaker form of ``argmemonly``
``inlinehint``
This attribute indicates that the source code contained a hint that
inlining this function is desirable (such as the "inline" keyword in
C/C++). It is just a hint; it imposes no requirements on the
inliner.
``jumptable``
This attribute indicates that the function should be added to a
jump-instruction table at code-generation time, and that all address-taken
references to this function should be replaced with a reference to the
appropriate jump-instruction-table function pointer. Note that this creates
a new pointer for the original function, which means that code that depends
on function-pointer identity can break. So, any function annotated with
``jumptable`` must also be ``unnamed_addr``.
``minsize``
This attribute suggests that optimization passes and code generator
passes make choices that keep the code size of this function as small
as possible and perform optimizations that may sacrifice runtime
performance in order to minimize the size of the generated code.
``naked``
This attribute disables prologue / epilogue emission for the
function. This can have very system-specific consequences.
``no-jump-tables``
When this attribute is set to true, the jump tables and lookup tables that
can be generated from a switch case lowering are disabled.
``nobuiltin``
This indicates that the callee function at a call site is not recognized as
a built-in function. LLVM will retain the original call and not replace it
with equivalent code based on the semantics of the built-in function, unless
the call site uses the ``builtin`` attribute. This is valid at call sites
and on function declarations and definitions.
``noduplicate``
This attribute indicates that calls to the function cannot be
duplicated. A call to a ``noduplicate`` function may be moved
within its parent function, but may not be duplicated within
its parent function.
A function containing a ``noduplicate`` call may still
be an inlining candidate, provided that the call is not
duplicated by inlining. That implies that the function has
internal linkage and only has one call site, so the original
call is dead after inlining.
``noimplicitfloat``
This attributes disables implicit floating point instructions.
``noinline``
This attribute indicates that the inliner should never inline this
function in any situation. This attribute may not be used together
with the ``alwaysinline`` attribute.
``nonlazybind``
This attribute suppresses lazy symbol binding for the function. This
may make calls to the function faster, at the cost of extra program
startup time if the function is not called during program startup.
``noredzone``
This attribute indicates that the code generator should not use a
red zone, even if the target-specific ABI normally permits it.
``noreturn``
This function attribute indicates that the function never returns
normally. This produces undefined behavior at runtime if the
function ever does dynamically return.
``norecurse``
This function attribute indicates that the function does not call itself
either directly or indirectly down any possible call path. This produces
undefined behavior at runtime if the function ever does recurse.
``nounwind``
This function attribute indicates that the function never raises an
exception. If the function does raise an exception, its runtime
behavior is undefined. However, functions marked nounwind may still
trap or generate asynchronous exceptions. Exception handling schemes
that are recognized by LLVM to handle asynchronous exceptions, such
as SEH, will still provide their implementation defined semantics.
``optnone``
This function attribute indicates that most optimization passes will skip
this function, with the exception of interprocedural optimization passes.
Code generation defaults to the "fast" instruction selector.
This attribute cannot be used together with the ``alwaysinline``
attribute; this attribute is also incompatible
with the ``minsize`` attribute and the ``optsize`` attribute.
This attribute requires the ``noinline`` attribute to be specified on
the function as well, so the function is never inlined into any caller.
Only functions with the ``alwaysinline`` attribute are valid
candidates for inlining into the body of this function.
``optsize``
This attribute suggests that optimization passes and code generator
passes make choices that keep the code size of this function low,
and otherwise do optimizations specifically to reduce code size as
long as they do not significantly impact runtime performance.
``"patchable-function"``
This attribute tells the code generator that the code
generated for this function needs to follow certain conventions that
make it possible for a runtime function to patch over it later.
The exact effect of this attribute depends on its string value,
for which there currently is one legal possibility:
* ``"prologue-short-redirect"`` - This style of patchable
function is intended to support patching a function prologue to
redirect control away from the function in a thread safe
manner. It guarantees that the first instruction of the
function will be large enough to accommodate a short jump
instruction, and will be sufficiently aligned to allow being
fully changed via an atomic compare-and-swap instruction.
While the first requirement can be satisfied by inserting large
enough NOP, LLVM can and will try to re-purpose an existing
instruction (i.e. one that would have to be emitted anyway) as
the patchable instruction larger than a short jump.
``"prologue-short-redirect"`` is currently only supported on
x86-64.
This attribute by itself does not imply restrictions on
inter-procedural optimizations. All of the semantic effects the
patching may have to be separately conveyed via the linkage type.
``"probe-stack"``
This attribute indicates that the function will trigger a guard region
in the end of the stack. It ensures that accesses to the stack must be
no further apart than the size of the guard region to a previous
access of the stack. It takes one required string value, the name of
the stack probing function that will be called.
If a function that has a ``"probe-stack"`` attribute is inlined into
a function with another ``"probe-stack"`` attribute, the resulting
function has the ``"probe-stack"`` attribute of the caller. If a
function that has a ``"probe-stack"`` attribute is inlined into a
function that has no ``"probe-stack"`` attribute at all, the resulting
function has the ``"probe-stack"`` attribute of the callee.
``readnone``
On a function, this attribute indicates that the function computes its
result (or decides to unwind an exception) based strictly on its arguments,
without dereferencing any pointer arguments or otherwise accessing
any mutable state (e.g. memory, control registers, etc) visible to
caller functions. It does not write through any pointer arguments
(including ``byval`` arguments) and never changes any state visible
to callers. This means while it cannot unwind exceptions by calling
the ``C++`` exception throwing methods (since they write to memory), there may
be non-``C++`` mechanisms that throw exceptions without writing to LLVM
visible memory.
On an argument, this attribute indicates that the function does not
dereference that pointer argument, even though it may read or write the
memory that the pointer points to if accessed through other pointers.
``readonly``
On a function, this attribute indicates that the function does not write
through any pointer arguments (including ``byval`` arguments) or otherwise
modify any state (e.g. memory, control registers, etc) visible to
caller functions. It may dereference pointer arguments and read
state that may be set in the caller. A readonly function always
returns the same value (or unwinds an exception identically) when
called with the same set of arguments and global state. This means while it
cannot unwind exceptions by calling the ``C++`` exception throwing methods
(since they write to memory), there may be non-``C++`` mechanisms that throw
exceptions without writing to LLVM visible memory.
On an argument, this attribute indicates that the function does not write
through this pointer argument, even though it may write to the memory that
the pointer points to.
``"stack-probe-size"``
This attribute controls the behavior of stack probes: either
the ``"probe-stack"`` attribute, or ABI-required stack probes, if any.
It defines the size of the guard region. It ensures that if the function
may use more stack space than the size of the guard region, stack probing
sequence will be emitted. It takes one required integer value, which
is 4096 by default.
If a function that has a ``"stack-probe-size"`` attribute is inlined into
a function with another ``"stack-probe-size"`` attribute, the resulting
function has the ``"stack-probe-size"`` attribute that has the lower
numeric value. If a function that has a ``"stack-probe-size"`` attribute is
inlined into a function that has no ``"stack-probe-size"`` attribute
at all, the resulting function has the ``"stack-probe-size"`` attribute
of the callee.
``writeonly``
On a function, this attribute indicates that the function may write to but
does not read from memory.
On an argument, this attribute indicates that the function may write to but
does not read through this pointer argument (even though it may read from
the memory that the pointer points to).
``argmemonly``
This attribute indicates that the only memory accesses inside function are
loads and stores from objects pointed to by its pointer-typed arguments,
with arbitrary offsets. Or in other words, all memory operations in the
function can refer to memory only using pointers based on its function
arguments.
Note that ``argmemonly`` can be used together with ``readonly`` attribute
in order to specify that function reads only from its arguments.
``returns_twice``
This attribute indicates that this function can return twice. The C
``setjmp`` is an example of such a function. The compiler disables
some optimizations (like tail calls) in the caller of these
functions.
``safestack``
This attribute indicates that
`SafeStack <http://clang.llvm.org/docs/SafeStack.html>`_
protection is enabled for this function.
If a function that has a ``safestack`` attribute is inlined into a
function that doesn't have a ``safestack`` attribute or which has an
``ssp``, ``sspstrong`` or ``sspreq`` attribute, then the resulting
function will have a ``safestack`` attribute.
``sanitize_address``
This attribute indicates that AddressSanitizer checks
(dynamic address safety analysis) are enabled for this function.
``sanitize_memory``
This attribute indicates that MemorySanitizer checks (dynamic detection
of accesses to uninitialized memory) are enabled for this function.
``sanitize_thread``
This attribute indicates that ThreadSanitizer checks
(dynamic thread safety analysis) are enabled for this function.
``speculatable``
This function attribute indicates that the function does not have any
effects besides calculating its result and does not have undefined behavior.
Note that ``speculatable`` is not enough to conclude that along any
particular execution path the number of calls to this function will not be
externally observable. This attribute is only valid on functions
and declarations, not on individual call sites. If a function is
incorrectly marked as speculatable and really does exhibit
undefined behavior, the undefined behavior may be observed even
if the call site is dead code.
``ssp``
This attribute indicates that the function should emit a stack
smashing protector. It is in the form of a "canary" --- a random value
placed on the stack before the local variables that's checked upon
return from the function to see if it has been overwritten. A
heuristic is used to determine if a function needs stack protectors
or not. The heuristic used will enable protectors for functions with:
- Character arrays larger than ``ssp-buffer-size`` (default 8).
- Aggregates containing character arrays larger than ``ssp-buffer-size``.
- Calls to alloca() with variable sizes or constant sizes greater than
``ssp-buffer-size``.
Variables that are identified as requiring a protector will be arranged
on the stack such that they are adjacent to the stack protector guard.
If a function that has an ``ssp`` attribute is inlined into a
function that doesn't have an ``ssp`` attribute, then the resulting
function will have an ``ssp`` attribute.
``sspreq``
This attribute indicates that the function should *always* emit a
stack smashing protector. This overrides the ``ssp`` function
attribute.
Variables that are identified as requiring a protector will be arranged
on the stack such that they are adjacent to the stack protector guard.
The specific layout rules are:
#. Large arrays and structures containing large arrays
(``>= ssp-buffer-size``) are closest to the stack protector.
#. Small arrays and structures containing small arrays
(``< ssp-buffer-size``) are 2nd closest to the protector.
#. Variables that have had their address taken are 3rd closest to the
protector.
If a function that has an ``sspreq`` attribute is inlined into a
function that doesn't have an ``sspreq`` attribute or which has an
``ssp`` or ``sspstrong`` attribute, then the resulting function will have
an ``sspreq`` attribute.
``sspstrong``
This attribute indicates that the function should emit a stack smashing
protector. This attribute causes a strong heuristic to be used when
determining if a function needs stack protectors. The strong heuristic
will enable protectors for functions with:
- Arrays of any size and type
- Aggregates containing an array of any size and type.
- Calls to alloca().
- Local variables that have had their address taken.
Variables that are identified as requiring a protector will be arranged
on the stack such that they are adjacent to the stack protector guard.
The specific layout rules are:
#. Large arrays and structures containing large arrays
(``>= ssp-buffer-size``) are closest to the stack protector.
#. Small arrays and structures containing small arrays
(``< ssp-buffer-size``) are 2nd closest to the protector.
#. Variables that have had their address taken are 3rd closest to the
protector.
This overrides the ``ssp`` function attribute.
If a function that has an ``sspstrong`` attribute is inlined into a
function that doesn't have an ``sspstrong`` attribute, then the
resulting function will have an ``sspstrong`` attribute.
``strictfp``
This attribute indicates that the function was called from a scope that
requires strict floating point semantics. LLVM will not attempt any
optimizations that require assumptions about the floating point rounding
mode or that might alter the state of floating point status flags that
might otherwise be set or cleared by calling this function.
``"thunk"``
This attribute indicates that the function will delegate to some other
function with a tail call. The prototype of a thunk should not be used for
optimization purposes. The caller is expected to cast the thunk prototype to
match the thunk target prototype.
``uwtable``
This attribute indicates that the ABI being targeted requires that
an unwind table entry be produced for this function even if we can
show that no exceptions passes by it. This is normally the case for
the ELF x86-64 abi, but it can be disabled for some compilation
units.
.. _glattrs:
Global Attributes
-----------------
Attributes may be set to communicate additional information about a global variable.
Unlike :ref:`function attributes <fnattrs>`, attributes on a global variable
are grouped into a single :ref:`attribute group <attrgrp>`.
.. _opbundles:
Operand Bundles
---------------
Operand bundles are tagged sets of SSA values that can be associated
with certain LLVM instructions (currently only ``call`` s and
``invoke`` s). In a way they are like metadata, but dropping them is
incorrect and will change program semantics.
Syntax::
operand bundle set ::= '[' operand bundle (, operand bundle )* ']'
operand bundle ::= tag '(' [ bundle operand ] (, bundle operand )* ')'
bundle operand ::= SSA value
tag ::= string constant
Operand bundles are **not** part of a function's signature, and a
given function may be called from multiple places with different kinds
of operand bundles. This reflects the fact that the operand bundles
are conceptually a part of the ``call`` (or ``invoke``), not the
callee being dispatched to.
Operand bundles are a generic mechanism intended to support
runtime-introspection-like functionality for managed languages. While
the exact semantics of an operand bundle depend on the bundle tag,
there are certain limitations to how much the presence of an operand
bundle can influence the semantics of a program. These restrictions
are described as the semantics of an "unknown" operand bundle. As
long as the behavior of an operand bundle is describable within these
restrictions, LLVM does not need to have special knowledge of the
operand bundle to not miscompile programs containing it.
- The bundle operands for an unknown operand bundle escape in unknown
ways before control is transferred to the callee or invokee.
- Calls and invokes with operand bundles have unknown read / write
effect on the heap on entry and exit (even if the call target is
``readnone`` or ``readonly``), unless they're overridden with
callsite specific attributes.
- An operand bundle at a call site cannot change the implementation
of the called function. Inter-procedural optimizations work as
usual as long as they take into account the first two properties.
More specific types of operand bundles are described below.
.. _deopt_opbundles:
Deoptimization Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Deoptimization operand bundles are characterized by the ``"deopt"``
operand bundle tag. These operand bundles represent an alternate
"safe" continuation for the call site they're attached to, and can be
used by a suitable runtime to deoptimize the compiled frame at the
specified call site. There can be at most one ``"deopt"`` operand
bundle attached to a call site. Exact details of deoptimization is
out of scope for the language reference, but it usually involves
rewriting a compiled frame into a set of interpreted frames.
From the compiler's perspective, deoptimization operand bundles make
the call sites they're attached to at least ``readonly``. They read
through all of their pointer typed operands (even if they're not
otherwise escaped) and the entire visible heap. Deoptimization
operand bundles do not capture their operands except during
deoptimization, in which case control will not be returned to the
compiled frame.
The inliner knows how to inline through calls that have deoptimization
operand bundles. Just like inlining through a normal call site
involves composing the normal and exceptional continuations, inlining
through a call site with a deoptimization operand bundle needs to
appropriately compose the "safe" deoptimization continuation. The
inliner does this by prepending the parent's deoptimization
continuation to every deoptimization continuation in the inlined body.
E.g. inlining ``@f`` into ``@g`` in the following example
.. code-block:: llvm
define void @f() {
call void @x() ;; no deopt state
call void @y() [ "deopt"(i32 10) ]
call void @y() [ "deopt"(i32 10), "unknown"(i8* null) ]
ret void
}
define void @g() {
call void @f() [ "deopt"(i32 20) ]
ret void
}
will result in
.. code-block:: llvm
define void @g() {
call void @x() ;; still no deopt state
call void @y() [ "deopt"(i32 20, i32 10) ]
call void @y() [ "deopt"(i32 20, i32 10), "unknown"(i8* null) ]
ret void
}
It is the frontend's responsibility to structure or encode the
deoptimization state in a way that syntactically prepending the
caller's deoptimization state to the callee's deoptimization state is
semantically equivalent to composing the caller's deoptimization
continuation after the callee's deoptimization continuation.
.. _ob_funclet:
Funclet Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^
Funclet operand bundles are characterized by the ``"funclet"``
operand bundle tag. These operand bundles indicate that a call site
is within a particular funclet. There can be at most one
``"funclet"`` operand bundle attached to a call site and it must have
exactly one bundle operand.
If any funclet EH pads have been "entered" but not "exited" (per the
`description in the EH doc\ <ExceptionHandling.html#wineh-constraints>`_),
it is undefined behavior to execute a ``call`` or ``invoke`` which:
* does not have a ``"funclet"`` bundle and is not a ``call`` to a nounwind
intrinsic, or
* has a ``"funclet"`` bundle whose operand is not the most-recently-entered
not-yet-exited funclet EH pad.
Similarly, if no funclet EH pads have been entered-but-not-yet-exited,
executing a ``call`` or ``invoke`` with a ``"funclet"`` bundle is undefined behavior.
GC Transition Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
GC transition operand bundles are characterized by the
``"gc-transition"`` operand bundle tag. These operand bundles mark a
call as a transition between a function with one GC strategy to a
function with a different GC strategy. If coordinating the transition
between GC strategies requires additional code generation at the call
site, these bundles may contain any values that are needed by the
generated code. For more details, see :ref:`GC Transitions
<gc_transition_args>`.
.. _moduleasm:
Module-Level Inline Assembly
----------------------------
Modules may contain "module-level inline asm" blocks, which corresponds
to the GCC "file scope inline asm" blocks. These blocks are internally
concatenated by LLVM and treated as a single unit, but may be separated
in the ``.ll`` file if desired. The syntax is very simple:
.. code-block:: llvm
module asm "inline asm code goes here"
module asm "more can go here"
The strings can contain any character by escaping non-printable
characters. The escape sequence used is simply "\\xx" where "xx" is the
two digit hex code for the number.
Note that the assembly string *must* be parseable by LLVM's integrated assembler
(unless it is disabled), even when emitting a ``.s`` file.
.. _langref_datalayout:
Data Layout
-----------
A module may specify a target specific data layout string that specifies
how data is to be laid out in memory. The syntax for the data layout is
simply:
.. code-block:: llvm
target datalayout = "layout specification"
The *layout specification* consists of a list of specifications
separated by the minus sign character ('-'). Each specification starts
with a letter and may include other information after the letter to
define some aspect of the data layout. The specifications accepted are
as follows:
``E``
Specifies that the target lays out data in big-endian form. That is,
the bits with the most significance have the lowest address
location.
``e``
Specifies that the target lays out data in little-endian form. That
is, the bits with the least significance have the lowest address
location.
``S<size>``
Specifies the natural alignment of the stack in bits. Alignment
promotion of stack variables is limited to the natural stack
alignment to avoid dynamic stack realignment. The stack alignment
must be a multiple of 8-bits. If omitted, the natural stack
alignment defaults to "unspecified", which does not prevent any
alignment promotions.
``A<address space>``
Specifies the address space of objects created by '``alloca``'.
Defaults to the default address space of 0.
``p[n]:<size>:<abi>:<pref>``
This specifies the *size* of a pointer and its ``<abi>`` and
``<pref>``\erred alignments for address space ``n``. All sizes are in
bits. The address space, ``n``, is optional, and if not specified,
denotes the default address space 0. The value of ``n`` must be
in the range [1,2^23).
``i<size>:<abi>:<pref>``
This specifies the alignment for an integer type of a given bit
``<size>``. The value of ``<size>`` must be in the range [1,2^23).
``v<size>:<abi>:<pref>``
This specifies the alignment for a vector type of a given bit
``<size>``.
``f<size>:<abi>:<pref>``
This specifies the alignment for a floating point type of a given bit
``<size>``. Only values of ``<size>`` that are supported by the target
will work. 32 (float) and 64 (double) are supported on all targets; 80
or 128 (different flavors of long double) are also supported on some
targets.
``a:<abi>:<pref>``
This specifies the alignment for an object of aggregate type.
``m:<mangling>``
If present, specifies that llvm names are mangled in the output. The
options are
* ``e``: ELF mangling: Private symbols get a ``.L`` prefix.
* ``m``: Mips mangling: Private symbols get a ``$`` prefix.
* ``o``: Mach-O mangling: Private symbols get ``L`` prefix. Other
symbols get a ``_`` prefix.
* ``w``: Windows COFF prefix: Similar to Mach-O, but stdcall and fastcall
functions also get a suffix based on the frame size.
* ``x``: Windows x86 COFF prefix: Similar to Windows COFF, but use a ``_``
prefix for ``__cdecl`` functions.
``n<size1>:<size2>:<size3>...``
This specifies a set of native integer widths for the target CPU in
bits. For example, it might contain ``n32`` for 32-bit PowerPC,
``n32:64`` for PowerPC 64, or ``n8:16:32:64`` for X86-64. Elements of
this set are considered to support most general arithmetic operations
efficiently.
``ni:<address space0>:<address space1>:<address space2>...``
This specifies pointer types with the specified address spaces
as :ref:`Non-Integral Pointer Type <nointptrtype>` s. The ``0``
address space cannot be specified as non-integral.
On every specification that takes a ``<abi>:<pref>``, specifying the
``<pref>`` alignment is optional. If omitted, the preceding ``:``
should be omitted too and ``<pref>`` will be equal to ``<abi>``.
When constructing the data layout for a given target, LLVM starts with a
default set of specifications which are then (possibly) overridden by
the specifications in the ``datalayout`` keyword. The default
specifications are given in this list:
- ``E`` - big endian
- ``p:64:64:64`` - 64-bit pointers with 64-bit alignment.
- ``p[n]:64:64:64`` - Other address spaces are assumed to be the
same as the default address space.
