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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 which 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.
#. 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 are
included in this numbering. For example, if the entry basic block is not
given a label name, 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]* @.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
!1 = metadata !{i32 42}
!foo = !{!1, null}
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 an 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.
``linker_private``
Similar to ``private``, but the symbol is passed through the
assembler and evaluated by the linker. Unlike normal strong symbols,
they are removed by the linker from the final linked image
(executable or dynamic library).
``linker_private_weak``
Similar to "``linker_private``", but the symbol is weak. Note that
``linker_private_weak`` symbols are subject to coalescing by the
linker. The symbols are removed by the linker from the final linked
image (executable or dynamic library).
``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. 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 are otherwise the same as
``linkonce_odr``. 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.
``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.
The next two types of linkage are targeted for Microsoft Windows
platform only. They are designed to support importing (exporting)
symbols from (to) DLLs (Dynamic Link Libraries).
``dllimport``
"``dllimport``" linkage 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``" linkage 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 linkage exists for defining a dll interface, the
compiler, assembler and linker know it is externally referenced and
must refrain from deleting the symbol.
It is illegal for a function *declaration* to have any linkage type
other than ``external``, ``dllimport`` 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.
"``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).
"``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.
.. _namedtypes:
Named Types
-----------
LLVM IR allows you to specify name aliases for certain types. This can
make it easier to read the IR and make the IR more condensed
(particularly when recursive types are involved). An example of a name
specification is:
.. code-block:: llvm
%mytype = type { %mytype*, i32 }
You may give a name to any :ref:`type <typesystem>` except
":ref:`void <t_void>`". Type name aliases may be used anywhere a type is
expected with the syntax "%mytype".
Note that type names are aliases for the structural type that they
indicate, and that you can therefore specify multiple names for the same
type. This often leads to confusing behavior when dumping out a .ll
file. Since LLVM IR uses structural typing, the name is not part of the
type. When printing out LLVM IR, the printer will pick *one name* to
render all types of a particular shape. This means that if you have code
where two different source types end up having the same LLVM type, that
the dumper will sometimes print the "wrong" or unexpected type. This is
an important design point and isn't going to change.
.. _globalvars:
Global Variables
----------------
Global variables define regions of memory allocated at compilation time
instead of run-time.
Global variables definitions must be initialized, may have an explicit section
to be placed in, and may have an optional explicit alignment specified.
Global variables in other translation units can also be declared, in which
case they don't have an initializer.
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.
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 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.
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.
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.
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:`visibility
style <visibility>`, 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:`garbage
collector name <gc>`, an optional :ref:`prefix <prefixdata>`, 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:`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 list of arguments, an optional alignment, an optional
:ref:`garbage collector name <gc>` and an optional :ref:`prefix <prefixdata>`.
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.
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 know to not
be significant and two identical functions can be merged.
Syntax::
define [linkage] [visibility]
[cconv] [ret attrs]
<ResultType> @<FunctionName> ([argument list])
[fn Attrs] [section "name"] [align N]
[gc] [prefix Constant] { ... }
.. _langref_aliases:
Aliases
-------
Aliases act as "second name" for the aliasee value (which can be either
function, global variable, another alias or bitcast of global value).
Aliases may have an optional :ref:`linkage type <linkage>`, and an optional
:ref:`visibility style <visibility>`.
Syntax::
@<Name> = alias [Linkage] [Visibility] <AliaseeTy> @<Aliasee>
The linkage must be one of ``private``, ``linker_private``,
``linker_private_weak``, ``internal``, ``linkonce``, ``weak``,
``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
might not correctly handle dropping a weak symbol that is aliased by a non-weak
alias.
.. _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.
Syntax::
; Some unnamed metadata nodes, which are referenced by the named metadata.
!0 = metadata !{metadata !"zero"}
!1 = metadata !{metadata !"one"}
!2 = metadata !{metadata !"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 (which is usually 32-bits, but is 8-bits for a i1 on x86-64) 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 during 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``
+
+.. Warning:: This feature is unstable and not fully implemented.
+
+ The ``inalloca`` argument attribute allows the caller to get the
+ address of an outgoing argument to a ``call`` or ``invoke`` before
+ it executes. It is similar to ``byval`` in that it is used to pass
+ arguments by value, but it guarantees that the argument will not be
+ copied.
+
+ To be :ref:`well formed <wellformed>`, the caller must pass in an
+ alloca value into an ``inalloca`` parameter, and an alloca may be
+ used as an ``inalloca`` argument at most once. The attribute can
+ only be applied to parameters that would be passed in memory and not
+ registers. The ``inalloca`` attribute cannot be used in conjunction
+ with other attributes that affect argument storage, like ``inreg``,
+ ``nest``, ``sret``, or ``byval``. The ``inalloca`` stack space is
+ considered to be clobbered by any call that uses it, so any
+ ``inalloca`` parameters cannot be marked ``readonly``.
+
+ Allocas passed with ``inalloca`` to a call must be in the opposite
+ order of the parameter list, meaning that the rightmost argument
+ must be allocated first. If a call has inalloca arguments, no other
+ allocas can occur between the first alloca used by the call and the
+ call site, unless they are are cleared by calls to
+ :ref:`llvm.stackrestore <int_stackrestore>`. Violating these rules
+ results in undefined behavior at runtime.
+
+ 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 may only be applied to
the first parameter. This is not a valid attribute for return
values.
``noalias``
This indicates that pointer values :ref:`based <pointeraliasing>` on
the argument or return value do not alias pointer values which are
not *based* on it, ignoring certain "irrelevant" dependencies. For a
call to the parent function, dependencies between memory references
from before or after the call and from those during the call are
"irrelevant" to the ``noalias`` keyword for the arguments and return
value used in that call. 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 `alias
analysis <AliasAnalysis.html#MustMayNo>`_.
Note that this definition of ``noalias`` is intentionally similar
to the definition of ``restrict`` in C99 for function arguments,
though it is slightly weaker.
For function return values, C99's ``restrict`` is not meaningful,
while LLVM's ``noalias`` is