- ``S0`` - natural stack alignment is unspecified
- ``i1:8:8`` - i1 is 8-bit (byte) aligned
- ``i8:8:8`` - i8 is 8-bit (byte) aligned
- ``i16:16:16`` - i16 is 16-bit aligned
- ``i32:32:32`` - i32 is 32-bit aligned
- ``i64:32:64`` - i64 has ABI alignment of 32-bits but preferred
alignment of 64-bits
- ``f16:16:16`` - half is 16-bit aligned
- ``f32:32:32`` - float is 32-bit aligned
- ``f64:64:64`` - double is 64-bit aligned
- ``f128:128:128`` - quad is 128-bit aligned
- ``v64:64:64`` - 64-bit vector is 64-bit aligned
- ``v128:128:128`` - 128-bit vector is 128-bit aligned
- ``a:0:64`` - aggregates are 64-bit aligned
When LLVM is determining the alignment for a given type, it uses the
following rules:
#. If the type sought is an exact match for one of the specifications,
that specification is used.
#. If no match is found, and the type sought is an integer type, then
the smallest integer type that is larger than the bitwidth of the
sought type is used. If none of the specifications are larger than
the bitwidth then the largest integer type is used. For example,
given the default specifications above, the i7 type will use the
alignment of i8 (next largest) while both i65 and i256 will use the
alignment of i64 (largest specified).
#. If no match is found, and the type sought is a vector type, then the
largest vector type that is smaller than the sought vector type will
be used as a fall back. This happens because <128 x double> can be
implemented in terms of 64 <2 x double>, for example.
The function of the data layout string may not be what you expect.
Notably, this is not a specification from the frontend of what alignment
the code generator should use.
Instead, if specified, the target data layout is required to match what
the ultimate *code generator* expects. This string is used by the
mid-level optimizers to improve code, and this only works if it matches
what the ultimate code generator uses. There is no way to generate IR
that does not embed this target-specific detail into the IR. If you
don't specify the string, the default specifications will be used to
generate a Data Layout and the optimization phases will operate
accordingly and introduce target specificity into the IR with respect to
these default specifications.
.. _langref_triple:
Target Triple
-------------
A module may specify a target triple string that describes the target
host. The syntax for the target triple is simply:
.. code-block:: llvm
target triple = "x86_64-apple-macosx10.7.0"
The *target triple* string consists of a series of identifiers delimited
by the minus sign character ('-'). The canonical forms are:
::
ARCHITECTURE-VENDOR-OPERATING_SYSTEM
ARCHITECTURE-VENDOR-OPERATING_SYSTEM-ENVIRONMENT
This information is passed along to the backend so that it generates
code for the proper architecture. It's possible to override this on the
command line with the ``-mtriple`` command line option.
.. _pointeraliasing:
Pointer Aliasing Rules
----------------------
Any memory access must be done through a pointer value associated with
an address range of the memory access, otherwise the behavior is
undefined. Pointer values are associated with address ranges according
to the following rules:
- A pointer value is associated with the addresses associated with any
value it is *based* on.
- An address of a global variable is associated with the address range
of the variable's storage.
- The result value of an allocation instruction is associated with the
address range of the allocated storage.
- A null pointer in the default address-space is associated with no
address.
- An integer constant other than zero or a pointer value returned from
a function not defined within LLVM may be associated with address
ranges allocated through mechanisms other than those provided by
LLVM. Such ranges shall not overlap with any ranges of addresses
allocated by mechanisms provided by LLVM.
A pointer value is *based* on another pointer value according to the
following rules:
- A pointer value formed from a scalar ``getelementptr`` operation is *based* on
the pointer-typed operand of the ``getelementptr``.
- The pointer in lane *l* of the result of a vector ``getelementptr`` operation
is *based* on the pointer in lane *l* of the vector-of-pointers-typed operand
of the ``getelementptr``.
- The result value of a ``bitcast`` is *based* on the operand of the
``bitcast``.
- A pointer value formed by an ``inttoptr`` is *based* on all pointer
values that contribute (directly or indirectly) to the computation of
the pointer's value.
- The "*based* on" relationship is transitive.
Note that this definition of *"based"* is intentionally similar to the
definition of *"based"* in C99, though it is slightly weaker.
LLVM IR does not associate types with memory. The result type of a
``load`` merely indicates the size and alignment of the memory from
which to load, as well as the interpretation of the value. The first
operand type of a ``store`` similarly only indicates the size and
alignment of the store.
Consequently, type-based alias analysis, aka TBAA, aka
``-fstrict-aliasing``, is not applicable to general unadorned LLVM IR.
:ref:`Metadata <metadata>` may be used to encode additional information
which specialized optimization passes may use to implement type-based
alias analysis.
.. _volatile:
Volatile Memory Accesses
------------------------
Certain memory accesses, such as :ref:`load <i_load>`'s,
:ref:`store <i_store>`'s, and :ref:`llvm.memcpy <int_memcpy>`'s may be
marked ``volatile``. The optimizers must not change the number of
volatile operations or change their order of execution relative to other
volatile operations. The optimizers *may* change the order of volatile
operations relative to non-volatile operations. This is not Java's
"volatile" and has no cross-thread synchronization behavior.
IR-level volatile loads and stores cannot safely be optimized into
llvm.memcpy or llvm.memmove intrinsics even when those intrinsics are
flagged volatile. Likewise, the backend should never split or merge
target-legal volatile load/store instructions.
.. admonition:: Rationale
Platforms may rely on volatile loads and stores of natively supported
data width to be executed as single instruction. For example, in C
this holds for an l-value of volatile primitive type with native
hardware support, but not necessarily for aggregate types. The
frontend upholds these expectations, which are intentionally
unspecified in the IR. The rules above ensure that IR transformations
do not violate the frontend's contract with the language.
.. _memmodel:
Memory Model for Concurrent Operations
--------------------------------------
The LLVM IR does not define any way to start parallel threads of
execution or to register signal handlers. Nonetheless, there are
platform-specific ways to create them, and we define LLVM IR's behavior
in their presence. This model is inspired by the C++0x memory model.
For a more informal introduction to this model, see the :doc:`Atomics`.
We define a *happens-before* partial order as the least partial order
that
- Is a superset of single-thread program order, and
- When a *synchronizes-with* ``b``, includes an edge from ``a`` to
``b``. *Synchronizes-with* pairs are introduced by platform-specific
techniques, like pthread locks, thread creation, thread joining,
etc., and by atomic instructions. (See also :ref:`Atomic Memory Ordering
Constraints <ordering>`).
Note that program order does not introduce *happens-before* edges
between a thread and signals executing inside that thread.
Every (defined) read operation (load instructions, memcpy, atomic
loads/read-modify-writes, etc.) R reads a series of bytes written by
(defined) write operations (store instructions, atomic
stores/read-modify-writes, memcpy, etc.). For the purposes of this
section, initialized globals are considered to have a write of the
initializer which is atomic and happens before any other read or write
of the memory in question. For each byte of a read R, R\ :sub:`byte`
may see any write to the same byte, except:
- If write\ :sub:`1` happens before write\ :sub:`2`, and
write\ :sub:`2` happens before R\ :sub:`byte`, then
R\ :sub:`byte` does not see write\ :sub:`1`.
- If R\ :sub:`byte` happens before write\ :sub:`3`, then
R\ :sub:`byte` does not see write\ :sub:`3`.
Given that definition, R\ :sub:`byte` is defined as follows:
- If R is volatile, the result is target-dependent. (Volatile is
supposed to give guarantees which can support ``sig_atomic_t`` in
C/C++, and may be used for accesses to addresses that do not behave
like normal memory. It does not generally provide cross-thread
synchronization.)
- Otherwise, if there is no write to the same byte that happens before
R\ :sub:`byte`, R\ :sub:`byte` returns ``undef`` for that byte.
- Otherwise, if R\ :sub:`byte` may see exactly one write,
R\ :sub:`byte` returns the value written by that write.
- Otherwise, if R is atomic, and all the writes R\ :sub:`byte` may
see are atomic, it chooses one of the values written. See the :ref:`Atomic
Memory Ordering Constraints <ordering>` section for additional
constraints on how the choice is made.
- Otherwise R\ :sub:`byte` returns ``undef``.
R returns the value composed of the series of bytes it read. This
implies that some bytes within the value may be ``undef`` **without**
the entire value being ``undef``. Note that this only defines the
semantics of the operation; it doesn't mean that targets will emit more
than one instruction to read the series of bytes.
Note that in cases where none of the atomic intrinsics are used, this
model places only one restriction on IR transformations on top of what
is required for single-threaded execution: introducing a store to a byte
which might not otherwise be stored is not allowed in general.
(Specifically, in the case where another thread might write to and read
from an address, introducing a store can change a load that may see
exactly one write into a load that may see multiple writes.)
.. _ordering:
Atomic Memory Ordering Constraints
----------------------------------
Atomic instructions (:ref:`cmpxchg <i_cmpxchg>`,
:ref:`atomicrmw <i_atomicrmw>`, :ref:`fence <i_fence>`,
:ref:`atomic load <i_load>`, and :ref:`atomic store <i_store>`) take
ordering parameters that determine which other atomic instructions on
the same address they *synchronize with*. These semantics are borrowed
from Java and C++0x, but are somewhat more colloquial. If these
descriptions aren't precise enough, check those specs (see spec
references in the :doc:`atomics guide <Atomics>`).
:ref:`fence <i_fence>` instructions treat these orderings somewhat
differently since they don't take an address. See that instruction's
documentation for details.
For a simpler introduction to the ordering constraints, see the
:doc:`Atomics`.
``unordered``
The set of values that can be read is governed by the happens-before
partial order. A value cannot be read unless some operation wrote
it. This is intended to provide a guarantee strong enough to model
Java's non-volatile shared variables. This ordering cannot be
specified for read-modify-write operations; it is not strong enough
to make them atomic in any interesting way.
``monotonic``
In addition to the guarantees of ``unordered``, there is a single
total order for modifications by ``monotonic`` operations on each
address. All modification orders must be compatible with the
happens-before order. There is no guarantee that the modification
orders can be combined to a global total order for the whole program
(and this often will not be possible). The read in an atomic
read-modify-write operation (:ref:`cmpxchg <i_cmpxchg>` and
:ref:`atomicrmw <i_atomicrmw>`) reads the value in the modification
order immediately before the value it writes. If one atomic read
happens before another atomic read of the same address, the later
read must see the same value or a later value in the address's
modification order. This disallows reordering of ``monotonic`` (or
stronger) operations on the same address. If an address is written
``monotonic``-ally by one thread, and other threads ``monotonic``-ally
read that address repeatedly, the other threads must eventually see
the write. This corresponds to the C++0x/C1x
``memory_order_relaxed``.
``acquire``
In addition to the guarantees of ``monotonic``, a
*synchronizes-with* edge may be formed with a ``release`` operation.
This is intended to model C++'s ``memory_order_acquire``.
``release``
In addition to the guarantees of ``monotonic``, if this operation
writes a value which is subsequently read by an ``acquire``
operation, it *synchronizes-with* that operation. (This isn't a
complete description; see the C++0x definition of a release
sequence.) This corresponds to the C++0x/C1x
``memory_order_release``.
``acq_rel`` (acquire+release)
Acts as both an ``acquire`` and ``release`` operation on its
address. This corresponds to the C++0x/C1x ``memory_order_acq_rel``.
``seq_cst`` (sequentially consistent)
In addition to the guarantees of ``acq_rel`` (``acquire`` for an
operation that only reads, ``release`` for an operation that only
writes), there is a global total order on all
sequentially-consistent operations on all addresses, which is
consistent with the *happens-before* partial order and with the
modification orders of all the affected addresses. Each
sequentially-consistent read sees the last preceding write to the
same address in this global order. This corresponds to the C++0x/C1x
``memory_order_seq_cst`` and Java volatile.
.. _syncscope:
If an atomic operation is marked ``syncscope("singlethread")``, it only
*synchronizes with* and only participates in the seq\_cst total orderings of
other operations running in the same thread (for example, in signal handlers).
If an atomic operation is marked ``syncscope("<target-scope>")``, where
``<target-scope>`` is a target specific synchronization scope, then it is target
dependent if it *synchronizes with* and participates in the seq\_cst total
orderings of other operations.
Otherwise, an atomic operation that is not marked ``syncscope("singlethread")``
or ``syncscope("<target-scope>")`` *synchronizes with* and participates in the
seq\_cst total orderings of other operations that are not marked
``syncscope("singlethread")`` or ``syncscope("<target-scope>")``.
.. _fastmath:
Fast-Math Flags
---------------
LLVM IR floating-point binary ops (:ref:`fadd <i_fadd>`,
:ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,
:ref:`frem <i_frem>`, :ref:`fcmp <i_fcmp>`) and :ref:`call <i_call>`
instructions have the following flags that can be set to enable
otherwise unsafe floating point transformations.
``nnan``
No NaNs - Allow optimizations to assume the arguments and result are not
NaN. Such optimizations are required to retain defined behavior over
NaNs, but the value of the result is undefined.
``ninf``
No Infs - Allow optimizations to assume the arguments and result are not
+/-Inf. Such optimizations are required to retain defined behavior over
+/-Inf, but the value of the result is undefined.
``nsz``
No Signed Zeros - Allow optimizations to treat the sign of a zero
argument or result as insignificant.
``arcp``
Allow Reciprocal - Allow optimizations to use the reciprocal of an
argument rather than perform division.
``contract``
Allow floating-point contraction (e.g. fusing a multiply followed by an
addition into a fused multiply-and-add).
``fast``
Fast - Allow algebraically equivalent transformations that may
dramatically change results in floating point (e.g. reassociate). This
flag implies all the others.
.. _uselistorder:
Use-list Order Directives
-------------------------
Use-list directives encode the in-memory order of each use-list, allowing the
order to be recreated. ``<order-indexes>`` is a comma-separated list of
indexes that are assigned to the referenced value's uses. The referenced
value's use-list is immediately sorted by these indexes.
Use-list directives may appear at function scope or global scope. They are not
instructions, and have no effect on the semantics of the IR. When they're at
function scope, they must appear after the terminator of the final basic block.
If basic blocks have their address taken via ``blockaddress()`` expressions,
``uselistorder_bb`` can be used to reorder their use-lists from outside their
function's scope.
:Syntax:
::
uselistorder <ty> <value>, { <order-indexes> }
uselistorder_bb @function, %block { <order-indexes> }
:Examples:
::
define void @foo(i32 %arg1, i32 %arg2) {
entry:
; ... instructions ...
bb:
; ... instructions ...
; At function scope.
uselistorder i32 %arg1, { 1, 0, 2 }
uselistorder label %bb, { 1, 0 }
}
; At global scope.
uselistorder i32* @global, { 1, 2, 0 }
uselistorder i32 7, { 1, 0 }
uselistorder i32 (i32) @bar, { 1, 0 }
uselistorder_bb @foo, %bb, { 5, 1, 3, 2, 0, 4 }
.. _source_filename:
Source Filename
---------------
The *source filename* string is set to the original module identifier,
which will be the name of the compiled source file when compiling from
source through the clang front end, for example. It is then preserved through
the IR and bitcode.
This is currently necessary to generate a consistent unique global
identifier for local functions used in profile data, which prepends the
source file name to the local function name.
The syntax for the source file name is simply:
.. code-block:: text
source_filename = "/path/to/source.c"
.. _typesystem:
Type System
===========
The LLVM type system is one of the most important features of the
intermediate representation. Being typed enables a number of
optimizations to be performed on the intermediate representation
directly, without having to do extra analyses on the side before the
transformation. A strong type system makes it easier to read the
generated code and enables novel analyses and transformations that are
not feasible to perform on normal three address code representations.
.. _t_void:
Void Type
---------
:Overview:
The void type does not represent any value and has no size.
:Syntax:
::
void
.. _t_function:
Function Type
-------------
:Overview:
The function type can be thought of as a function signature. It consists of a
return type and a list of formal parameter types. The return type of a function
type is a void type or first class type --- except for :ref:`label <t_label>`
and :ref:`metadata <t_metadata>` types.
:Syntax:
::
<returntype> (<parameter list>)
...where '``<parameter list>``' is a comma-separated list of type
specifiers. Optionally, the parameter list may include a type ``...``, which
indicates that the function takes a variable number of arguments. Variable
argument functions can access their arguments with the :ref:`variable argument
handling intrinsic <int_varargs>` functions. '``<returntype>``' is any type
except :ref:`label <t_label>` and :ref:`metadata <t_metadata>`.
:Examples:
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``i32 (i32)`` | function taking an ``i32``, returning an ``i32`` |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``float (i16, i32 *) *`` | :ref:`Pointer <t_pointer>` to a function that takes an ``i16`` and a :ref:`pointer <t_pointer>` to ``i32``, returning ``float``. |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``i32 (i8*, ...)`` | A vararg function that takes at least one :ref:`pointer <t_pointer>` to ``i8`` (char in C), which returns an integer. This is the signature for ``printf`` in LLVM. |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{i32, i32} (i32)`` | A function taking an ``i32``, returning a :ref:`structure <t_struct>` containing two ``i32`` values |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
.. _t_firstclass:
First Class Types
-----------------
The :ref:`first class <t_firstclass>` types are perhaps the most important.
Values of these types are the only ones which can be produced by
instructions.
.. _t_single_value:
Single Value Types
^^^^^^^^^^^^^^^^^^
These are the types that are valid in registers from CodeGen's perspective.
.. _t_integer:
Integer Type
""""""""""""
:Overview:
The integer type is a very simple type that simply specifies an
arbitrary bit width for the integer type desired. Any bit width from 1
bit to 2\ :sup:`23`\ -1 (about 8 million) can be specified.
:Syntax:
::
iN
The number of bits the integer will occupy is specified by the ``N``
value.
Examples:
*********
+----------------+------------------------------------------------+
| ``i1`` | a single-bit integer. |
+----------------+------------------------------------------------+
| ``i32`` | a 32-bit integer. |
+----------------+------------------------------------------------+
| ``i1942652`` | a really big integer of over 1 million bits. |
+----------------+------------------------------------------------+
.. _t_floating:
Floating Point Types
""""""""""""""""""""
.. list-table::
:header-rows: 1
* - Type
- Description
* - ``half``
- 16-bit floating point value
* - ``float``
- 32-bit floating point value
* - ``double``
- 64-bit floating point value
* - ``fp128``
- 128-bit floating point value (112-bit mantissa)
* - ``x86_fp80``
- 80-bit floating point value (X87)
* - ``ppc_fp128``
- 128-bit floating point value (two 64-bits)
X86_mmx Type
""""""""""""
:Overview:
The x86_mmx type represents a value held in an MMX register on an x86
machine. The operations allowed on it are quite limited: parameters and
return values, load and store, and bitcast. User-specified MMX
instructions are represented as intrinsic or asm calls with arguments
and/or results of this type. There are no arrays, vectors or constants
of this type.
:Syntax:
::
x86_mmx
.. _t_pointer:
Pointer Type
""""""""""""
:Overview:
The pointer type is used to specify memory locations. Pointers are
commonly used to reference objects in memory.
Pointer types may have an optional address space attribute defining the
numbered address space where the pointed-to object resides. The default
address space is number zero. The semantics of non-zero address spaces
are target-specific.
Note that LLVM does not permit pointers to void (``void*``) nor does it
permit pointers to labels (``label*``). Use ``i8*`` instead.
:Syntax:
::
<type> *
:Examples:
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``[4 x i32]*`` | A :ref:`pointer <t_pointer>` to :ref:`array <t_array>` of four ``i32`` values. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``i32 (i32*) *`` | A :ref:`pointer <t_pointer>` to a :ref:`function <t_function>` that takes an ``i32*``, returning an ``i32``. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``i32 addrspace(5)*`` | A :ref:`pointer <t_pointer>` to an ``i32`` value that resides in address space #5. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
.. _t_vector:
Vector Type
"""""""""""
:Overview:
A vector type is a simple derived type that represents a vector of
elements. Vector types are used when multiple primitive data are
operated in parallel using a single instruction (SIMD). A vector type
requires a size (number of elements) and an underlying primitive data
type. Vector types are considered :ref:`first class <t_firstclass>`.
:Syntax:
::
< <# elements> x <elementtype> >
The number of elements is a constant integer value larger than 0;
elementtype may be any integer, floating point or pointer type. Vectors
of size zero are not allowed.
:Examples:
+-------------------+--------------------------------------------------+
| ``<4 x i32>`` | Vector of 4 32-bit integer values. |
+-------------------+--------------------------------------------------+
| ``<8 x float>`` | Vector of 8 32-bit floating-point values. |
+-------------------+--------------------------------------------------+
| ``<2 x i64>`` | Vector of 2 64-bit integer values. |
+-------------------+--------------------------------------------------+
| ``<4 x i64*>`` | Vector of 4 pointers to 64-bit integer values. |
+-------------------+--------------------------------------------------+
.. _t_label:
Label Type
^^^^^^^^^^
:Overview:
The label type represents code labels.
:Syntax:
::
label
.. _t_token:
Token Type
^^^^^^^^^^
:Overview:
The token type is used when a value is associated with an instruction
but all uses of the value must not attempt to introspect or obscure it.
As such, it is not appropriate to have a :ref:`phi <i_phi>` or
:ref:`select <i_select>` of type token.
:Syntax:
::
token
.. _t_metadata:
Metadata Type
^^^^^^^^^^^^^
:Overview:
The metadata type represents embedded metadata. No derived types may be
created from metadata except for :ref:`function <t_function>` arguments.
:Syntax:
::
metadata
.. _t_aggregate:
Aggregate Types
^^^^^^^^^^^^^^^
Aggregate Types are a subset of derived types that can contain multiple
member types. :ref:`Arrays <t_array>` and :ref:`structs <t_struct>` are
aggregate types. :ref:`Vectors <t_vector>` are not considered to be
aggregate types.
.. _t_array:
Array Type
""""""""""
:Overview:
The array type is a very simple derived type that arranges elements
sequentially in memory. The array type requires a size (number of
elements) and an underlying data type.
:Syntax:
::
[<# elements> x <elementtype>]
The number of elements is a constant integer value; ``elementtype`` may
be any type with a size.
:Examples:
+------------------+--------------------------------------+
| ``[40 x i32]`` | Array of 40 32-bit integer values. |
+------------------+--------------------------------------+
| ``[41 x i32]`` | Array of 41 32-bit integer values. |
+------------------+--------------------------------------+
| ``[4 x i8]`` | Array of 4 8-bit integer values. |
+------------------+--------------------------------------+
Here are some examples of multidimensional arrays:
+-----------------------------+----------------------------------------------------------+
| ``[3 x [4 x i32]]`` | 3x4 array of 32-bit integer values. |
+-----------------------------+----------------------------------------------------------+
| ``[12 x [10 x float]]`` | 12x10 array of single precision floating point values. |
+-----------------------------+----------------------------------------------------------+
| ``[2 x [3 x [4 x i16]]]`` | 2x3x4 array of 16-bit integer values. |
+-----------------------------+----------------------------------------------------------+
There is no restriction on indexing beyond the end of the array implied
by a static type (though there are restrictions on indexing beyond the
bounds of an allocated object in some cases). This means that
single-dimension 'variable sized array' addressing can be implemented in
LLVM with a zero length array type. An implementation of 'pascal style
arrays' in LLVM could use the type "``{ i32, [0 x float]}``", for
example.
.. _t_struct:
Structure Type
""""""""""""""
:Overview:
The structure type is used to represent a collection of data members
together in memory. The elements of a structure may be any type that has
a size.
Structures in memory are accessed using '``load``' and '``store``' by
getting a pointer to a field with the '``getelementptr``' instruction.
Structures in registers are accessed using the '``extractvalue``' and
'``insertvalue``' instructions.
Structures may optionally be "packed" structures, which indicate that
the alignment of the struct is one byte, and that there is no padding
between the elements. In non-packed structs, padding between field types
is inserted as defined by the DataLayout string in the module, which is
required to match what the underlying code generator expects.
Structures can either be "literal" or "identified". A literal structure
is defined inline with other types (e.g. ``{i32, i32}*``) whereas
identified types are always defined at the top level with a name.
Literal types are uniqued by their contents and can never be recursive
or opaque since there is no way to write one. Identified types can be
recursive, can be opaqued, and are never uniqued.
:Syntax:
::
%T1 = type { <type list> } ; Identified normal struct type
%T2 = type <{ <type list> }> ; Identified packed struct type
:Examples:
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{ i32, i32, i32 }`` | A triple of three ``i32`` values |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{ float, i32 (i32) * }`` | A pair, where the first element is a ``float`` and the second element is a :ref:`pointer <t_pointer>` to a :ref:`function <t_function>` that takes an ``i32``, returning an ``i32``. |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``<{ i8, i32 }>`` | A packed struct known to be 5 bytes in size. |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
.. _t_opaque:
Opaque Structure Types
""""""""""""""""""""""
:Overview:
Opaque structure types are used to represent named structure types that
do not have a body specified. This corresponds (for example) to the C
notion of a forward declared structure.
:Syntax:
::
%X = type opaque
%52 = type opaque
:Examples:
+--------------+-------------------+
| ``opaque`` | An opaque type. |
+--------------+-------------------+
.. _constants:
Constants
=========
LLVM has several different basic types of constants. This section
describes them all and their syntax.
Simple Constants
----------------
**Boolean constants**
The two strings '``true``' and '``false``' are both valid constants
of the ``i1`` type.
**Integer constants**
Standard integers (such as '4') are constants of the
:ref:`integer <t_integer>` type. Negative numbers may be used with
integer types.
**Floating point constants**
Floating point constants use standard decimal notation (e.g.
123.421), exponential notation (e.g. 1.23421e+2), or a more precise
hexadecimal notation (see below). The assembler requires the exact
decimal value of a floating-point constant. For example, the
assembler accepts 1.25 but rejects 1.3 because 1.3 is a repeating
decimal in binary. Floating point constants must have a :ref:`floating
point <t_floating>` type.
**Null pointer constants**
The identifier '``null``' is recognized as a null pointer constant
and must be of :ref:`pointer type <t_pointer>`.
**Token constants**
The identifier '``none``' is recognized as an empty token constant
and must be of :ref:`token type <t_token>`.
The one non-intuitive notation for constants is the hexadecimal form of
floating point constants. For example, the form
'``double 0x432ff973cafa8000``' is equivalent to (but harder to read
than) '``double 4.5e+15``'. The only time hexadecimal floating point
constants are required (and the only time that they are generated by the
disassembler) is when a floating point constant must be emitted but it
cannot be represented as a decimal floating point number in a reasonable
number of digits. For example, NaN's, infinities, and other special
values are represented in their IEEE hexadecimal format so that assembly
and disassembly do not cause any bits to change in the constants.
When using the hexadecimal form, constants of types half, float, and
double are represented using the 16-digit form shown above (which
matches the IEEE754 representation for double); half and float values
must, however, be exactly representable as IEEE 754 half and single
precision, respectively. Hexadecimal format is always used for long
double, and there are three forms of long double. The 80-bit format used
by x86 is represented as ``0xK`` followed by 20 hexadecimal digits. The
128-bit format used by PowerPC (two adjacent doubles) is represented by
``0xM`` followed by 32 hexadecimal digits. The IEEE 128-bit format is
represented by ``0xL`` followed by 32 hexadecimal digits. Long doubles
will only work if they match the long double format on your target.
The IEEE 16-bit format (half precision) is represented by ``0xH``
followed by 4 hexadecimal digits. All hexadecimal formats are big-endian
(sign bit at the left).
There are no constants of type x86_mmx.
.. _complexconstants:
Complex Constants
-----------------
Complex constants are a (potentially recursive) combination of simple
constants and smaller complex constants.
**Structure constants**
Structure constants are represented with notation similar to
structure type definitions (a comma separated list of elements,
surrounded by braces (``{}``)). For example:
"``{ i32 4, float 17.0, i32* @G }``", where "``@G``" is declared as
"``@G = external global i32``". Structure constants must have
:ref:`structure type <t_struct>`, and the number and types of elements
must match those specified by the type.
**Array constants**
Array constants are represented with notation similar to array type
definitions (a comma separated list of elements, surrounded by
square brackets (``[]``)). For example:
"``[ i32 42, i32 11, i32 74 ]``". Array constants must have
:ref:`array type <t_array>`, and the number and types of elements must
match those specified by the type. As a special case, character array
constants may also be represented as a double-quoted string using the ``c``
prefix. For example: "``c"Hello World\0A\00"``".
**Vector constants**
Vector constants are represented with notation similar to vector
type definitions (a comma separated list of elements, surrounded by
less-than/greater-than's (``<>``)). For example:
"``< i32 42, i32 11, i32 74, i32 100 >``". Vector constants
must have :ref:`vector type <t_vector>`, and the number and types of
elements must match those specified by the type.
**Zero initialization**
The string '``zeroinitializer``' can be used to zero initialize a
value to zero of *any* type, including scalar and
:ref:`aggregate <t_aggregate>` types. This is often used to avoid
having to print large zero initializers (e.g. for large arrays) and
is always exactly equivalent to using explicit zero initializers.
**Metadata node**
A metadata node is a constant tuple without types. For example:
"``!{!0, !{!2, !0}, !"test"}``". Metadata can reference constant values,
for example: "``!{!0, i32 0, i8* @global, i64 (i64)* @function, !"str"}``".
Unlike other typed constants that are meant to be interpreted as part of
the instruction stream, metadata is a place to attach additional
information such as debug info.
Global Variable and Function Addresses
--------------------------------------
The addresses of :ref:`global variables <globalvars>` and
:ref:`functions <functionstructure>` are always implicitly valid
(link-time) constants. These constants are explicitly referenced when
the :ref:`identifier for the global <identifiers>` is used and always have
:ref:`pointer <t_pointer>` type. For example, the following is a legal LLVM
file:
.. code-block:: llvm
@X = global i32 17
@Y = global i32 42
@Z = global [2 x i32*] [ i32* @X, i32* @Y ]
.. _undefvalues:
Undefined Values
----------------
The string '``undef``' can be used anywhere a constant is expected, and
indicates that the user of the value may receive an unspecified
bit-pattern. Undefined values may be of any type (other than '``label``'
or '``void``') and be used anywhere a constant is permitted.
Undefined values are useful because they indicate to the compiler that
the program is well defined no matter what value is used. This gives the
compiler more freedom to optimize. Here are some examples of
(potentially surprising) transformations that are valid (in pseudo IR):
.. code-block:: llvm
%A = add %X, undef
%B = sub %X, undef
%C = xor %X, undef
Safe:
%A = undef
%B = undef
%C = undef
This is safe because all of the output bits are affected by the undef
bits. Any output bit can have a zero or one depending on the input bits.
.. code-block:: llvm
%A = or %X, undef
%B = and %X, undef
Safe:
%A = -1
%B = 0
Safe:
%A = %X ;; By choosing undef as 0
%B = %X ;; By choosing undef as -1
Unsafe:
%A = undef
%B = undef
These logical operations have bits that are not always affected by the
input. For example, if ``%X`` has a zero bit, then the output of the
'``and``' operation will always be a zero for that bit, no matter what
the corresponding bit from the '``undef``' is. As such, it is unsafe to
optimize or assume that the result of the '``and``' is '``undef``'.
However, it is safe to assume that all bits of the '``undef``' could be
0, and optimize the '``and``' to 0. Likewise, it is safe to assume that
all the bits of the '``undef``' operand to the '``or``' could be set,
allowing the '``or``' to be folded to -1.
.. code-block:: llvm
%A = select undef, %X, %Y
%B = select undef, 42, %Y
%C = select %X, %Y, undef
Safe:
%A = %X (or %Y)
%B = 42 (or %Y)
%C = %Y
Unsafe:
%A = undef
%B = undef
%C = undef
This set of examples shows that undefined '``select``' (and conditional
branch) conditions can go *either way*, but they have to come from one
of the two operands. In the ``%A`` example, if ``%X`` and ``%Y`` were
both known to have a clear low bit, then ``%A`` would have to have a
cleared low bit. However, in the ``%C`` example, the optimizer is
allowed to assume that the '``undef``' operand could be the same as
``%Y``, allowing the whole '``select``' to be eliminated.
.. code-block:: text
%A = xor undef, undef
%B = undef
%C = xor %B, %B
%D = undef
%E = icmp slt %D, 4
%F = icmp gte %D, 4
Safe:
%A = undef
%B = undef
%C = undef
%D = undef
%E = undef
%F = undef
This example points out that two '``undef``' operands are not
necessarily the same. This can be surprising to people (and also matches
C semantics) where they assume that "``X^X``" is always zero, even if
``X`` is undefined. This isn't true for a number of reasons, but the
short answer is that an '``undef``' "variable" can arbitrarily change
its value over its "live range". This is true because the variable
doesn't actually *have a live range*. Instead, the value is logically
read from arbitrary registers that happen to be around when needed, so
the value is not necessarily consistent over time. In fact, ``%A`` and
``%C`` need to have the same semantics or the core LLVM "replace all
uses with" concept would not hold.
.. code-block:: llvm
%A = fdiv undef, %X
%B = fdiv %X, undef
Safe:
%A = undef
b: unreachable
These examples show the crucial difference between an *undefined value*
and *undefined behavior*. An undefined value (like '``undef``') is
allowed to have an arbitrary bit-pattern. This means that the ``%A``
operation can be constant folded to '``undef``', because the '``undef``'
could be an SNaN, and ``fdiv`` is not (currently) defined on SNaN's.
However, in the second example, we can make a more aggressive
assumption: because the ``undef`` is allowed to be an arbitrary value,
we are allowed to assume that it could be zero. Since a divide by zero
has *undefined behavior*, we are allowed to assume that the operation
does not execute at all. This allows us to delete the divide and all
code after it. Because the undefined operation "can't happen", the
optimizer can assume that it occurs in dead code.
.. code-block:: text
a: store undef -> %X
b: store %X -> undef
Safe:
a: <deleted>
b: unreachable
These examples reiterate the ``fdiv`` example: a store *of* an undefined
value can be assumed to not have any effect; we can assume that the
value is overwritten with bits that happen to match what was already
there. However, a store *to* an undefined location could clobber
arbitrary memory, therefore, it has undefined behavior.
.. _poisonvalues:
Poison Values
-------------
Poison values are similar to :ref:`undef values <undefvalues>`, however
they also represent the fact that an instruction or constant expression
that cannot evoke side effects has nevertheless detected a condition
that results in undefined behavior.
There is currently no way of representing a poison value in the IR; they
only exist when produced by operations such as :ref:`add <i_add>` with
the ``nsw`` flag.
Poison value behavior is defined in terms of value *dependence*:
- Values other than :ref:`phi <i_phi>` nodes depend on their operands.
- :ref:`Phi <i_phi>` nodes depend on the operand corresponding to
their dynamic predecessor basic block.
- Function arguments depend on the corresponding actual argument values
in the dynamic callers of their functions.
- :ref:`Call <i_call>` instructions depend on the :ref:`ret <i_ret>`
instructions that dynamically transfer control back to them.
- :ref:`Invoke <i_invoke>` instructions depend on the
:ref:`ret <i_ret>`, :ref:`resume <i_resume>`, or exception-throwing
call instructions that dynamically transfer control back to them.
- Non-volatile loads and stores depend on the most recent stores to all
of the referenced memory addresses, following the order in the IR
(including loads and stores implied by intrinsics such as
:ref:`@llvm.memcpy <int_memcpy>`.)
- An instruction with externally visible side effects depends on the
most recent preceding instruction with externally visible side
effects, following the order in the IR. (This includes :ref:`volatile
operations <volatile>`.)
- An instruction *control-depends* on a :ref:`terminator
instruction <terminators>` if the terminator instruction has
multiple successors and the instruction is always executed when
control transfers to one of the successors, and may not be executed
when control is transferred to another.
- Additionally, an instruction also *control-depends* on a terminator
instruction if the set of instructions it otherwise depends on would
be different if the terminator had transferred control to a different
successor.
- Dependence is transitive.
Poison values have the same behavior as :ref:`undef values <undefvalues>`,
with the additional effect that any instruction that has a *dependence*
on a poison value has undefined behavior.
Here are some examples:
.. code-block:: llvm
entry:
%poison = sub nuw i32 0, 1 ; Results in a poison value.
%still_poison = and i32 %poison, 0 ; 0, but also poison.
%poison_yet_again = getelementptr i32, i32* @h, i32 %still_poison
store i32 0, i32* %poison_yet_again ; memory at @h[0] is poisoned
store i32 %poison, i32* @g ; Poison value stored to memory.
%poison2 = load i32, i32* @g ; Poison value loaded back from memory.
store volatile i32 %poison, i32* @g ; External observation; undefined behavior.
%narrowaddr = bitcast i32* @g to i16*
%wideaddr = bitcast i32* @g to i64*
%poison3 = load i16, i16* %narrowaddr ; Returns a poison value.
%poison4 = load i64, i64* %wideaddr ; Returns a poison value.
%cmp = icmp slt i32 %poison, 0 ; Returns a poison value.
br i1 %cmp, label %true, label %end ; Branch to either destination.
true:
store volatile i32 0, i32* @g ; This is control-dependent on %cmp, so
; it has undefined behavior.
br label %end
end:
%p = phi i32 [ 0, %entry ], [ 1, %true ]
; Both edges into this PHI are
; control-dependent on %cmp, so this
; always results in a poison value.
store volatile i32 0, i32* @g ; This would depend on the store in %true
; if %cmp is true, or the store in %entry
; otherwise, so this is undefined behavior.
br i1 %cmp, label %second_true, label %second_end
; The same branch again, but this time the
; true block doesn't have side effects.
second_true:
; No side effects!
ret void
second_end:
store volatile i32 0, i32* @g ; This time, the instruction always depends
; on the store in %end. Also, it is
; control-equivalent to %end, so this is
; well-defined (ignoring earlier undefined
; behavior in this example).
.. _blockaddress:
Addresses of Basic Blocks
-------------------------
``blockaddress(@function, %block)``
The '``blockaddress``' constant computes the address of the specified
basic block in the specified function, and always has an ``i8*`` type.
Taking the address of the entry block is illegal.
This value only has defined behavior when used as an operand to the
':ref:`indirectbr <i_indirectbr>`' instruction, or for comparisons
against null. Pointer equality tests between labels addresses results in
undefined behavior --- though, again, comparison against null is ok, and
no label is equal to the null pointer. This may be passed around as an
opaque pointer sized value as long as the bits are not inspected. This
allows ``ptrtoint`` and arithmetic to be performed on these values so
long as the original value is reconstituted before the ``indirectbr``
instruction.
Finally, some targets may provide defined semantics when using the value
as the operand to an inline assembly, but that is target specific.
.. _constantexprs:
Constant Expressions
--------------------
Constant expressions are used to allow expressions involving other
constants to be used as constants. Constant expressions may be of any
:ref:`first class <t_firstclass>` type and may involve any LLVM operation
that does not have side effects (e.g. load and call are not supported).
The following is the syntax for constant expressions:
``trunc (CST to TYPE)``
Perform the :ref:`trunc operation <i_trunc>` on constants.
``zext (CST to TYPE)``
Perform the :ref:`zext operation <i_zext>` on constants.
``sext (CST to TYPE)``
Perform the :ref:`sext operation <i_sext>` on constants.
``fptrunc (CST to TYPE)``
Truncate a floating point constant to another floating point type.
The size of CST must be larger than the size of TYPE. Both types
must be floating point.
``fpext (CST to TYPE)``
Floating point extend a constant to another type. The size of CST
must be smaller or equal to the size of TYPE. Both types must be
floating point.
``fptoui (CST to TYPE)``
Convert a floating point constant to the corresponding unsigned
integer constant. TYPE must be a scalar or vector integer type. CST
must be of scalar or vector floating point type. Both CST and TYPE
must be scalars, or vectors of the same number of elements. If the
value won't fit in the integer type, the results are undefined.
``fptosi (CST to TYPE)``
Convert a floating point constant to the corresponding signed
integer constant. TYPE must be a scalar or vector integer type. CST
must be of scalar or vector floating point type. Both CST and TYPE
must be scalars, or vectors of the same number of elements. If the
value won't fit in the integer type, the results are undefined.
``uitofp (CST to TYPE)``
Convert an unsigned integer constant to the corresponding floating
point constant. TYPE must be a scalar or vector floating point type.
CST must be of scalar or vector integer type. Both CST and TYPE must
be scalars, or vectors of the same number of elements. If the value
won't fit in the floating point type, the results are undefined.
``sitofp (CST to TYPE)``
Convert a signed integer constant to the corresponding floating
point constant. TYPE must be a scalar or vector floating point type.
CST must be of scalar or vector integer type. Both CST and TYPE must
be scalars, or vectors of the same number of elements. If the value
won't fit in the floating point type, the results are undefined.
``ptrtoint (CST to TYPE)``
Perform the :ref:`ptrtoint operation <i_ptrtoint>` on constants.
``inttoptr (CST to TYPE)``
Perform the :ref:`inttoptr operation <i_inttoptr>` on constants.
This one is *really* dangerous!
``bitcast (CST to TYPE)``
Convert a constant, CST, to another TYPE.
The constraints of the operands are the same as those for the
:ref:`bitcast instruction <i_bitcast>`.
``addrspacecast (CST to TYPE)``
Convert a constant pointer or constant vector of pointer, CST, to another
TYPE in a different address space. The constraints of the operands are the
same as those for the :ref:`addrspacecast instruction <i_addrspacecast>`.
``getelementptr (TY, CSTPTR, IDX0, IDX1, ...)``, ``getelementptr inbounds (TY, CSTPTR, IDX0, IDX1, ...)``
Perform the :ref:`getelementptr operation <i_getelementptr>` on
constants. As with the :ref:`getelementptr <i_getelementptr>`
instruction, the index list may have one or more indexes, which are
required to make sense for the type of "pointer to TY".
``select (COND, VAL1, VAL2)``
Perform the :ref:`select operation <i_select>` on constants.
``icmp COND (VAL1, VAL2)``
Perform the :ref:`icmp operation <i_icmp>` on constants.
``fcmp COND (VAL1, VAL2)``
Perform the :ref:`fcmp operation <i_fcmp>` on constants.
``extractelement (VAL, IDX)``
Perform the :ref:`extractelement operation <i_extractelement>` on
constants.
``insertelement (VAL, ELT, IDX)``
Perform the :ref:`insertelement operation <i_insertelement>` on
constants.
``shufflevector (VEC1, VEC2, IDXMASK)``
Perform the :ref:`shufflevector operation <i_shufflevector>` on
constants.
``extractvalue (VAL, IDX0, IDX1, ...)``
Perform the :ref:`extractvalue operation <i_extractvalue>` on
constants. The index list is interpreted in a similar manner as
indices in a ':ref:`getelementptr <i_getelementptr>`' operation. At
least one index value must be specified.
``insertvalue (VAL, ELT, IDX0, IDX1, ...)``
Perform the :ref:`insertvalue operation <i_insertvalue>` on constants.
The index list is interpreted in a similar manner as indices in a
':ref:`getelementptr <i_getelementptr>`' operation. At least one index
value must be specified.
``OPCODE (LHS, RHS)``
Perform the specified operation of the LHS and RHS constants. OPCODE
may be any of the :ref:`binary <binaryops>` or :ref:`bitwise
binary <bitwiseops>` operations. The constraints on operands are
the same as those for the corresponding instruction (e.g. no bitwise
operations on floating point values are allowed).
Other Values
============
.. _inlineasmexprs:
Inline Assembler Expressions
----------------------------
LLVM supports inline assembler expressions (as opposed to :ref:`Module-Level
Inline Assembly <moduleasm>`) through the use of a special value. This value
represents the inline assembler as a template string (containing the
instructions to emit), a list of operand constraints (stored as a string), a
flag that indicates whether or not the inline asm expression has side effects,
and a flag indicating whether the function containing the asm needs to align its
stack conservatively.
The template string supports argument substitution of the operands using "``$``"
followed by a number, to indicate substitution of the given register/memory
location, as specified by the constraint string. "``${NUM:MODIFIER}``" may also
be used, where ``MODIFIER`` is a target-specific annotation for how to print the
operand (See :ref:`inline-asm-modifiers`).
A literal "``$``" may be included by using "``$$``" in the template. To include
other special characters into the output, the usual "``\XX``" escapes may be
used, just as in other strings. Note that after template substitution, the
resulting assembly string is parsed by LLVM's integrated assembler unless it is
disabled -- even when emitting a ``.s`` file -- and thus must contain assembly
syntax known to LLVM.
LLVM also supports a few more substitions useful for writing inline assembly:
- ``${:uid}``: Expands to a decimal integer unique to this inline assembly blob.
This substitution is useful when declaring a local label. Many standard
compiler optimizations, such as inlining, may duplicate an inline asm blob.
Adding a blob-unique identifier ensures that the two labels will not conflict
during assembly. This is used to implement `GCC's %= special format
string <https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_.
- ``${:comment}``: Expands to the comment character of the current target's
assembly dialect. This is usually ``#``, but many targets use other strings,
such as ``;``, ``//``, or ``!``.
- ``${:private}``: Expands to the assembler private label prefix. Labels with
this prefix will not appear in the symbol table of the assembled object.
Typically the prefix is ``L``, but targets may use other strings. ``.L`` is
relatively popular.
LLVM's support for inline asm is modeled closely on the requirements of Clang's
GCC-compatible inline-asm support. Thus, the feature-set and the constraint and
modifier codes listed here are similar or identical to those in GCC's inline asm
support. However, to be clear, the syntax of the template and constraint strings
described here is *not* the same as the syntax accepted by GCC and Clang, and,
while most constraint letters are passed through as-is by Clang, some get
translated to other codes when converting from the C source to the LLVM
assembly.
An example inline assembler expression is:
.. code-block:: llvm
i32 (i32) asm "bswap $0", "=r,r"
Inline assembler expressions may **only** be used as the callee operand
of a :ref:`call <i_call>` or an :ref:`invoke <i_invoke>` instruction.
Thus, typically we have:
.. code-block:: llvm
%X = call i32 asm "bswap $0", "=r,r"(i32 %Y)
Inline asms with side effects not visible in the constraint list must be
marked as having side effects. This is done through the use of the
'``sideeffect``' keyword, like so:
.. code-block:: llvm
call void asm sideeffect "eieio", ""()
In some cases inline asms will contain code that will not work unless
the stack is aligned in some way, such as calls or SSE instructions on
x86, yet will not contain code that does that alignment within the asm.
The compiler should make conservative assumptions about what the asm
might contain and should generate its usual stack alignment code in the
prologue if the '``alignstack``' keyword is present:
.. code-block:: llvm
call void asm alignstack "eieio", ""()
Inline asms also support using non-standard assembly dialects. The
assumed dialect is ATT. When the '``inteldialect``' keyword is present,
the inline asm is using the Intel dialect. Currently, ATT and Intel are
the only supported dialects. An example is:
.. code-block:: llvm
call void asm inteldialect "eieio", ""()
If multiple keywords appear the '``sideeffect``' keyword must come
first, the '``alignstack``' keyword second and the '``inteldialect``'
keyword last.
Inline Asm Constraint String
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The constraint list is a comma-separated string, each element containing one or
more constraint codes.
For each element in the constraint list an appropriate register or memory
operand will be chosen, and it will be made available to assembly template
string expansion as ``$0`` for the first constraint in the list, ``$1`` for the
second, etc.
There are three different types of constraints, which are distinguished by a
prefix symbol in front of the constraint code: Output, Input, and Clobber. The
constraints must always be given in that order: outputs first, then inputs, then
clobbers. They cannot be intermingled.
There are also three different categories of constraint codes:
- Register constraint. This is either a register class, or a fixed physical
register. This kind of constraint will allocate a register, and if necessary,
bitcast the argument or result to the appropriate type.
- Memory constraint. This kind of constraint is for use with an instruction
taking a memory operand. Different constraints allow for different addressing
modes used by the target.
- Immediate value constraint. This kind of constraint is for an integer or other
immediate value which can be rendered directly into an instruction. The
various target-specific constraints allow the selection of a value in the
proper range for the instruction you wish to use it with.
Output constraints
""""""""""""""""""
Output constraints are specified by an "``=``" prefix (e.g. "``=r``"). This
indicates that the assembly will write to this operand, and the operand will
then be made available as a return value of the ``asm`` expression. Output
constraints do not consume an argument from the call instruction. (Except, see
below about indirect outputs).
Normally, it is expected that no output locations are written to by the assembly
expression until *all* of the inputs have been read. As such, LLVM may assign
the same register to an output and an input. If this is not safe (e.g. if the
assembly contains two instructions, where the first writes to one output, and
the second reads an input and writes to a second output), then the "``&``"
modifier must be used (e.g. "``=&r``") to specify that the output is an
"early-clobber" output. Marking an output as "early-clobber" ensures that LLVM
will not use the same register for any inputs (other than an input tied to this
output).
Input constraints
"""""""""""""""""
Input constraints do not have a prefix -- just the constraint codes. Each input
constraint will consume one argument from the call instruction. It is not
permitted for the asm to write to any input register or memory location (unless
that input is tied to an output). Note also that multiple inputs may all be
assigned to the same register, if LLVM can determine that they necessarily all
contain the same value.
Instead of providing a Constraint Code, input constraints may also "tie"
themselves to an output constraint, by providing an integer as the constraint
string. Tied inputs still consume an argument from the call instruction, and
take up a position in the asm template numbering as is usual -- they will simply
be constrained to always use the same register as the output they've been tied
to. For example, a constraint string of "``=r,0``" says to assign a register for
output, and use that register as an input as well (it being the 0'th
constraint).
It is permitted to tie an input to an "early-clobber" output. In that case, no
*other* input may share the same register as the input tied to the early-clobber
(even when the other input has the same value).
You may only tie an input to an output which has a register constraint, not a
memory constraint. Only a single input may be tied to an output.
There is also an "interesting" feature which deserves a bit of explanation: if a
register class constraint allocates a register which is too small for the value
type operand provided as input, the input value will be split into multiple
registers, and all of them passed to the inline asm.
However, this feature is often not as useful as you might think.
Firstly, the registers are *not* guaranteed to be consecutive. So, on those
architectures that have instructions which operate on multiple consecutive
instructions, this is not an appropriate way to support them. (e.g. the 32-bit
SparcV8 has a 64-bit load, which instruction takes a single 32-bit register. The
hardware then loads into both the named register, and the next register. This
feature of inline asm would not be useful to support that.)
A few of the targets provide a template string modifier allowing explicit access
to the second register of a two-register operand (e.g. MIPS ``L``, ``M``, and
``D``). On such an architecture, you can actually access the second allocated
register (yet, still, not any subsequent ones). But, in that case, you're still
probably better off simply splitting the value into two separate operands, for
clarity. (e.g. see the description of the ``A`` constraint on X86, which,
despite existing only for use with this feature, is not really a good idea to
use)
Indirect inputs and outputs
"""""""""""""""""""""""""""
Indirect output or input constraints can be specified by the "``*``" modifier
(which goes after the "``=``" in case of an output). This indicates that the asm
will write to or read from the contents of an *address* provided as an input
argument. (Note that in this way, indirect outputs act more like an *input* than
an output: just like an input, they consume an argument of the call expression,
rather than producing a return value. An indirect output constraint is an
"output" only in that the asm is expected to write to the contents of the input
memory location, instead of just read from it).
This is most typically used for memory constraint, e.g. "``=*m``", to pass the
address of a variable as a value.
It is also possible to use an indirect *register* constraint, but only on output
(e.g. "``=*r``"). This will cause LLVM to allocate a register for an output
value normally, and then, separately emit a store to the address provided as
input, after the provided inline asm. (It's not clear what value this
functionality provides, compared to writing the store explicitly after the asm
statement, and it can only produce worse code, since it bypasses many
optimization passes. I would recommend not using it.)
Clobber constraints
"""""""""""""""""""
A clobber constraint is indicated by a "``~``" prefix. A clobber does not
consume an input operand, nor generate an output. Clobbers cannot use any of the
general constraint code letters -- they may use only explicit register
constraints, e.g. "``~{eax}``". The one exception is that a clobber string of
"``~{memory}``" indicates that the assembly writes to arbitrary undeclared
memory locations -- not only the memory pointed to by a declared indirect
output.
Note that clobbering named registers that are also present in output
constraints is not legal.
Constraint Codes
""""""""""""""""
After a potential prefix comes constraint code, or codes.
A Constraint Code is either a single letter (e.g. "``r``"), a "``^``" character
followed by two letters (e.g. "``^wc``"), or "``{``" register-name "``}``"
(e.g. "``{eax}``").
The one and two letter constraint codes are typically chosen to be the same as
GCC's constraint codes.
A single constraint may include one or more than constraint code in it, leaving
it up to LLVM to choose which one to use. This is included mainly for
compatibility with the translation of GCC inline asm coming from clang.
There are two ways to specify alternatives, and either or both may be used in an
inline asm constraint list:
1) Append the codes to each other, making a constraint code set. E.g. "``im``"
or "``{eax}m``". This means "choose any of the options in the set". The
choice of constraint is made independently for each constraint in the
constraint list.
2) Use "``|``" between constraint code sets, creating alternatives. Every
constraint in the constraint list must have the same number of alternative
sets. With this syntax, the same alternative in *all* of the items in the
constraint list will be chosen together.
Putting those together, you might have a two operand constraint string like
``"rm|r,ri|rm"``. This indicates that if operand 0 is ``r`` or ``m``, then
operand 1 may be one of ``r`` or ``i``. If operand 0 is ``r``, then operand 1
may be one of ``r`` or ``m``. But, operand 0 and 1 cannot both be of type m.
However, the use of either of the alternatives features is *NOT* recommended, as
LLVM is not able to make an intelligent choice about which one to use. (At the
point it currently needs to choose, not enough information is available to do so
in a smart way.) Thus, it simply tries to make a choice that's most likely to
compile, not one that will be optimal performance. (e.g., given "``rm``", it'll
always choose to use memory, not registers). And, if given multiple registers,
or multiple register classes, it will simply choose the first one. (In fact, it
doesn't currently even ensure explicitly specified physical registers are
unique, so specifying multiple physical registers as alternatives, like
``{r11}{r12},{r11}{r12}``, will assign r11 to both operands, not at all what was
intended.)
Supported Constraint Code List
""""""""""""""""""""""""""""""
The constraint codes are, in general, expected to behave the same way they do in
GCC. LLVM's support is often implemented on an 'as-needed' basis, to support C
inline asm code which was supported by GCC. A mismatch in behavior between LLVM
and GCC likely indicates a bug in LLVM.
Some constraint codes are typically supported by all targets:
- ``r``: A register in the target's general purpose register class.
- ``m``: A memory address operand. It is target-specific what addressing modes
are supported, typical examples are register, or register + register offset,
or register + immediate offset (of some target-specific size).
- ``i``: An integer constant (of target-specific width). Allows either a simple
immediate, or a relocatable value.
- ``n``: An integer constant -- *not* including relocatable values.
- ``s``: An integer constant, but allowing *only* relocatable values.
- ``X``: Allows an operand of any kind, no constraint whatsoever. Typically
useful to pass a label for an asm branch or call.
.. FIXME: but that surely isn't actually okay to jump out of an asm
block without telling llvm about the control transfer???)
- ``{register-name}``: Requires exactly the named physical register.
Other constraints are target-specific:
AArch64:
- ``z``: An immediate integer 0. Outputs ``WZR`` or ``XZR``, as appropriate.
- ``I``: An immediate integer valid for an ``ADD`` or ``SUB`` instruction,
i.e. 0 to 4095 with optional shift by 12.
- ``J``: An immediate integer that, when negated, is valid for an ``ADD`` or
``SUB`` instruction, i.e. -1 to -4095 with optional left shift by 12.
- ``K``: An immediate integer that is valid for the 'bitmask immediate 32' of a
logical instruction like ``AND``, ``EOR``, or ``ORR`` with a 32-bit register.
- ``L``: An immediate integer that is valid for the 'bitmask immediate 64' of a
logical instruction like ``AND``, ``EOR``, or ``ORR`` with a 64-bit register.
- ``M``: An immediate integer for use with the ``MOV`` assembly alias on a
32-bit register. This is a superset of ``K``: in addition to the bitmask
immediate, also allows immediate integers which can be loaded with a single
``MOVZ`` or ``MOVL`` instruction.
- ``N``: An immediate integer for use with the ``MOV`` assembly alias on a
64-bit register. This is a superset of ``L``.
- ``Q``: Memory address operand must be in a single register (no
offsets). (However, LLVM currently does this for the ``m`` constraint as
well.)
- ``r``: A 32 or 64-bit integer register (W* or X*).
- ``w``: A 32, 64, or 128-bit floating-point/SIMD register.
- ``x``: A lower 128-bit floating-point/SIMD register (``V0`` to ``V15``).
AMDGPU:
- ``r``: A 32 or 64-bit integer register.
- ``[0-9]v``: The 32-bit VGPR register, number 0-9.
- ``[0-9]s``: The 32-bit SGPR register, number 0-9.
All ARM modes:
- ``Q``, ``Um``, ``Un``, ``Uq``, ``Us``, ``Ut``, ``Uv``, ``Uy``: Memory address
operand. Treated the same as operand ``m``, at the moment.
ARM and ARM's Thumb2 mode:
- ``j``: An immediate integer between 0 and 65535 (valid for ``MOVW``)
- ``I``: An immediate integer valid for a data-processing instruction.
- ``J``: An immediate integer between -4095 and 4095.
- ``K``: An immediate integer whose bitwise inverse is valid for a
data-processing instruction. (Can be used with template modifier "``B``" to
print the inverted value).
- ``L``: An immediate integer whose negation is valid for a data-processing
instruction. (Can be used with template modifier "``n``" to print the negated
value).
- ``M``: A power of two or a integer between 0 and 32.
- ``N``: Invalid immediate constraint.
- ``O``: Invalid immediate constraint.
- ``r``: A general-purpose 32-bit integer register (``r0-r15``).
- ``l``: In Thumb2 mode, low 32-bit GPR registers (``r0-r7``). In ARM mode, same
as ``r``.
- ``h``: In Thumb2 mode, a high 32-bit GPR register (``r8-r15``). In ARM mode,
invalid.
- ``w``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s31``,
``d0-d31``, or ``q0-q15``.
- ``x``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s15``,
``d0-d7``, or ``q0-q3``.
- ``t``: A floating-point/SIMD register, only supports 32-bit values:
``s0-s31``.
ARM's Thumb1 mode:
- ``I``: An immediate integer between 0 and 255.
- ``J``: An immediate integer between -255 and -1.
- ``K``: An immediate integer between 0 and 255, with optional left-shift by
some amount.
- ``L``: An immediate integer between -7 and 7.
- ``M``: An immediate integer which is a multiple of 4 between 0 and 1020.
- ``N``: An immediate integer between 0 and 31.
- ``O``: An immediate integer which is a multiple of 4 between -508 and 508.
- ``r``: A low 32-bit GPR register (``r0-r7``).
- ``l``: A low 32-bit GPR register (``r0-r7``).
- ``h``: A high GPR register (``r0-r7``).
- ``w``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s31``,
``d0-d31``, or ``q0-q15``.
- ``x``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s15``,
``d0-d7``, or ``q0-q3``.
- ``t``: A floating-point/SIMD register, only supports 32-bit values:
``s0-s31``.
Hexagon:
- ``o``, ``v``: A memory address operand, treated the same as constraint ``m``,
at the moment.
- ``r``: A 32 or 64-bit register.
MSP430:
- ``r``: An 8 or 16-bit register.
MIPS:
- ``I``: An immediate signed 16-bit integer.
- ``J``: An immediate integer zero.
- ``K``: An immediate unsigned 16-bit integer.
- ``L``: An immediate 32-bit integer, where the lower 16 bits are 0.
- ``N``: An immediate integer between -65535 and -1.
- ``O``: An immediate signed 15-bit integer.
- ``P``: An immediate integer between 1 and 65535.
- ``m``: A memory address operand. In MIPS-SE mode, allows a base address
register plus 16-bit immediate offset. In MIPS mode, just a base register.
- ``R``: A memory address operand. In MIPS-SE mode, allows a base address
register plus a 9-bit signed offset. In MIPS mode, the same as constraint
``m``.
- ``ZC``: A memory address operand, suitable for use in a ``pref``, ``ll``, or
``sc`` instruction on the given subtarget (details vary).
- ``r``, ``d``, ``y``: A 32 or 64-bit GPR register.
- ``f``: A 32 or 64-bit FPU register (``F0-F31``), or a 128-bit MSA register
(``W0-W31``). In the case of MSA registers, it is recommended to use the ``w``
argument modifier for compatibility with GCC.
- ``c``: A 32-bit or 64-bit GPR register suitable for indirect jump (always
``25``).
- ``l``: The ``lo`` register, 32 or 64-bit.
- ``x``: Invalid.
NVPTX:
- ``b``: A 1-bit integer register.
- ``c`` or ``h``: A 16-bit integer register.
- ``r``: A 32-bit integer register.
- ``l`` or ``N``: A 64-bit integer register.
- ``f``: A 32-bit float register.
- ``d``: A 64-bit float register.
PowerPC:
- ``I``: An immediate signed 16-bit integer.
- ``J``: An immediate unsigned 16-bit integer, shifted left 16 bits.
- ``K``: An immediate unsigned 16-bit integer.
- ``L``: An immediate signed 16-bit integer, shifted left 16 bits.
- ``M``: An immediate integer greater than 31.
- ``N``: An immediate integer that is an exact power of 2.
- ``O``: The immediate integer constant 0.
- ``P``: An immediate integer constant whose negation is a signed 16-bit
constant.
- ``es``, ``o``, ``Q``, ``Z``, ``Zy``: A memory address operand, currently
treated the same as ``m``.
- ``r``: A 32 or 64-bit integer register.
- ``b``: A 32 or 64-bit integer register, excluding ``R0`` (that is:
``R1-R31``).
- ``f``: A 32 or 64-bit float register (``F0-F31``), or when QPX is enabled, a
128 or 256-bit QPX register (``Q0-Q31``; aliases the ``F`` registers).
- ``v``: For ``4 x f32`` or ``4 x f64`` types, when QPX is enabled, a
128 or 256-bit QPX register (``Q0-Q31``), otherwise a 128-bit
altivec vector register (``V0-V31``).
.. FIXME: is this a bug that v accepts QPX registers? I think this
is supposed to only use the altivec vector registers?
- ``y``: Condition register (``CR0-CR7``).
- ``wc``: An individual CR bit in a CR register.
- ``wa``, ``wd``, ``wf``: Any 128-bit VSX vector register, from the full VSX
register set (overlapping both the floating-point and vector register files).
- ``ws``: A 32 or 64-bit floating point register, from the full VSX register
set.
Sparc:
- ``I``: An immediate 13-bit signed integer.
- ``r``: A 32-bit integer register.
- ``f``: Any floating-point register on SparcV8, or a floating point
register in the "low" half of the registers on SparcV9.
- ``e``: Any floating point register. (Same as ``f`` on SparcV8.)
SystemZ:
- ``I``: An immediate unsigned 8-bit integer.
- ``J``: An immediate unsigned 12-bit integer.
- ``K``: An immediate signed 16-bit integer.
- ``L``: An immediate signed 20-bit integer.
- ``M``: An immediate integer 0x7fffffff.
- ``Q``: A memory address operand with a base address and a 12-bit immediate
unsigned displacement.
- ``R``: A memory address operand with a base address, a 12-bit immediate
unsigned displacement, and an index register.
- ``S``: A memory address operand with a base address and a 20-bit immediate
signed displacement.
- ``T``: A memory address operand with a base address, a 20-bit immediate
signed displacement, and an index register.
- ``r`` or ``d``: A 32, 64, or 128-bit integer register.
- ``a``: A 32, 64, or 128-bit integer address register (excludes R0, which in an
address context evaluates as zero).
- ``h``: A 32-bit value in the high part of a 64bit data register
(LLVM-specific)
- ``f``: A 32, 64, or 128-bit floating point register.
X86:
- ``I``: An immediate integer between 0 and 31.
- ``J``: An immediate integer between 0 and 64.
- ``K``: An immediate signed 8-bit integer.
- ``L``: An immediate integer, 0xff or 0xffff or (in 64-bit mode only)
0xffffffff.
- ``M``: An immediate integer between 0 and 3.
- ``N``: An immediate unsigned 8-bit integer.
- ``O``: An immediate integer between 0 and 127.
- ``e``: An immediate 32-bit signed integer.
- ``Z``: An immediate 32-bit unsigned integer.
- ``o``, ``v``: Treated the same as ``m``, at the moment.
- ``q``: An 8, 16, 32, or 64-bit register which can be accessed as an 8-bit
``l`` integer register. On X86-32, this is the ``a``, ``b``, ``c``, and ``d``
registers, and on X86-64, it is all of the integer registers.
- ``Q``: An 8, 16, 32, or 64-bit register which can be accessed as an 8-bit
``h`` integer register. This is the ``a``, ``b``, ``c``, and ``d`` registers.
- ``r`` or ``l``: An 8, 16, 32, or 64-bit integer register.
- ``R``: An 8, 16, 32, or 64-bit "legacy" integer register -- one which has
existed since i386, and can be accessed without the REX prefix.
- ``f``: A 32, 64, or 80-bit '387 FPU stack pseudo-register.
- ``y``: A 64-bit MMX register, if MMX is enabled.
- ``x``: If SSE is enabled: a 32 or 64-bit scalar operand, or 128-bit vector
operand in a SSE register. If AVX is also enabled, can also be a 256-bit
vector operand in an AVX register. If AVX-512 is also enabled, can also be a
512-bit vector operand in an AVX512 register, Otherwise, an error.
- ``Y``: The same as ``x``, if *SSE2* is enabled, otherwise an error.
- ``A``: Special case: allocates EAX first, then EDX, for a single operand (in
32-bit mode, a 64-bit integer operand will get split into two registers). It
is not recommended to use this constraint, as in 64-bit mode, the 64-bit
operand will get allocated only to RAX -- if two 32-bit operands are needed,
you're better off splitting it yourself, before passing it to the asm
statement.
XCore:
- ``r``: A 32-bit integer register.
.. _inline-asm-modifiers:
Asm template argument modifiers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In the asm template string, modifiers can be used on the operand reference, like
"``${0:n}``".
The modifiers are, in general, expected to behave the same way they do in
GCC. LLVM's support is often implemented on an 'as-needed' basis, to support C
inline asm code which was supported by GCC. A mismatch in behavior between LLVM
and GCC likely indicates a bug in LLVM.
Target-independent:
- ``c``: Print an immediate integer constant unadorned, without
the target-specific immediate punctuation (e.g. no ``$`` prefix).
- ``n``: Negate and print immediate integer constant unadorned, without the
target-specific immediate punctuation (e.g. no ``$`` prefix).
- ``l``: Print as an unadorned label, without the target-specific label
punctuation (e.g. no ``$`` prefix).
AArch64:
- ``w``: Print a GPR register with a ``w*`` name instead of ``x*`` name. E.g.,
instead of ``x30``, print ``w30``.
- ``x``: Print a GPR register with a ``x*`` name. (this is the default, anyhow).
- ``b``, ``h``, ``s``, ``d``, ``q``: Print a floating-point/SIMD register with a
``b*``, ``h*``, ``s*``, ``d*``, or ``q*`` name, rather than the default of
``v*``.
AMDGPU:
- ``r``: No effect.
ARM:
- ``a``: Print an operand as an address (with ``[`` and ``]`` surrounding a
register).
- ``P``: No effect.
- ``q``: No effect.
- ``y``: Print a VFP single-precision register as an indexed double (e.g. print
as ``d4[1]`` instead of ``s9``)
- ``B``: Bitwise invert and print an immediate integer constant without ``#``
prefix.
- ``L``: Print the low 16-bits of an immediate integer constant.
- ``M``: Print as a register set suitable for ldm/stm. Also prints *all*
register operands subsequent to the specified one (!), so use carefully.
- ``Q``: Print the low-order register of a register-pair, or the low-order
register of a two-register operand.
- ``R``: Print the high-order register of a register-pair, or the high-order
register of a two-register operand.
- ``H``: Print the second register of a register-pair. (On a big-endian system,
``H`` is equivalent to ``Q``, and on little-endian system, ``H`` is equivalent
to ``R``.)
.. FIXME: H doesn't currently support printing the second register
of a two-register operand.
- ``e``: Print the low doubleword register of a NEON quad register.
- ``f``: Print the high doubleword register of a NEON quad register.
- ``m``: Print the base register of a memory operand without the ``[`` and ``]``
adornment.
Hexagon:
- ``L``: Print the second register of a two-register operand. Requires that it
has been allocated consecutively to the first.
.. FIXME: why is it restricted to consecutive ones? And there's
nothing that ensures that happens, is there?
- ``I``: Print the letter 'i' if the operand is an integer constant, otherwise
nothing. Used to print 'addi' vs 'add' instructions.
MSP430:
No additional modifiers.
MIPS:
- ``X``: Print an immediate integer as hexadecimal
- ``x``: Print the low 16 bits of an immediate integer as hexadecimal.
- ``d``: Print an immediate integer as decimal.
- ``m``: Subtract one and print an immediate integer as decimal.
- ``z``: Print $0 if an immediate zero, otherwise print normally.
- ``L``: Print the low-order register of a two-register operand, or prints the
address of the low-order word of a double-word memory operand.
.. FIXME: L seems to be missing memory operand support.
- ``M``: Print the high-order register of a two-register operand, or prints the
address of the high-order word of a double-word memory operand.
.. FIXME: M seems to be missing memory operand support.
- ``D``: Print the second register of a two-register operand, or prints the
second word of a double-word memory operand. (On a big-endian system, ``D`` is
equivalent to ``L``, and on little-endian system, ``D`` is equivalent to
``M``.)
- ``w``: No effect. Provided for compatibility with GCC which requires this
modifier in order to print MSA registers (``W0-W31``) with the ``f``
constraint.
NVPTX:
- ``r``: No effect.
PowerPC:
- ``L``: Print the second register of a two-register operand. Requires that it
has been allocated consecutively to the first.
.. FIXME: why is it restricted to consecutive ones? And there's
nothing that ensures that happens, is there?
- ``I``: Print the letter 'i' if the operand is an integer constant, otherwise
nothing. Used to print 'addi' vs 'add' instructions.
- ``y``: For a memory operand, prints formatter for a two-register X-form
instruction. (Currently always prints ``r0,OPERAND``).
- ``U``: Prints 'u' if the memory operand is an update form, and nothing
otherwise. (NOTE: LLVM does not support update form, so this will currently
always print nothing)
- ``X``: Prints 'x' if the memory operand is an indexed form. (NOTE: LLVM does
not support indexed form, so this will currently always print nothing)
Sparc:
- ``r``: No effect.
SystemZ:
SystemZ implements only ``n``, and does *not* support any of the other
target-independent modifiers.
X86:
- ``c``: Print an unadorned integer or symbol name. (The latter is
target-specific behavior for this typically target-independent modifier).
- ``A``: Print a register name with a '``*``' before it.
- ``b``: Print an 8-bit register name (e.g. ``al``); do nothing on a memory
operand.
- ``h``: Print the upper 8-bit register name (e.g. ``ah``); do nothing on a
memory operand.
- ``w``: Print the 16-bit register name (e.g. ``ax``); do nothing on a memory
operand.
- ``k``: Print the 32-bit register name (e.g. ``eax``); do nothing on a memory
operand.
- ``q``: Print the 64-bit register name (e.g. ``rax``), if 64-bit registers are
available, otherwise the 32-bit register name; do nothing on a memory operand.
- ``n``: Negate and print an unadorned integer, or, for operands other than an
immediate integer (e.g. a relocatable symbol expression), print a '-' before
the operand. (The behavior for relocatable symbol expressions is a
target-specific behavior for this typically target-independent modifier)
- ``H``: Print a memory reference with additional offset +8.
- ``P``: Print a memory reference or operand for use as the argument of a call
instruction. (E.g. omit ``(rip)``, even though it's PC-relative.)
XCore:
No additional modifiers.
Inline Asm Metadata
^^^^^^^^^^^^^^^^^^^
The call instructions that wrap inline asm nodes may have a
"``!srcloc``" MDNode attached to it that contains a list of constant
integers. If present, the code generator will use the integer as the
location cookie value when report errors through the ``LLVMContext``
error reporting mechanisms. This allows a front-end to correlate backend
errors that occur with inline asm back to the source code that produced
it. For example:
.. code-block:: llvm
call void asm sideeffect "something bad", ""(), !srcloc !42
...
!42 = !{ i32 1234567 }
It is up to the front-end to make sense of the magic numbers it places
in the IR. If the MDNode contains multiple constants, the code generator
will use the one that corresponds to the line of the asm that the error
occurs on.
.. _metadata:
Metadata
========
LLVM IR allows metadata to be attached to instructions in the program
that can convey extra information about the code to the optimizers and
code generator. One example application of metadata is source-level
debug information. There are two metadata primitives: strings and nodes.
Metadata does not have a type, and is not a value. If referenced from a
``call`` instruction, it uses the ``metadata`` type.
All metadata are identified in syntax by a exclamation point ('``!``').
.. _metadata-string:
Metadata Nodes and Metadata Strings
-----------------------------------
A metadata string is a string surrounded by double quotes. It can
contain any character by escaping non-printable characters with
"``\xx``" where "``xx``" is the two digit hex code. For example:
"``!"test\00"``".
Metadata nodes are represented with notation similar to structure
constants (a comma separated list of elements, surrounded by braces and
preceded by an exclamation point). Metadata nodes can have any values as
their operand. For example:
.. code-block:: llvm
!{ !"test\00", i32 10}
Metadata nodes that aren't uniqued use the ``distinct`` keyword. For example:
.. code-block:: text
!0 = distinct !{!"test\00", i32 10}
``distinct`` nodes are useful when nodes shouldn't be merged based on their
content. They can also occur when transformations cause uniquing collisions
when metadata operands change.
A :ref:`named metadata <namedmetadatastructure>` is a collection of
metadata nodes, which can be looked up in the module symbol table. For
example:
.. code-block:: llvm
!foo = !{!4, !3}
Metadata can be used as function arguments. Here the ``llvm.dbg.value``
intrinsic is using three metadata arguments:
.. code-block:: llvm
call void @llvm.dbg.value(metadata !24, metadata !25, metadata !26)
Metadata can be attached to an instruction. Here metadata ``!21`` is attached
to the ``add`` instruction using the ``!dbg`` identifier:
.. code-block:: llvm
%indvar.next = add i64 %indvar, 1, !dbg !21
Metadata can also be attached to a function or a global variable. Here metadata
``!22`` is attached to the ``f1`` and ``f2 functions, and the globals ``g1``
and ``g2`` using the ``!dbg`` identifier:
.. code-block:: llvm
declare !dbg !22 void @f1()
define void @f2() !dbg !22 {
ret void
}
@g1 = global i32 0, !dbg !22
@g2 = external global i32, !dbg !22
A transformation is required to drop any metadata attachment that it does not
know or know it can't preserve. Currently there is an exception for metadata
attachment to globals for ``!type`` and ``!absolute_symbol`` which can't be
unconditionally dropped unless the global is itself deleted.
Metadata attached to a module using named metadata may not be dropped, with
the exception of debug metadata (named metadata with the name ``!llvm.dbg.*``).
More information about specific metadata nodes recognized by the
optimizers and code generator is found below.
.. _specialized-metadata:
Specialized Metadata Nodes
^^^^^^^^^^^^^^^^^^^^^^^^^^
Specialized metadata nodes are custom data structures in metadata (as opposed
to generic tuples). Their fields are labelled, and can be specified in any
order.
These aren't inherently debug info centric, but currently all the specialized
metadata nodes are related to debug info.
.. _DICompileUnit:
DICompileUnit
"""""""""""""
``DICompileUnit`` nodes represent a compile unit. The ``enums:``,
``retainedTypes:``, ``globals:``, ``imports:`` and ``macros:`` fields are tuples
containing the debug info to be emitted along with the compile unit, regardless
of code optimizations (some nodes are only emitted if there are references to
them from instructions). The ``debugInfoForProfiling:`` field is a boolean
indicating whether or not line-table discriminators are updated to provide
more-accurate debug info for profiling results.
.. code-block:: text
!0 = !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang",
isOptimized: true, flags: "-O2", runtimeVersion: 2,
splitDebugFilename: "abc.debug", emissionKind: FullDebug,
enums: !2, retainedTypes: !3, globals: !4, imports: !5,
macros: !6, dwoId: 0x0abcd)
Compile unit descriptors provide the root scope for objects declared in a
specific compilation unit. File descriptors are defined using this scope. These
descriptors are collected by a named metadata node ``!llvm.dbg.cu``. They keep
track of global variables, type information, and imported entities (declarations
and namespaces).
.. _DIFile:
DIFile
""""""
``DIFile`` nodes represent files. The ``filename:`` can include slashes.
.. code-block:: none
!0 = !DIFile(filename: "path/to/file", directory: "/path/to/dir",
checksumkind: CSK_MD5,
checksum: "000102030405060708090a0b0c0d0e0f")
Files are sometimes used in ``scope:`` fields, and are the only valid target
for ``file:`` fields.
Valid values for ``checksumkind:`` field are: {CSK_None, CSK_MD5, CSK_SHA1}
.. _DIBasicType:
DIBasicType
"""""""""""
``DIBasicType`` nodes represent primitive types, such as ``int``, ``bool`` and
``float``. ``tag:`` defaults to ``DW_TAG_base_type``.
.. code-block:: text
!0 = !DIBasicType(name: "unsigned char", size: 8, align: 8,
encoding: DW_ATE_unsigned_char)
!1 = !DIBasicType(tag: DW_TAG_unspecified_type, name: "decltype(nullptr)")
The ``encoding:`` describes the details of the type. Usually it's one of the
following:
.. code-block:: text
DW_ATE_address = 1
DW_ATE_boolean = 2
DW_ATE_float = 4
DW_ATE_signed = 5
DW_ATE_signed_char = 6
DW_ATE_unsigned = 7
DW_ATE_unsigned_char = 8
.. _DISubroutineType:
DISubroutineType
""""""""""""""""
``DISubroutineType`` nodes represent subroutine types. Their ``types:`` field
refers to a tuple; the first operand is the return type, while the rest are the
types of the formal arguments in order. If the first operand is ``null``, that
represents a function with no return value (such as ``void foo() {}`` in C++).
.. code-block:: text
!0 = !BasicType(name: "int", size: 32, align: 32, DW_ATE_signed)
!1 = !BasicType(name: "char", size: 8, align: 8, DW_ATE_signed_char)
!2 = !DISubroutineType(types: !{null, !0, !1}) ; void (int, char)
.. _DIDerivedType:
DIDerivedType
"""""""""""""
``DIDerivedType`` nodes represent types derived from other types, such as
qualified types.
.. code-block:: text
!0 = !DIBasicType(name: "unsigned char", size: 8, align: 8,
encoding: DW_ATE_unsigned_char)
!1 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !0, size: 32,
align: 32)
The following ``tag:`` values are valid:
.. code-block:: text
DW_TAG_member = 13
DW_TAG_pointer_type = 15
DW_TAG_reference_type = 16
DW_TAG_typedef = 22
DW_TAG_inheritance = 28
DW_TAG_ptr_to_member_type = 31
DW_TAG_const_type = 38
DW_TAG_friend = 42
DW_TAG_volatile_type = 53
DW_TAG_restrict_type = 55
DW_TAG_atomic_type = 71
.. _DIDerivedTypeMember:
``DW_TAG_member`` is used to define a member of a :ref:`composite type
<DICompositeType>`. The type of the member is the ``baseType:``. The
``offset:`` is the member's bit offset. If the composite type has an ODR
``identifier:`` and does not set ``flags: DIFwdDecl``, then the member is
uniqued based only on its ``name:`` and ``scope:``.
``DW_TAG_inheritance`` and ``DW_TAG_friend`` are used in the ``elements:``
field of :ref:`composite types <DICompositeType>` to describe parents and
friends.
``DW_TAG_typedef`` is used to provide a name for the ``baseType:``.
``DW_TAG_pointer_type``, ``DW_TAG_reference_type``, ``DW_TAG_const_type``,
``DW_TAG_volatile_type``, ``DW_TAG_restrict_type`` and ``DW_TAG_atomic_type``
are used to qualify the ``baseType:``.
Note that the ``void *`` type is expressed as a type derived from NULL.
.. _DICompositeType:
DICompositeType
"""""""""""""""
``DICompositeType`` nodes represent types composed of other types, like
structures and unions. ``elements:`` points to a tuple of the composed types.
If the source language supports ODR, the ``identifier:`` field gives the unique
identifier used for type merging between modules. When specified,
:ref:`subprogram declarations <DISubprogramDeclaration>` and :ref:`member
derived types <DIDerivedTypeMember>` that reference the ODR-type in their
``scope:`` change uniquing rules.
For a given ``identifier:``, there should only be a single composite type that
does not have ``flags: DIFlagFwdDecl`` set. LLVM tools that link modules
together will unique such definitions at parse time via the ``identifier:``
field, even if the nodes are ``distinct``.
.. code-block:: text
!0 = !DIEnumerator(name: "SixKind", value: 7)
!1 = !DIEnumerator(name: "SevenKind", value: 7)
!2 = !DIEnumerator(name: "NegEightKind", value: -8)
!3 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "Enum", file: !12,
line: 2, size: 32, align: 32, identifier: "_M4Enum",
elements: !{!0, !1, !2})
The following ``tag:`` values are valid:
.. code-block:: text
DW_TAG_array_type = 1
DW_TAG_class_type = 2
DW_TAG_enumeration_type = 4
DW_TAG_structure_type = 19
DW_TAG_union_type = 23
For ``DW_TAG_array_type``, the ``elements:`` should be :ref:`subrange
descriptors <DISubrange>`, each representing the range of subscripts at that
level of indexing. The ``DIFlagVector`` flag to ``flags:`` indicates that an
array type is a native packed vector.
For ``DW_TAG_enumeration_type``, the ``elements:`` should be :ref:`enumerator
descriptors <DIEnumerator>`, each representing the definition of an enumeration
value for the set. All enumeration type descriptors are collected in the
``enums:`` field of the :ref:`compile unit <DICompileUnit>`.
For ``DW_TAG_structure_type``, ``DW_TAG_class_type``, and
``DW_TAG_union_type``, the ``elements:`` should be :ref:`derived types
<DIDerivedType>` with ``tag: DW_TAG_member``, ``tag: DW_TAG_inheritance``, or
``tag: DW_TAG_friend``; or :ref:`subprograms <DISubprogram>` with
``isDefinition: false``.
.. _DISubrange:
DISubrange
""""""""""
``DISubrange`` nodes are the elements for ``DW_TAG_array_type`` variants of
:ref:`DICompositeType`. ``count: -1`` indicates an empty array.
.. code-block:: llvm
!0 = !DISubrange(count: 5, lowerBound: 0) ; array counting from 0
!1 = !DISubrange(count: 5, lowerBound: 1) ; array counting from 1
!2 = !DISubrange(count: -1) ; empty array.
.. _DIEnumerator:
DIEnumerator
""""""""""""
``DIEnumerator`` nodes are the elements for ``DW_TAG_enumeration_type``
variants of :ref:`DICompositeType`.
.. code-block:: llvm
!0 = !DIEnumerator(name: "SixKind", value: 7)
!1 = !DIEnumerator(name: "SevenKind", value: 7)
!2 = !DIEnumerator(name: "NegEightKind", value: -8)
DITemplateTypeParameter
"""""""""""""""""""""""
``DITemplateTypeParameter`` nodes represent type parameters to generic source
language constructs. They are used (optionally) in :ref:`DICompositeType` and
:ref:`DISubprogram` ``templateParams:`` fields.
.. code-block:: llvm
!0 = !DITemplateTypeParameter(name: "Ty", type: !1)
DITemplateValueParameter
""""""""""""""""""""""""
``DITemplateValueParameter`` nodes represent value parameters to generic source
language constructs. ``tag:`` defaults to ``DW_TAG_template_value_parameter``,
but if specified can also be set to ``DW_TAG_GNU_template_template_param`` or
``DW_TAG_GNU_template_param_pack``. They are used (optionally) in
:ref:`DICompositeType` and :ref:`DISubprogram` ``templateParams:`` fields.
.. code-block:: llvm
!0 = !DITemplateValueParameter(name: "Ty", type: !1, value: i32 7)
DINamespace
"""""""""""
``DINamespace`` nodes represent namespaces in the source language.
.. code-block:: llvm
!0 = !DINamespace(name: "myawesomeproject", scope: !1, file: !2, line: 7)
DIGlobalVariable
""""""""""""""""
``DIGlobalVariable`` nodes represent global variables in the source language.
.. code-block:: llvm
!0 = !DIGlobalVariable(name: "foo", linkageName: "foo", scope: !1,
file: !2, line: 7, type: !3, isLocal: true,
isDefinition: false, variable: i32* @foo,
declaration: !4)
All global variables should be referenced by the `globals:` field of a
:ref:`compile unit <DICompileUnit>`.
.. _DISubprogram:
DISubprogram
""""""""""""
``DISubprogram`` nodes represent functions from the source language. A
``DISubprogram`` may be attached to a function definition using ``!dbg``
metadata. The ``variables:`` field points at :ref:`variables <DILocalVariable>`
that must be retained, even if their IR counterparts are optimized out of
the IR. The ``type:`` field must point at an :ref:`DISubroutineType`.
.. _DISubprogramDeclaration:
When ``isDefinition: false``, subprograms describe a declaration in the type
tree as opposed to a definition of a function. If the scope is a composite
type with an ODR ``identifier:`` and that does not set ``flags: DIFwdDecl``,
then the subprogram declaration is uniqued based only on its ``linkageName:``
and ``scope:``.
.. code-block:: text
define void @_Z3foov() !dbg !0 {
...
}
!0 = distinct !DISubprogram(name: "foo", linkageName: "_Zfoov", scope: !1,
file: !2, line: 7, type: !3, isLocal: true,
isDefinition: true, scopeLine: 8,
containingType: !4,
virtuality: DW_VIRTUALITY_pure_virtual,
virtualIndex: 10, flags: DIFlagPrototyped,
isOptimized: true, unit: !5, templateParams: !6,
declaration: !7, variables: !8, thrownTypes: !9)
.. _DILexicalBlock:
DILexicalBlock
""""""""""""""
``DILexicalBlock`` nodes describe nested blocks within a :ref:`subprogram
<DISubprogram>`. The line number and column numbers are used to distinguish
two lexical blocks at same depth. They are valid targets for ``scope:``
fields.
.. code-block:: text
!0 = distinct !DILexicalBlock(scope: !1, file: !2, line: 7, column: 35)
Usually lexical blocks are ``distinct`` to prevent node merging based on
operands.
.. _DILexicalBlockFile:
DILexicalBlockFile
""""""""""""""""""
``DILexicalBlockFile`` nodes are used to discriminate between sections of a
:ref:`lexical block <DILexicalBlock>`. The ``file:`` field can be changed to
indicate textual inclusion, or the ``discriminator:`` field can be used to
discriminate between control flow within a single block in the source language.
.. code-block:: llvm
!0 = !DILexicalBlock(scope: !3, file: !4, line: 7, column: 35)
!1 = !DILexicalBlockFile(scope: !0, file: !4, discriminator: 0)
!2 = !DILexicalBlockFile(scope: !0, file: !4, discriminator: 1)
.. _DILocation:
DILocation
""""""""""
``DILocation`` nodes represent source debug locations. The ``scope:`` field is
mandatory, and points at an :ref:`DILexicalBlockFile`, an
:ref:`DILexicalBlock`, or an :ref:`DISubprogram`.
.. code-block:: llvm
!0 = !DILocation(line: 2900, column: 42, scope: !1, inlinedAt: !2)
.. _DILocalVariable:
DILocalVariable
"""""""""""""""
``DILocalVariable`` nodes represent local variables in the source language. If
the ``arg:`` field is set to non-zero, then this variable is a subprogram
parameter, and it will be included in the ``variables:`` field of its
:ref:`DISubprogram`.
.. code-block:: text
!0 = !DILocalVariable(name: "this", arg: 1, scope: !3, file: !2, line: 7,
type: !3, flags: DIFlagArtificial)
!1 = !DILocalVariable(name: "x", arg: 2, scope: !4, file: !2, line: 7,
type: !3)
!2 = !DILocalVariable(name: "y", scope: !5, file: !2, line: 7, type: !3)
DIExpression
""""""""""""
``DIExpression`` nodes represent expressions that are inspired by the DWARF
expression language. They are used in :ref:`debug intrinsics<dbg_intrinsics>`
(such as ``llvm.dbg.declare`` and ``llvm.dbg.value``) to describe how the
referenced LLVM variable relates to the source language variable.
The current supported vocabulary is limited:
- ``DW_OP_deref`` dereferences the top of the expression stack.
- ``DW_OP_plus`` pops the last two entries from the expression stack, adds
them together and appends the result to the expression stack.
- ``DW_OP_minus`` pops the last two entries from the expression stack, subtracts
the last entry from the second last entry and appends the result to the
expression stack.
- ``DW_OP_plus_uconst, 93`` adds ``93`` to the working expression.
- ``DW_OP_LLVM_fragment, 16, 8`` specifies the offset and size (``16`` and ``8``
here, respectively) of the variable fragment from the working expression. Note
that contrary to DW_OP_bit_piece, the offset is describing the the location
within the described source variable.
- ``DW_OP_swap`` swaps top two stack entries.
- ``DW_OP_xderef`` provides extended dereference mechanism. The entry at the top
of the stack is treated as an address. The second stack entry is treated as an
address space identifier.
- ``DW_OP_stack_value`` marks a constant value.
DWARF specifies three kinds of simple location descriptions: Register, memory,
and implicit location descriptions. Register and memory location descriptions
describe the *location* of a source variable (in the sense that a debugger might
modify its value), whereas implicit locations describe merely the *value* of a
source variable. DIExpressions also follow this model: A DIExpression that
doesn't have a trailing ``DW_OP_stack_value`` will describe an *address* when
combined with a concrete location.
-.. code-block:: llvm
+.. code-block:: text
!0 = !DIExpression(DW_OP_deref)
!1 = !DIExpression(DW_OP_plus_uconst, 3)
!1 = !DIExpression(DW_OP_constu, 3, DW_OP_plus)
!2 = !DIExpression(DW_OP_bit_piece, 3, 7)
!3 = !DIExpression(DW_OP_deref, DW_OP_constu, 3, DW_OP_plus, DW_OP_LLVM_fragment, 3, 7)
!4 = !DIExpression(DW_OP_constu, 2, DW_OP_swap, DW_OP_xderef)
!5 = !DIExpression(DW_OP_constu, 42, DW_OP_stack_value)
DIObjCProperty
""""""""""""""
``DIObjCProperty`` nodes represent Objective-C property nodes.
.. code-block:: llvm
!3 = !DIObjCProperty(name: "foo", file: !1, line: 7, setter: "setFoo",
getter: "getFoo", attributes: 7, type: !2)
DIImportedEntity
""""""""""""""""
``DIImportedEntity`` nodes represent entities (such as modules) imported into a
compile unit.
.. code-block:: text
!2 = !DIImportedEntity(tag: DW_TAG_imported_module, name: "foo", scope: !0,
entity: !1, line: 7)
DIMacro
"""""""
``DIMacro`` nodes represent definition or undefinition of a macro identifiers.
The ``name:`` field is the macro identifier, followed by macro parameters when
defining a function-like macro, and the ``value`` field is the token-string
used to expand the macro identifier.
.. code-block:: text
!2 = !DIMacro(macinfo: DW_MACINFO_define, line: 7, name: "foo(x)",
value: "((x) + 1)")
!3 = !DIMacro(macinfo: DW_MACINFO_undef, line: 30, name: "foo")
DIMacroFile
"""""""""""
``DIMacroFile`` nodes represent inclusion of source files.
The ``nodes:`` field is a list of ``DIMacro`` and ``DIMacroFile`` nodes that
appear in the included source file.
.. code-block:: text
!2 = !DIMacroFile(macinfo: DW_MACINFO_start_file, line: 7, file: !2,
nodes: !3)
'``tbaa``' Metadata
^^^^^^^^^^^^^^^^^^^
In LLVM IR, memory does not have types, so LLVM's own type system is not
suitable for doing type based alias analysis (TBAA). Instead, metadata is
added to the IR to describe a type system of a higher level language. This
can be used to implement C/C++ strict type aliasing rules, but it can also
be used to implement custom alias analysis behavior for other languages.
This description of LLVM's TBAA system is broken into two parts:
:ref:`Semantics<tbaa_node_semantics>` talks about high level issues, and
:ref:`Representation<tbaa_node_representation>` talks about the metadata
encoding of various entities.
It is always possible to trace any TBAA node to a "root" TBAA node (details
in the :ref:`Representation<tbaa_node_representation>` section). TBAA
nodes with different roots have an unknown aliasing relationship, and LLVM
conservatively infers ``MayAlias`` between them. The rules mentioned in
this section only pertain to TBAA nodes living under the same root.
.. _tbaa_node_semantics:
Semantics
"""""""""
The TBAA metadata system, referred to as "struct path TBAA" (not to be
confused with ``tbaa.struct``), consists of the following high level
concepts: *Type Descriptors*, further subdivided into scalar type
descriptors and struct type descriptors; and *Access Tags*.
**Type descriptors** describe the type system of the higher level language
being compiled. **Scalar type descriptors** describe types that do not
contain other types. Each scalar type has a parent type, which must also
be a scalar type or the TBAA root. Via this parent relation, scalar types
within a TBAA root form a tree. **Struct type descriptors** denote types
that contain a sequence of other type descriptors, at known offsets. These
contained type descriptors can either be struct type descriptors themselves
or scalar type descriptors.
**Access tags** are metadata nodes attached to load and store instructions.
Access tags use type descriptors to describe the *location* being accessed
in terms of the type system of the higher level language. Access tags are
tuples consisting of a base type, an access type and an offset. The base
type is a scalar type descriptor or a struct type descriptor, the access
type is a scalar type descriptor, and the offset is a constant integer.
The access tag ``(BaseTy, AccessTy, Offset)`` can describe one of two
things:
* If ``BaseTy`` is a struct type, the tag describes a memory access (load
or store) of a value of type ``AccessTy`` contained in the struct type
``BaseTy`` at offset ``Offset``.
* If ``BaseTy`` is a scalar type, ``Offset`` must be 0 and ``BaseTy`` and
``AccessTy`` must be the same; and the access tag describes a scalar
access with scalar type ``AccessTy``.
We first define an ``ImmediateParent`` relation on ``(BaseTy, Offset)``
tuples this way:
* If ``BaseTy`` is a scalar type then ``ImmediateParent(BaseTy, 0)`` is
``(ParentTy, 0)`` where ``ParentTy`` is the parent of the scalar type as
described in the TBAA metadata. ``ImmediateParent(BaseTy, Offset)`` is
undefined if ``Offset`` is non-zero.
* If ``BaseTy`` is a struct type then ``ImmediateParent(BaseTy, Offset)``
is ``(NewTy, NewOffset)`` where ``NewTy`` is the type contained in
``BaseTy`` at offset ``Offset`` and ``NewOffset`` is ``Offset`` adjusted
to be relative within that inner type.
A memory access with an access tag ``(BaseTy1, AccessTy1, Offset1)``
aliases a memory access with an access tag ``(BaseTy2, AccessTy2,
Offset2)`` if either ``(BaseTy1, Offset1)`` is reachable from ``(Base2,
Offset2)`` via the ``Parent`` relation or vice versa.
As a concrete example, the type descriptor graph for the following program
.. code-block:: c
struct Inner {
int i; // offset 0
float f; // offset 4
};
-
+
struct Outer {
float f; // offset 0
double d; // offset 4
struct Inner inner_a; // offset 12
};
-
+
void f(struct Outer* outer, struct Inner* inner, float* f, int* i, char* c) {
outer->f = 0; // tag0: (OuterStructTy, FloatScalarTy, 0)
outer->inner_a.i = 0; // tag1: (OuterStructTy, IntScalarTy, 12)
outer->inner_a.f = 0.0; // tag2: (OuterStructTy, IntScalarTy, 16)
*f = 0.0; // tag3: (FloatScalarTy, FloatScalarTy, 0)
}
is (note that in C and C++, ``char`` can be used to access any arbitrary
type):
.. code-block:: text
Root = "TBAA Root"
CharScalarTy = ("char", Root, 0)
FloatScalarTy = ("float", CharScalarTy, 0)
DoubleScalarTy = ("double", CharScalarTy, 0)
IntScalarTy = ("int", CharScalarTy, 0)
InnerStructTy = {"Inner" (IntScalarTy, 0), (FloatScalarTy, 4)}
OuterStructTy = {"Outer", (FloatScalarTy, 0), (DoubleScalarTy, 4),
(InnerStructTy, 12)}
with (e.g.) ``ImmediateParent(OuterStructTy, 12)`` = ``(InnerStructTy,
0)``, ``ImmediateParent(InnerStructTy, 0)`` = ``(IntScalarTy, 0)``, and
``ImmediateParent(IntScalarTy, 0)`` = ``(CharScalarTy, 0)``.
.. _tbaa_node_representation:
Representation
""""""""""""""
The root node of a TBAA type hierarchy is an ``MDNode`` with 0 operands or
with exactly one ``MDString`` operand.
Scalar type descriptors are represented as an ``MDNode`` s with two
operands. The first operand is an ``MDString`` denoting the name of the
struct type. LLVM does not assign meaning to the value of this operand, it
only cares about it being an ``MDString``. The second operand is an
``MDNode`` which points to the parent for said scalar type descriptor,
which is either another scalar type descriptor or the TBAA root. Scalar
type descriptors can have an optional third argument, but that must be the
constant integer zero.
Struct type descriptors are represented as ``MDNode`` s with an odd number
of operands greater than 1. The first operand is an ``MDString`` denoting
the name of the struct type. Like in scalar type descriptors the actual
value of this name operand is irrelevant to LLVM. After the name operand,
the struct type descriptors have a sequence of alternating ``MDNode`` and
``ConstantInt`` operands. With N starting from 1, the 2N - 1 th operand,
an ``MDNode``, denotes a contained field, and the 2N th operand, a
``ConstantInt``, is the offset of the said contained field. The offsets
must be in non-decreasing order.
Access tags are represented as ``MDNode`` s with either 3 or 4 operands.
The first operand is an ``MDNode`` pointing to the node representing the
base type. The second operand is an ``MDNode`` pointing to the node
representing the access type. The third operand is a ``ConstantInt`` that
states the offset of the access. If a fourth field is present, it must be
a ``ConstantInt`` valued at 0 or 1. If it is 1 then the access tag states
that the location being accessed is "constant" (meaning
``pointsToConstantMemory`` should return true; see `other useful
AliasAnalysis methods <AliasAnalysis.html#OtherItfs>`_). The TBAA root of
the access type and the base type of an access tag must be the same, and
that is the TBAA root of the access tag.
'``tbaa.struct``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^
The :ref:`llvm.memcpy <int_memcpy>` is often used to implement
aggregate assignment operations in C and similar languages, however it
is defined to copy a contiguous region of memory, which is more than
strictly necessary for aggregate types which contain holes due to
padding. Also, it doesn't contain any TBAA information about the fields
of the aggregate.
``!tbaa.struct`` metadata can describe which memory subregions in a
memcpy are padding and what the TBAA tags of the struct are.
The current metadata format is very simple. ``!tbaa.struct`` metadata
nodes are a list of operands which are in conceptual groups of three.
For each group of three, the first operand gives the byte offset of a
field in bytes, the second gives its size in bytes, and the third gives
its tbaa tag. e.g.:
.. code-block:: llvm
!4 = !{ i64 0, i64 4, !1, i64 8, i64 4, !2 }
This describes a struct with two fields. The first is at offset 0 bytes
with size 4 bytes, and has tbaa tag !1. The second is at offset 8 bytes
and has size 4 bytes and has tbaa tag !2.
Note that the fields need not be contiguous. In this example, there is a
4 byte gap between the two fields. This gap represents padding which
does not carry useful data and need not be preserved.
'``noalias``' and '``alias.scope``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``noalias`` and ``alias.scope`` metadata provide the ability to specify generic
noalias memory-access sets. This means that some collection of memory access
instructions (loads, stores, memory-accessing calls, etc.) that carry
``noalias`` metadata can specifically be specified not to alias with some other
collection of memory access instructions that carry ``alias.scope`` metadata.
Each type of metadata specifies a list of scopes where each scope has an id and
a domain.
When evaluating an aliasing query, if for some domain, the set
of scopes with that domain in one instruction's ``alias.scope`` list is a
subset of (or equal to) the set of scopes for that domain in another
instruction's ``noalias`` list, then the two memory accesses are assumed not to
alias.
Because scopes in one domain don't affect scopes in other domains, separate
domains can be used to compose multiple independent noalias sets. This is
used for example during inlining. As the noalias function parameters are
turned into noalias scope metadata, a new domain is used every time the
function is inlined.
The metadata identifying each domain is itself a list containing one or two
entries. The first entry is the name of the domain. Note that if the name is a
string then it can be combined across functions and translation units. A
self-reference can be used to create globally unique domain names. A
descriptive string may optionally be provided as a second list entry.
The metadata identifying each scope is also itself a list containing two or
three entries. The first entry is the name of the scope. Note that if the name
is a string then it can be combined across functions and translation units. A
self-reference can be used to create globally unique scope names. A metadata
reference to the scope's domain is the second entry. A descriptive string may
optionally be provided as a third list entry.
For example,
.. code-block:: llvm
; Two scope domains:
!0 = !{!0}
!1 = !{!1}
; Some scopes in these domains:
!2 = !{!2, !0}
!3 = !{!3, !0}
!4 = !{!4, !1}
; Some scope lists:
!5 = !{!4} ; A list containing only scope !4
!6 = !{!4, !3, !2}
!7 = !{!3}
; These two instructions don't alias:
%0 = load float, float* %c, align 4, !alias.scope !5
store float %0, float* %arrayidx.i, align 4, !noalias !5
; These two instructions also don't alias (for domain !1, the set of scopes
; in the !alias.scope equals that in the !noalias list):
%2 = load float, float* %c, align 4, !alias.scope !5
store float %2, float* %arrayidx.i2, align 4, !noalias !6
; These two instructions may alias (for domain !0, the set of scopes in
; the !noalias list is not a superset of, or equal to, the scopes in the
; !alias.scope list):
%2 = load float, float* %c, align 4, !alias.scope !6
store float %0, float* %arrayidx.i, align 4, !noalias !7
'``fpmath``' Metadata
^^^^^^^^^^^^^^^^^^^^^
``fpmath`` metadata may be attached to any instruction of floating point
type. It can be used to express the maximum acceptable error in the
result of that instruction, in ULPs, thus potentially allowing the
compiler to use a more efficient but less accurate method of computing
it. ULP is defined as follows:
If ``x`` is a real number that lies between two finite consecutive
floating-point numbers ``a`` and ``b``, without being equal to one
of them, then ``ulp(x) = |b - a|``, otherwise ``ulp(x)`` is the
distance between the two non-equal finite floating-point numbers
nearest ``x``. Moreover, ``ulp(NaN)`` is ``NaN``.
The metadata node shall consist of a single positive float type number
representing the maximum relative error, for example:
.. code-block:: llvm
!0 = !{ float 2.5 } ; maximum acceptable inaccuracy is 2.5 ULPs
.. _range-metadata:
'``range``' Metadata
^^^^^^^^^^^^^^^^^^^^
``range`` metadata may be attached only to ``load``, ``call`` and ``invoke`` of
integer types. It expresses the possible ranges the loaded value or the value
returned by the called function at this call site is in. The ranges are
represented with a flattened list of integers. The loaded value or the value
returned is known to be in the union of the ranges defined by each consecutive
pair. Each pair has the following properties:
- The type must match the type loaded by the instruction.
- The pair ``a,b`` represents the range ``[a,b)``.
- Both ``a`` and ``b`` are constants.
- The range is allowed to wrap.
- The range should not represent the full or empty set. That is,
``a!=b``.
In addition, the pairs must be in signed order of the lower bound and
they must be non-contiguous.
Examples:
.. code-block:: llvm
%a = load i8, i8* %x, align 1, !range !0 ; Can only be 0 or 1
%b = load i8, i8* %y, align 1, !range !1 ; Can only be 255 (-1), 0 or 1
%c = call i8 @foo(), !range !2 ; Can only be 0, 1, 3, 4 or 5
%d = invoke i8 @bar() to label %cont
unwind label %lpad, !range !3 ; Can only be -2, -1, 3, 4 or 5
...
!0 = !{ i8 0, i8 2 }
!1 = !{ i8 255, i8 2 }
!2 = !{ i8 0, i8 2, i8 3, i8 6 }
!3 = !{ i8 -2, i8 0, i8 3, i8 6 }
'``absolute_symbol``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``absolute_symbol`` metadata may be attached to a global variable
declaration. It marks the declaration as a reference to an absolute symbol,
which causes the backend to use absolute relocations for the symbol even
in position independent code, and expresses the possible ranges that the
global variable's *address* (not its value) is in, in the same format as
``range`` metadata, with the extension that the pair ``all-ones,all-ones``
may be used to represent the full set.
Example (assuming 64-bit pointers):
.. code-block:: llvm
@a = external global i8, !absolute_symbol !0 ; Absolute symbol in range [0,256)
@b = external global i8, !absolute_symbol !1 ; Absolute symbol in range [0,2^64)
...
!0 = !{ i64 0, i64 256 }
!1 = !{ i64 -1, i64 -1 }
'``callees``' Metadata
^^^^^^^^^^^^^^^^^^^^^^
``callees`` metadata may be attached to indirect call sites. If ``callees``
metadata is attached to a call site, and any callee is not among the set of
functions provided by the metadata, the behavior is undefined. The intent of
this metadata is to facilitate optimizations such as indirect-call promotion.
For example, in the code below, the call instruction may only target the
``add`` or ``sub`` functions:
.. code-block:: llvm
%result = call i64 %binop(i64 %x, i64 %y), !callees !0
...
!0 = !{i64 (i64, i64)* @add, i64 (i64, i64)* @sub}
'``unpredictable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``unpredictable`` metadata may be attached to any branch or switch
instruction. It can be used to express the unpredictability of control
flow. Similar to the llvm.expect intrinsic, it may be used to alter
optimizations related to compare and branch instructions. The metadata
is treated as a boolean value; if it exists, it signals that the branch
or switch that it is attached to is completely unpredictable.
'``llvm.loop``'
^^^^^^^^^^^^^^^
It is sometimes useful to attach information to loop constructs. Currently,
loop metadata is implemented as metadata attached to the branch instruction
in the loop latch block. This type of metadata refer to a metadata node that is
guaranteed to be separate for each loop. The loop identifier metadata is
specified with the name ``llvm.loop``.
The loop identifier metadata is implemented using a metadata that refers to
itself to avoid merging it with any other identifier metadata, e.g.,
during module linkage or function inlining. That is, each loop should refer
to their own identification metadata even if they reside in separate functions.
The following example contains loop identifier metadata for two separate loop
constructs:
.. code-block:: llvm
!0 = !{!0}
!1 = !{!1}
The loop identifier metadata can be used to specify additional
per-loop metadata. Any operands after the first operand can be treated
as user-defined metadata. For example the ``llvm.loop.unroll.count``
suggests an unroll factor to the loop unroller:
.. code-block:: llvm
br i1 %exitcond, label %._crit_edge, label %.lr.ph, !llvm.loop !0
...
!0 = !{!0, !1}
!1 = !{!"llvm.loop.unroll.count", i32 4}
'``llvm.loop.vectorize``' and '``llvm.loop.interleave``'
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.loop.vectorize`` or ``llvm.loop.interleave`` are
used to control per-loop vectorization and interleaving parameters such as
vectorization width and interleave count. These metadata should be used in
conjunction with ``llvm.loop`` loop identification metadata. The
``llvm.loop.vectorize`` and ``llvm.loop.interleave`` metadata are only
optimization hints and the optimizer will only interleave and vectorize loops if
it believes it is safe to do so. The ``llvm.mem.parallel_loop_access`` metadata
which contains information about loop-carried memory dependencies can be helpful
in determining the safety of these transformations.
'``llvm.loop.interleave.count``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata suggests an interleave count to the loop interleaver.
The first operand is the string ``llvm.loop.interleave.count`` and the
second operand is an integer specifying the interleave count. For
example:
.. code-block:: llvm
!0 = !{!"llvm.loop.interleave.count", i32 4}
Note that setting ``llvm.loop.interleave.count`` to 1 disables interleaving
multiple iterations of the loop. If ``llvm.loop.interleave.count`` is set to 0
then the interleave count will be determined automatically.
'``llvm.loop.vectorize.enable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata selectively enables or disables vectorization for the loop. The
first operand is the string ``llvm.loop.vectorize.enable`` and the second operand
is a bit. If the bit operand value is 1 vectorization is enabled. A value of
0 disables vectorization:
.. code-block:: llvm
!0 = !{!"llvm.loop.vectorize.enable", i1 0}
!1 = !{!"llvm.loop.vectorize.enable", i1 1}
'``llvm.loop.vectorize.width``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata sets the target width of the vectorizer. The first
operand is the string ``llvm.loop.vectorize.width`` and the second
operand is an integer specifying the width. For example:
.. code-block:: llvm
!0 = !{!"llvm.loop.vectorize.width", i32 4}
Note that setting ``llvm.loop.vectorize.width`` to 1 disables
vectorization of the loop. If ``llvm.loop.vectorize.width`` is set to
0 or if the loop does not have this metadata the width will be
determined automatically.
'``llvm.loop.unroll``'
^^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.loop.unroll`` are loop unrolling
optimization hints such as the unroll factor. ``llvm.loop.unroll``
metadata should be used in conjunction with ``llvm.loop`` loop
identification metadata. The ``llvm.loop.unroll`` metadata are only
optimization hints and the unrolling will only be performed if the
optimizer believes it is safe to do so.
'``llvm.loop.unroll.count``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata suggests an unroll factor to the loop unroller. The
first operand is the string ``llvm.loop.unroll.count`` and the second
operand is a positive integer specifying the unroll factor. For
example:
.. code-block:: llvm
!0 = !{!"llvm.loop.unroll.count", i32 4}
If the trip count of the loop is less than the unroll count the loop
will be partially unrolled.
'``llvm.loop.unroll.disable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata disables loop unrolling. The metadata has a single operand
which is the string ``llvm.loop.unroll.disable``. For example:
.. code-block:: llvm
!0 = !{!"llvm.loop.unroll.disable"}
'``llvm.loop.unroll.runtime.disable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata disables runtime loop unrolling. The metadata has a single
operand which is the string ``llvm.loop.unroll.runtime.disable``. For example:
.. code-block:: llvm
!0 = !{!"llvm.loop.unroll.runtime.disable"}
'``llvm.loop.unroll.enable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata suggests that the loop should be fully unrolled if the trip count
is known at compile time and partially unrolled if the trip count is not known
at compile time. The metadata has a single operand which is the string
``llvm.loop.unroll.enable``. For example:
.. code-block:: llvm
!0 = !{!"llvm.loop.unroll.enable"}
'``llvm.loop.unroll.full``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata suggests that the loop should be unrolled fully. The
metadata has a single operand which is the string ``llvm.loop.unroll.full``.
For example:
.. code-block:: llvm
!0 = !{!"llvm.loop.unroll.full"}
'``llvm.loop.licm_versioning.disable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata indicates that the loop should not be versioned for the purpose
of enabling loop-invariant code motion (LICM). The metadata has a single operand
which is the string ``llvm.loop.licm_versioning.disable``. For example:
.. code-block:: llvm
!0 = !{!"llvm.loop.licm_versioning.disable"}
'``llvm.loop.distribute.enable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Loop distribution allows splitting a loop into multiple loops. Currently,
this is only performed if the entire loop cannot be vectorized due to unsafe
memory dependencies. The transformation will attempt to isolate the unsafe
dependencies into their own loop.
This metadata can be used to selectively enable or disable distribution of the
loop. The first operand is the string ``llvm.loop.distribute.enable`` and the
second operand is a bit. If the bit operand value is 1 distribution is
enabled. A value of 0 disables distribution:
.. code-block:: llvm
!0 = !{!"llvm.loop.distribute.enable", i1 0}
!1 = !{!"llvm.loop.distribute.enable", i1 1}
This metadata should be used in conjunction with ``llvm.loop`` loop
identification metadata.
'``llvm.mem``'
^^^^^^^^^^^^^^^
Metadata types used to annotate memory accesses with information helpful
for optimizations are prefixed with ``llvm.mem``.
'``llvm.mem.parallel_loop_access``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``llvm.mem.parallel_loop_access`` metadata refers to a loop identifier,
or metadata containing a list of loop identifiers for nested loops.
The metadata is attached to memory accessing instructions and denotes that
no loop carried memory dependence exist between it and other instructions denoted
with the same loop identifier. The metadata on memory reads also implies that
if conversion (i.e. speculative execution within a loop iteration) is safe.
Precisely, given two instructions ``m1`` and ``m2`` that both have the
``llvm.mem.parallel_loop_access`` metadata, with ``L1`` and ``L2`` being the
set of loops associated with that metadata, respectively, then there is no loop
carried dependence between ``m1`` and ``m2`` for loops in both ``L1`` and
``L2``.
As a special case, if all memory accessing instructions in a loop have
``llvm.mem.parallel_loop_access`` metadata that refers to that loop, then the
loop has no loop carried memory dependences and is considered to be a parallel
loop.
Note that if not all memory access instructions have such metadata referring to
the loop, then the loop is considered not being trivially parallel. Additional
memory dependence analysis is required to make that determination. As a fail
safe mechanism, this causes loops that were originally parallel to be considered
sequential (if optimization passes that are unaware of the parallel semantics
insert new memory instructions into the loop body).
Example of a loop that is considered parallel due to its correct use of
both ``llvm.loop`` and ``llvm.mem.parallel_loop_access``
metadata types that refer to the same loop identifier metadata.
.. code-block:: llvm
for.body:
...
%val0 = load i32, i32* %arrayidx, !llvm.mem.parallel_loop_access !0
...
store i32 %val0, i32* %arrayidx1, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !0
for.end:
...
!0 = !{!0}
It is also possible to have nested parallel loops. In that case the
memory accesses refer to a list of loop identifier metadata nodes instead of
the loop identifier metadata node directly:
.. code-block:: llvm
outer.for.body:
...
%val1 = load i32, i32* %arrayidx3, !llvm.mem.parallel_loop_access !2
...
br label %inner.for.body
inner.for.body:
...
%val0 = load i32, i32* %arrayidx1, !llvm.mem.parallel_loop_access !0
...
store i32 %val0, i32* %arrayidx2, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %inner.for.end, label %inner.for.body, !llvm.loop !1
inner.for.end:
...
store i32 %val1, i32* %arrayidx4, !llvm.mem.parallel_loop_access !2
...
br i1 %exitcond, label %outer.for.end, label %outer.for.body, !llvm.loop !2
outer.for.end: ; preds = %for.body
...
!0 = !{!1, !2} ; a list of loop identifiers
!1 = !{!1} ; an identifier for the inner loop
!2 = !{!2} ; an identifier for the outer loop
'``irr_loop``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^
``irr_loop`` metadata may be attached to the terminator instruction of a basic
block that's an irreducible loop header (note that an irreducible loop has more
than once header basic blocks.) If ``irr_loop`` metadata is attached to the
terminator instruction of a basic block that is not really an irreducible loop
header, the behavior is undefined. The intent of this metadata is to improve the
accuracy of the block frequency propagation. For example, in the code below, the
block ``header0`` may have a loop header weight (relative to the other headers of
the irreducible loop) of 100:
.. code-block:: llvm
header0:
...
br i1 %cmp, label %t1, label %t2, !irr_loop !0
...
!0 = !{"loop_header_weight", i64 100}
Irreducible loop header weights are typically based on profile data.
'``invariant.group``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``invariant.group`` metadata may be attached to ``load``/``store`` instructions.
-The existence of the ``invariant.group`` metadata on the instruction tells
-the optimizer that every ``load`` and ``store`` to the same pointer operand
-within the same invariant group can be assumed to load or store the same
-value (but see the ``llvm.invariant.group.barrier`` intrinsic which affects
+The existence of the ``invariant.group`` metadata on the instruction tells
+the optimizer that every ``load`` and ``store`` to the same pointer operand
+within the same invariant group can be assumed to load or store the same
+value (but see the ``llvm.invariant.group.barrier`` intrinsic which affects
when two pointers are considered the same). Pointers returned by bitcast or
getelementptr with only zero indices are considered the same.
Examples:
.. code-block:: llvm
@unknownPtr = external global i8
...
%ptr = alloca i8
store i8 42, i8* %ptr, !invariant.group !0
call void @foo(i8* %ptr)
-
+
%a = load i8, i8* %ptr, !invariant.group !0 ; Can assume that value under %ptr didn't change
call void @foo(i8* %ptr)
%b = load i8, i8* %ptr, !invariant.group !1 ; Can't assume anything, because group changed
-
- %newPtr = call i8* @getPointer(i8* %ptr)
+
+ %newPtr = call i8* @getPointer(i8* %ptr)
%c = load i8, i8* %newPtr, !invariant.group !0 ; Can't assume anything, because we only have information about %ptr
-
+
%unknownValue = load i8, i8* @unknownPtr
store i8 %unknownValue, i8* %ptr, !invariant.group !0 ; Can assume that %unknownValue == 42
-
+
call void @foo(i8* %ptr)
%newPtr2 = call i8* @llvm.invariant.group.barrier(i8* %ptr)
%d = load i8, i8* %newPtr2, !invariant.group !0 ; Can't step through invariant.group.barrier to get value of %ptr
-
+
...
declare void @foo(i8*)
declare i8* @getPointer(i8*)
declare i8* @llvm.invariant.group.barrier(i8*)
-
+
!0 = !{!"magic ptr"}
!1 = !{!"other ptr"}
The invariant.group metadata must be dropped when replacing one pointer by
another based on aliasing information. This is because invariant.group is tied
to the SSA value of the pointer operand.
.. code-block:: llvm
-
+
%v = load i8, i8* %x, !invariant.group !0
; if %x mustalias %y then we can replace the above instruction with
%v = load i8, i8* %y
'``type``' Metadata
^^^^^^^^^^^^^^^^^^^
See :doc:`TypeMetadata`.
'``associated``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^
The ``associated`` metadata may be attached to a global object
declaration with a single argument that references another global object.
This metadata prevents discarding of the global object in linker GC
unless the referenced object is also discarded. The linker support for
this feature is spotty. For best compatibility, globals carrying this
metadata may also:
- Be in a comdat with the referenced global.
- Be in @llvm.compiler.used.
- Have an explicit section with a name which is a valid C identifier.
It does not have any effect on non-ELF targets.
Example:
-.. code-block:: llvm
+.. code-block:: text
$a = comdat any
@a = global i32 1, comdat $a
@b = internal global i32 2, comdat $a, section "abc", !associated !0
!0 = !{i32* @a}
'``prof``' Metadata
^^^^^^^^^^^^^^^^^^^
The ``prof`` metadata is used to record profile data in the IR.
The first operand of the metadata node indicates the profile metadata
type. There are currently 3 types:
:ref:`branch_weights<prof_node_branch_weights>`,
:ref:`function_entry_count<prof_node_function_entry_count>`, and
:ref:`VP<prof_node_VP>`.
.. _prof_node_branch_weights:
branch_weights
""""""""""""""
Branch weight metadata attached to a branch, select, switch or call instruction
represents the likeliness of the associated branch being taken.
For more information, see :doc:`BranchWeightMetadata`.
.. _prof_node_function_entry_count:
function_entry_count
""""""""""""""""""""
Function entry count metadata can be attached to function definitions
to record the number of times the function is called. Used with BFI
information, it is also used to derive the basic block profile count.
For more information, see :doc:`BranchWeightMetadata`.
.. _prof_node_VP:
VP
""
VP (value profile) metadata can be attached to instructions that have
value profile information. Currently this is indirect calls (where it
records the hottest callees) and calls to memory intrinsics such as memcpy,
memmove, and memset (where it records the hottest byte lengths).
Each VP metadata node contains "VP" string, then a uint32_t value for the value
profiling kind, a uint64_t value for the total number of times the instruction
is executed, followed by uint64_t value and execution count pairs.
The value profiling kind is 0 for indirect call targets and 1 for memory
operations. For indirect call targets, each profile value is a hash
of the callee function name, and for memory operations each value is the
byte length.
Note that the value counts do not need to add up to the total count
listed in the third operand (in practice only the top hottest values
are tracked and reported).
Indirect call example:
.. code-block:: llvm
call void %f(), !prof !1
!1 = !{!"VP", i32 0, i64 1600, i64 7651369219802541373, i64 1030, i64 -4377547752858689819, i64 410}
Note that the VP type is 0 (the second operand), which indicates this is
an indirect call value profile data. The third operand indicates that the
indirect call executed 1600 times. The 4th and 6th operands give the
hashes of the 2 hottest target functions' names (this is the same hash used
to represent function names in the profile database), and the 5th and 7th
operands give the execution count that each of the respective prior target
functions was called.
Module Flags Metadata
=====================
Information about the module as a whole is difficult to convey to LLVM's
subsystems. The LLVM IR isn't sufficient to transmit this information.
The ``llvm.module.flags`` named metadata exists in order to facilitate
this. These flags are in the form of key / value pairs --- much like a
dictionary --- making it easy for any subsystem who cares about a flag to
look it up.
The ``llvm.module.flags`` metadata contains a list of metadata triplets.
Each triplet has the following form:
- The first element is a *behavior* flag, which specifies the behavior
when two (or more) modules are merged together, and it encounters two
(or more) metadata with the same ID. The supported behaviors are
described below.
- The second element is a metadata string that is a unique ID for the
metadata. Each module may only have one flag entry for each unique ID (not
including entries with the **Require** behavior).
- The third element is the value of the flag.
When two (or more) modules are merged together, the resulting
``llvm.module.flags`` metadata is the union of the modules' flags. That is, for
each unique metadata ID string, there will be exactly one entry in the merged
modules ``llvm.module.flags`` metadata table, and the value for that entry will
be determined by the merge behavior flag, as described below. The only exception
is that entries with the *Require* behavior are always preserved.
The following behaviors are supported:
.. list-table::
:header-rows: 1
:widths: 10 90
* - Value
- Behavior
* - 1
- **Error**
Emits an error if two values disagree, otherwise the resulting value
is that of the operands.
* - 2
- **Warning**
Emits a warning if two values disagree. The result value will be the
operand for the flag from the first module being linked.
* - 3
- **Require**
Adds a requirement that another module flag be present and have a
specified value after linking is performed. The value must be a
metadata pair, where the first element of the pair is the ID of the
module flag to be restricted, and the second element of the pair is
the value the module flag should be restricted to. This behavior can
be used to restrict the allowable results (via triggering of an
error) of linking IDs with the **Override** behavior.
* - 4
- **Override**
Uses the specified value, regardless of the behavior or value of the
other module. If both modules specify **Override**, but the values
differ, an error will be emitted.
* - 5
- **Append**
Appends the two values, which are required to be metadata nodes.
* - 6
- **AppendUnique**
Appends the two values, which are required to be metadata
nodes. However, duplicate entries in the second list are dropped
during the append operation.
* - 7
- **Max**
Takes the max of the two values, which are required to be integers.
It is an error for a particular unique flag ID to have multiple behaviors,
except in the case of **Require** (which adds restrictions on another metadata
value) or **Override**.
An example of module flags:
.. code-block:: llvm
!0 = !{ i32 1, !"foo", i32 1 }
!1 = !{ i32 4, !"bar", i32 37 }
!2 = !{ i32 2, !"qux", i32 42 }
!3 = !{ i32 3, !"qux",
!{
!"foo", i32 1
}
}
!llvm.module.flags = !{ !0, !1, !2, !3 }
- Metadata ``!0`` has the ID ``!"foo"`` and the value '1'. The behavior
if two or more ``!"foo"`` flags are seen is to emit an error if their
values are not equal.
- Metadata ``!1`` has the ID ``!"bar"`` and the value '37'. The
behavior if two or more ``!"bar"`` flags are seen is to use the value
'37'.
- Metadata ``!2`` has the ID ``!"qux"`` and the value '42'. The
behavior if two or more ``!"qux"`` flags are seen is to emit a
warning if their values are not equal.
- Metadata ``!3`` has the ID ``!"qux"`` and the value:
::
!{ !"foo", i32 1 }
The behavior is to emit an error if the ``llvm.module.flags`` does not
contain a flag with the ID ``!"foo"`` that has the value '1' after linking is
performed.
Objective-C Garbage Collection Module Flags Metadata
----------------------------------------------------
On the Mach-O platform, Objective-C stores metadata about garbage
collection in a special section called "image info". The metadata
consists of a version number and a bitmask specifying what types of
garbage collection are supported (if any) by the file. If two or more
modules are linked together their garbage collection metadata needs to
be merged rather than appended together.
The Objective-C garbage collection module flags metadata consists of the
following key-value pairs:
.. list-table::
:header-rows: 1
:widths: 30 70
* - Key
- Value
* - ``Objective-C Version``
- **[Required]** --- The Objective-C ABI version. Valid values are 1 and 2.
* - ``Objective-C Image Info Version``
- **[Required]** --- The version of the image info section. Currently
always 0.
* - ``Objective-C Image Info Section``
- **[Required]** --- The section to place the metadata. Valid values are
``"__OBJC, __image_info, regular"`` for Objective-C ABI version 1, and
``"__DATA,__objc_imageinfo, regular, no_dead_strip"`` for
Objective-C ABI version 2.
* - ``Objective-C Garbage Collection``
- **[Required]** --- Specifies whether garbage collection is supported or
not. Valid values are 0, for no garbage collection, and 2, for garbage
collection supported.
* - ``Objective-C GC Only``
- **[Optional]** --- Specifies that only garbage collection is supported.
If present, its value must be 6. This flag requires that the
``Objective-C Garbage Collection`` flag have the value 2.
Some important flag interactions:
- If a module with ``Objective-C Garbage Collection`` set to 0 is
merged with a module with ``Objective-C Garbage Collection`` set to
2, then the resulting module has the
``Objective-C Garbage Collection`` flag set to 0.
- A module with ``Objective-C Garbage Collection`` set to 0 cannot be
merged with a module with ``Objective-C GC Only`` set to 6.
C type width Module Flags Metadata
----------------------------------
The ARM backend emits a section into each generated object file describing the
options that it was compiled with (in a compiler-independent way) to prevent
linking incompatible objects, and to allow automatic library selection. Some
of these options are not visible at the IR level, namely wchar_t width and enum
width.
To pass this information to the backend, these options are encoded in module
flags metadata, using the following key-value pairs:
.. list-table::
:header-rows: 1
:widths: 30 70
* - Key
- Value
* - short_wchar
- * 0 --- sizeof(wchar_t) == 4
* 1 --- sizeof(wchar_t) == 2
* - short_enum
- * 0 --- Enums are at least as large as an ``int``.
* 1 --- Enums are stored in the smallest integer type which can
represent all of its values.
For example, the following metadata section specifies that the module was
compiled with a ``wchar_t`` width of 4 bytes, and the underlying type of an
enum is the smallest type which can represent all of its values::
!llvm.module.flags = !{!0, !1}
!0 = !{i32 1, !"short_wchar", i32 1}
!1 = !{i32 1, !"short_enum", i32 0}
Automatic Linker Flags Named Metadata
=====================================
Some targets support embedding flags to the linker inside individual object
files. Typically this is used in conjunction with language extensions which
allow source files to explicitly declare the libraries they depend on, and have
these automatically be transmitted to the linker via object files.
These flags are encoded in the IR using named metadata with the name
``!llvm.linker.options``. Each operand is expected to be a metadata node
which should be a list of other metadata nodes, each of which should be a
list of metadata strings defining linker options.
For example, the following metadata section specifies two separate sets of
linker options, presumably to link against ``libz`` and the ``Cocoa``
framework::
!0 = !{ !"-lz" },
!1 = !{ !"-framework", !"Cocoa" } } }
!llvm.linker.options = !{ !0, !1 }
The metadata encoding as lists of lists of options, as opposed to a collapsed
list of options, is chosen so that the IR encoding can use multiple option
strings to specify e.g., a single library, while still having that specifier be
preserved as an atomic element that can be recognized by a target specific
assembly writer or object file emitter.
Each individual option is required to be either a valid option for the target's
linker, or an option that is reserved by the target specific assembly writer or
object file emitter. No other aspect of these options is defined by the IR.
.. _intrinsicglobalvariables:
Intrinsic Global Variables
==========================
LLVM has a number of "magic" global variables that contain data that
affect code generation or other IR semantics. These are documented here.
All globals of this sort should have a section specified as
"``llvm.metadata``". This section and all globals that start with
"``llvm.``" are reserved for use by LLVM.
.. _gv_llvmused:
The '``llvm.used``' Global Variable
-----------------------------------
The ``@llvm.used`` global is an array which has
:ref:`appending linkage <linkage_appending>`. This array contains a list of
pointers to named global variables, functions and aliases which may optionally
have a pointer cast formed of bitcast or getelementptr. For example, a legal
use of it is:
.. code-block:: llvm
@X = global i8 4
@Y = global i32 123
@llvm.used = appending global [2 x i8*] [
i8* @X,
i8* bitcast (i32* @Y to i8*)
], section "llvm.metadata"
If a symbol appears in the ``@llvm.used`` list, then the compiler, assembler,
and linker are required to treat the symbol as if there is a reference to the
symbol that it cannot see (which is why they have to be named). For example, if
a variable has internal linkage and no references other than that from the
``@llvm.used`` list, it cannot be deleted. This is commonly used to represent
references from inline asms and other things the compiler cannot "see", and
corresponds to "``attribute((used))``" in GNU C.
On some targets, the code generator must emit a directive to the
assembler or object file to prevent the assembler and linker from
molesting the symbol.
.. _gv_llvmcompilerused:
The '``llvm.compiler.used``' Global Variable
--------------------------------------------
The ``@llvm.compiler.used`` directive is the same as the ``@llvm.used``
directive, except that it only prevents the compiler from touching the
symbol. On targets that support it, this allows an intelligent linker to
optimize references to the symbol without being impeded as it would be
by ``@llvm.used``.
This is a rare construct that should only be used in rare circumstances,
and should not be exposed to source languages.
.. _gv_llvmglobalctors:
The '``llvm.global_ctors``' Global Variable
-------------------------------------------
.. code-block:: llvm
%0 = type { i32, void ()*, i8* }
@llvm.global_ctors = appending global [1 x %0] [%0 { i32 65535, void ()* @ctor, i8* @data }]
The ``@llvm.global_ctors`` array contains a list of constructor
functions, priorities, and an optional associated global or function.
The functions referenced by this array will be called in ascending order
of priority (i.e. lowest first) when the module is loaded. The order of
functions with the same priority is not defined.
If the third field is present, non-null, and points to a global variable
or function, the initializer function will only run if the associated
data from the current module is not discarded.
.. _llvmglobaldtors:
The '``llvm.global_dtors``' Global Variable
-------------------------------------------
.. code-block:: llvm
%0 = type { i32, void ()*, i8* }
@llvm.global_dtors = appending global [1 x %0] [%0 { i32 65535, void ()* @dtor, i8* @data }]
The ``@llvm.global_dtors`` array contains a list of destructor
functions, priorities, and an optional associated global or function.
The functions referenced by this array will be called in descending
order of priority (i.e. highest first) when the module is unloaded. The
order of functions with the same priority is not defined.
If the third field is present, non-null, and points to a global variable
or function, the destructor function will only run if the associated
data from the current module is not discarded.
Instruction Reference
=====================
The LLVM instruction set consists of several different classifications
of instructions: :ref:`terminator instructions <terminators>`, :ref:`binary
instructions <binaryops>`, :ref:`bitwise binary
instructions <bitwiseops>`, :ref:`memory instructions <memoryops>`, and
:ref:`other instructions <otherops>`.
.. _terminators:
Terminator Instructions
-----------------------
As mentioned :ref:`previously <functionstructure>`, every basic block in a
program ends with a "Terminator" instruction, which indicates which
block should be executed after the current block is finished. These
terminator instructions typically yield a '``void``' value: they produce
control flow, not values (the one exception being the
':ref:`invoke <i_invoke>`' instruction).
The terminator instructions are: ':ref:`ret <i_ret>`',
':ref:`br <i_br>`', ':ref:`switch <i_switch>`',
':ref:`indirectbr <i_indirectbr>`', ':ref:`invoke <i_invoke>`',
':ref:`resume <i_resume>`', ':ref:`catchswitch <i_catchswitch>`',
':ref:`catchret <i_catchret>`',
':ref:`cleanupret <i_cleanupret>`',
and ':ref:`unreachable <i_unreachable>`'.
.. _i_ret:
'``ret``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
ret <type> <value> ; Return a value from a non-void function
ret void ; Return from void function
Overview:
"""""""""
The '``ret``' instruction is used to return control flow (and optionally
a value) from a function back to the caller.
There are two forms of the '``ret``' instruction: one that returns a
value and then causes control flow, and one that just causes control
flow to occur.
Arguments:
""""""""""
The '``ret``' instruction optionally accepts a single argument, the
return value. The type of the return value must be a ':ref:`first
class <t_firstclass>`' type.
A function is not :ref:`well formed <wellformed>` if it it has a non-void
return type and contains a '``ret``' instruction with no return value or
a return value with a type that does not match its type, or if it has a
void return type and contains a '``ret``' instruction with a return
value.
Semantics:
""""""""""
When the '``ret``' instruction is executed, control flow returns back to
the calling function's context. If the caller is a
":ref:`call <i_call>`" instruction, execution continues at the
instruction after the call. If the caller was an
":ref:`invoke <i_invoke>`" instruction, execution continues at the
beginning of the "normal" destination block. If the instruction returns
a value, that value shall set the call or invoke instruction's return
value.
Example:
""""""""
.. code-block:: llvm
ret i32 5 ; Return an integer value of 5
ret void ; Return from a void function
ret { i32, i8 } { i32 4, i8 2 } ; Return a struct of values 4 and 2
.. _i_br:
'``br``' Instruction
^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
br i1 <cond>, label <iftrue>, label <iffalse>
br label <dest> ; Unconditional branch
Overview:
"""""""""
The '``br``' instruction is used to cause control flow to transfer to a
different basic block in the current function. There are two forms of
this instruction, corresponding to a conditional branch and an
unconditional branch.
Arguments:
""""""""""
The conditional branch form of the '``br``' instruction takes a single
'``i1``' value and two '``label``' values. The unconditional form of the
'``br``' instruction takes a single '``label``' value as a target.
Semantics:
""""""""""
Upon execution of a conditional '``br``' instruction, the '``i1``'
argument is evaluated. If the value is ``true``, control flows to the
'``iftrue``' ``label`` argument. If "cond" is ``false``, control flows
to the '``iffalse``' ``label`` argument.
Example:
""""""""
.. code-block:: llvm
Test:
%cond = icmp eq i32 %a, %b
br i1 %cond, label %IfEqual, label %IfUnequal
IfEqual:
ret i32 1
IfUnequal:
ret i32 0
.. _i_switch:
'``switch``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]
Overview:
"""""""""
The '``switch``' instruction is used to transfer control flow to one of
several different places. It is a generalization of the '``br``'
instruction, allowing a branch to occur to one of many possible
destinations.
Arguments:
""""""""""
The '``switch``' instruction uses three parameters: an integer
comparison value '``value``', a default '``label``' destination, and an
array of pairs of comparison value constants and '``label``'s. The table
is not allowed to contain duplicate constant entries.
Semantics:
""""""""""
The ``switch`` instruction specifies a table of values and destinations.
When the '``switch``' instruction is executed, this table is searched
for the given value. If the value is found, control flow is transferred
to the corresponding destination; otherwise, control flow is transferred
to the default destination.
Implementation:
"""""""""""""""
Depending on properties of the target machine and the particular
``switch`` instruction, this instruction may be code generated in
different ways. For example, it could be generated as a series of
chained conditional branches or with a lookup table.
Example:
""""""""
.. code-block:: llvm
; Emulate a conditional br instruction
%Val = zext i1 %value to i32
switch i32 %Val, label %truedest [ i32 0, label %falsedest ]
; Emulate an unconditional br instruction
switch i32 0, label %dest [ ]
; Implement a jump table:
switch i32 %val, label %otherwise [ i32 0, label %onzero
i32 1, label %onone
i32 2, label %ontwo ]
.. _i_indirectbr:
'``indirectbr``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
indirectbr <somety>* <address>, [ label <dest1>, label <dest2>, ... ]
Overview:
"""""""""
The '``indirectbr``' instruction implements an indirect branch to a
label within the current function, whose address is specified by
"``address``". Address must be derived from a
:ref:`blockaddress <blockaddress>` constant.
Arguments:
""""""""""
The '``address``' argument is the address of the label to jump to. The
rest of the arguments indicate the full set of possible destinations
that the address may point to. Blocks are allowed to occur multiple
times in the destination list, though this isn't particularly useful.
This destination list is required so that dataflow analysis has an
accurate understanding of the CFG.
Semantics:
""""""""""
Control transfers to the block specified in the address argument. All
possible destination blocks must be listed in the label list, otherwise
this instruction has undefined behavior. This implies that jumps to
labels defined in other functions have undefined behavior as well.
Implementation:
"""""""""""""""
This is typically implemented with a jump through a register.
Example:
""""""""
.. code-block:: llvm
indirectbr i8* %Addr, [ label %bb1, label %bb2, label %bb3 ]
.. _i_invoke:
'``invoke``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = invoke [cconv] [ret attrs] <ty>|<fnty> <fnptrval>(<function args>) [fn attrs]
[operand bundles] to label <normal label> unwind label <exception label>
Overview:
"""""""""
The '``invoke``' instruction causes control to transfer to a specified
function, with the possibility of control flow transfer to either the
'``normal``' label or the '``exception``' label. If the callee function
returns with the "``ret``" instruction, control flow will return to the
"normal" label. If the callee (or any indirect callees) returns via the
":ref:`resume <i_resume>`" instruction or other exception handling
mechanism, control is interrupted and continued at the dynamically
nearest "exception" label.
The '``exception``' label is a `landing
pad <ExceptionHandling.html#overview>`_ for the exception. As such,
'``exception``' label is required to have the
":ref:`landingpad <i_landingpad>`" instruction, which contains the
information about the behavior of the program after unwinding happens,
as its first non-PHI instruction. The restrictions on the
"``landingpad``" instruction's tightly couples it to the "``invoke``"
instruction, so that the important information contained within the
"``landingpad``" instruction can't be lost through normal code motion.
Arguments:
""""""""""
This instruction requires several arguments:
#. The optional "cconv" marker indicates which :ref:`calling
convention <callingconv>` the call should use. If none is
specified, the call defaults to using C calling conventions.
#. The optional :ref:`Parameter Attributes <paramattrs>` list for return
values. Only '``zeroext``', '``signext``', and '``inreg``' attributes
are valid here.
#. '``ty``': the type of the call instruction itself which is also the
type of the return value. Functions that return no value are marked
``void``.
#. '``fnty``': shall be the signature of the function being invoked. The
argument types must match the types implied by this signature. This
type can be omitted if the function is not varargs.
#. '``fnptrval``': An LLVM value containing a pointer to a function to
be invoked. In most cases, this is a direct function invocation, but
indirect ``invoke``'s are just as possible, calling an arbitrary pointer
to function value.
#. '``function args``': argument list whose types match the function
signature argument types and parameter attributes. All arguments must
be of :ref:`first class <t_firstclass>` type. If the function signature
indicates the function accepts a variable number of arguments, the
extra arguments can be specified.
#. '``normal label``': the label reached when the called function
executes a '``ret``' instruction.
#. '``exception label``': the label reached when a callee returns via
the :ref:`resume <i_resume>` instruction or other exception handling
mechanism.
#. The optional :ref:`function attributes <fnattrs>` list.
#. The optional :ref:`operand bundles <opbundles>` list.
Semantics:
""""""""""
This instruction is designed to operate as a standard '``call``'
instruction in most regards. The primary difference is that it
establishes an association with a label, which is used by the runtime
library to unwind the stack.
This instruction is used in languages with destructors to ensure that
proper cleanup is performed in the case of either a ``longjmp`` or a
thrown exception. Additionally, this is important for implementation of
'``catch``' clauses in high-level languages that support them.
For the purposes of the SSA form, the definition of the value returned
by the '``invoke``' instruction is deemed to occur on the edge from the
current block to the "normal" label. If the callee unwinds then no
return value is available.
Example:
""""""""
.. code-block:: llvm
%retval = invoke i32 @Test(i32 15) to label %Continue
unwind label %TestCleanup ; i32:retval set
%retval = invoke coldcc i32 %Testfnptr(i32 15) to label %Continue
unwind label %TestCleanup ; i32:retval set
.. _i_resume:
'``resume``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
resume <type> <value>
Overview:
"""""""""
The '``resume``' instruction is a terminator instruction that has no
successors.
Arguments:
""""""""""
The '``resume``' instruction requires one argument, which must have the
same type as the result of any '``landingpad``' instruction in the same
function.
Semantics:
""""""""""
The '``resume``' instruction resumes propagation of an existing
(in-flight) exception whose unwinding was interrupted with a
:ref:`landingpad <i_landingpad>` instruction.
Example:
""""""""
.. code-block:: llvm
resume { i8*, i32 } %exn
.. _i_catchswitch:
'``catchswitch``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<resultval> = catchswitch within <parent> [ label <handler1>, label <handler2>, ... ] unwind to caller
<resultval> = catchswitch within <parent> [ label <handler1>, label <handler2>, ... ] unwind label <default>
Overview:
"""""""""
The '``catchswitch``' instruction is used by `LLVM's exception handling system
<ExceptionHandling.html#overview>`_ to describe the set of possible catch handlers
that may be executed by the :ref:`EH personality routine <personalityfn>`.
Arguments:
""""""""""
The ``parent`` argument is the token of the funclet that contains the
``catchswitch`` instruction. If the ``catchswitch`` is not inside a funclet,
this operand may be the token ``none``.
The ``default`` argument is the label of another basic block beginning with
either a ``cleanuppad`` or ``catchswitch`` instruction. This unwind destination
must be a legal target with respect to the ``parent`` links, as described in
the `exception handling documentation\ <ExceptionHandling.html#wineh-constraints>`_.
The ``handlers`` are a nonempty list of successor blocks that each begin with a
:ref:`catchpad <i_catchpad>` instruction.
Semantics:
""""""""""
Executing this instruction transfers control to one of the successors in
``handlers``, if appropriate, or continues to unwind via the unwind label if
present.
The ``catchswitch`` is both a terminator and a "pad" instruction, meaning that
it must be both the first non-phi instruction and last instruction in the basic
block. Therefore, it must be the only non-phi instruction in the block.
Example:
""""""""
.. code-block:: text
dispatch1:
%cs1 = catchswitch within none [label %handler0, label %handler1] unwind to caller
dispatch2:
%cs2 = catchswitch within %parenthandler [label %handler0] unwind label %cleanup
.. _i_catchret:
'``catchret``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
catchret from <token> to label <normal>
Overview:
"""""""""
The '``catchret``' instruction is a terminator instruction that has a
single successor.
Arguments:
""""""""""
The first argument to a '``catchret``' indicates which ``catchpad`` it
exits. It must be a :ref:`catchpad <i_catchpad>`.
The second argument to a '``catchret``' specifies where control will
transfer to next.
Semantics:
""""""""""
The '``catchret``' instruction ends an existing (in-flight) exception whose
unwinding was interrupted with a :ref:`catchpad <i_catchpad>` instruction. The
:ref:`personality function <personalityfn>` gets a chance to execute arbitrary
code to, for example, destroy the active exception. Control then transfers to
``normal``.
The ``token`` argument must be a token produced by a ``catchpad`` instruction.
If the specified ``catchpad`` is not the most-recently-entered not-yet-exited
funclet pad (as described in the `EH documentation\ <ExceptionHandling.html#wineh-constraints>`_),
the ``catchret``'s behavior is undefined.
Example:
""""""""
.. code-block:: text
catchret from %catch label %continue
.. _i_cleanupret:
'``cleanupret``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
cleanupret from <value> unwind label <continue>
cleanupret from <value> unwind to caller
Overview:
"""""""""
The '``cleanupret``' instruction is a terminator instruction that has
an optional successor.
Arguments:
""""""""""
The '``cleanupret``' instruction requires one argument, which indicates
which ``cleanuppad`` it exits, and must be a :ref:`cleanuppad <i_cleanuppad>`.
If the specified ``cleanuppad`` is not the most-recently-entered not-yet-exited
funclet pad (as described in the `EH documentation\ <ExceptionHandling.html#wineh-constraints>`_),
the ``cleanupret``'s behavior is undefined.
The '``cleanupret``' instruction also has an optional successor, ``continue``,
which must be the label of another basic block beginning with either a
``cleanuppad`` or ``catchswitch`` instruction. This unwind destination must
be a legal target with respect to the ``parent`` links, as described in the
`exception handling documentation\ <ExceptionHandling.html#wineh-constraints>`_.
Semantics:
""""""""""
The '``cleanupret``' instruction indicates to the
:ref:`personality function <personalityfn>` that one
:ref:`cleanuppad <i_cleanuppad>` it transferred control to has ended.
It transfers control to ``continue`` or unwinds out of the function.
Example:
""""""""
.. code-block:: text
cleanupret from %cleanup unwind to caller
cleanupret from %cleanup unwind label %continue
.. _i_unreachable:
'``unreachable``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
unreachable
Overview:
"""""""""
The '``unreachable``' instruction has no defined semantics. This
instruction is used to inform the optimizer that a particular portion of
the code is not reachable. This can be used to indicate that the code
after a no-return function cannot be reached, and other facts.
Semantics:
""""""""""
The '``unreachable``' instruction has no defined semantics.
.. _binaryops:
Binary Operations
-----------------
Binary operators are used to do most of the computation in a program.
They require two operands of the same type, execute an operation on
them, and produce a single value. The operands might represent multiple
data, as is the case with the :ref:`vector <t_vector>` data type. The
result value has the same type as its operands.
There are several different binary operators:
.. _i_add:
'``add``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = add <ty> <op1>, <op2> ; yields ty:result
<result> = add nuw <ty> <op1>, <op2> ; yields ty:result
<result> = add nsw <ty> <op1>, <op2> ; yields ty:result
<result> = add nuw nsw <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``add``' instruction returns the sum of its two operands.
Arguments:
""""""""""
The two arguments to the '``add``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The value produced is the integer sum of the two operands.
If the sum has unsigned overflow, the result returned is the
mathematical result modulo 2\ :sup:`n`\ , where n is the bit width of
the result.
Because LLVM integers use a two's complement representation, this
instruction is appropriate for both signed and unsigned integers.
``nuw`` and ``nsw`` stand for "No Unsigned Wrap" and "No Signed Wrap",
respectively. If the ``nuw`` and/or ``nsw`` keywords are present, the
result value of the ``add`` is a :ref:`poison value <poisonvalues>` if
unsigned and/or signed overflow, respectively, occurs.
Example:
""""""""
.. code-block:: text
<result> = add i32 4, %var ; yields i32:result = 4 + %var
.. _i_fadd:
'``fadd``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fadd [fast-math flags]* <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``fadd``' instruction returns the sum of its two operands.
Arguments:
""""""""""
The two arguments to the '``fadd``' instruction must be :ref:`floating
point <t_floating>` or :ref:`vector <t_vector>` of floating point values.
Both arguments must have identical types.
Semantics:
""""""""""
The value produced is the floating point sum of the two operands. This
instruction can also take any number of :ref:`fast-math flags <fastmath>`,
which are optimization hints to enable otherwise unsafe floating point
optimizations:
Example:
""""""""
.. code-block:: text
<result> = fadd float 4.0, %var ; yields float:result = 4.0 + %var
'``sub``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = sub <ty> <op1>, <op2> ; yields ty:result
<result> = sub nuw <ty> <op1>, <op2> ; yields ty:result
<result> = sub nsw <ty> <op1>, <op2> ; yields ty:result
<result> = sub nuw nsw <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``sub``' instruction returns the difference of its two operands.
Note that the '``sub``' instruction is used to represent the '``neg``'
instruction present in most other intermediate representations.
Arguments:
""""""""""
The two arguments to the '``sub``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The value produced is the integer difference of the two operands.
If the difference has unsigned overflow, the result returned is the
mathematical result modulo 2\ :sup:`n`\ , where n is the bit width of
the result.
Because LLVM integers use a two's complement representation, this
instruction is appropriate for both signed and unsigned integers.
``nuw`` and ``nsw`` stand for "No Unsigned Wrap" and "No Signed Wrap",
respectively. If the ``nuw`` and/or ``nsw`` keywords are present, the
result value of the ``sub`` is a :ref:`poison value <poisonvalues>` if
unsigned and/or signed overflow, respectively, occurs.
Example:
""""""""
.. code-block:: text
<result> = sub i32 4, %var ; yields i32:result = 4 - %var
<result> = sub i32 0, %val ; yields i32:result = -%var
.. _i_fsub:
'``fsub``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fsub [fast-math flags]* <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``fsub``' instruction returns the difference of its two operands.
Note that the '``fsub``' instruction is used to represent the '``fneg``'
instruction present in most other intermediate representations.
Arguments:
""""""""""
The two arguments to the '``fsub``' instruction must be :ref:`floating
point <t_floating>` or :ref:`vector <t_vector>` of floating point values.
Both arguments must have identical types.
Semantics:
""""""""""
The value produced is the floating point difference of the two operands.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating point optimizations:
Example:
""""""""
.. code-block:: text
<result> = fsub float 4.0, %var ; yields float:result = 4.0 - %var
<result> = fsub float -0.0, %val ; yields float:result = -%var
'``mul``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = mul <ty> <op1>, <op2> ; yields ty:result
<result> = mul nuw <ty> <op1>, <op2> ; yields ty:result
<result> = mul nsw <ty> <op1>, <op2> ; yields ty:result
<result> = mul nuw nsw <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``mul``' instruction returns the product of its two operands.
Arguments:
""""""""""
The two arguments to the '``mul``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The value produced is the integer product of the two operands.
If the result of the multiplication has unsigned overflow, the result
returned is the mathematical result modulo 2\ :sup:`n`\ , where n is the
bit width of the result.
Because LLVM integers use a two's complement representation, and the
result is the same width as the operands, this instruction returns the
correct result for both signed and unsigned integers. If a full product
(e.g. ``i32`` * ``i32`` -> ``i64``) is needed, the operands should be
sign-extended or zero-extended as appropriate to the width of the full
product.
``nuw`` and ``nsw`` stand for "No Unsigned Wrap" and "No Signed Wrap",
respectively. If the ``nuw`` and/or ``nsw`` keywords are present, the
result value of the ``mul`` is a :ref:`poison value <poisonvalues>` if
unsigned and/or signed overflow, respectively, occurs.
Example:
""""""""
.. code-block:: text
<result> = mul i32 4, %var ; yields i32:result = 4 * %var
.. _i_fmul:
'``fmul``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fmul [fast-math flags]* <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``fmul``' instruction returns the product of its two operands.
Arguments:
""""""""""
The two arguments to the '``fmul``' instruction must be :ref:`floating
point <t_floating>` or :ref:`vector <t_vector>` of floating point values.
Both arguments must have identical types.
Semantics:
""""""""""
The value produced is the floating point product of the two operands.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating point optimizations:
Example:
""""""""
.. code-block:: text
<result> = fmul float 4.0, %var ; yields float:result = 4.0 * %var
'``udiv``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = udiv <ty> <op1>, <op2> ; yields ty:result
<result> = udiv exact <ty> <op1>, <op2> ; yields ty:result
Overview:
"""""""""
The '``udiv``' instruction returns the quotient of its two operands.
Arguments:
""""""""""
The two arguments to the '``udiv``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The value produced is the unsigned integer quotient of the two operands.
Note that unsigned integer division and signed integer division are
distinct operations; for signed integer division, use '``sdiv``'.
Division by zero is undefined behavior. For vectors, if any element
of the divisor is zero, the operation has undefined behavior.
If the ``exact`` keyword is present, the result value of the ``udiv`` is
a :ref:`poison value <poisonvalues>` if %op1 is not a multiple of %op2 (as
such, "((a udiv exact b) mul b) == a").
Example:
""""""""
.. code-block:: text
<result> = udiv i32 4, %var ; yields i32:result = 4 / %var
'``sdiv``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax: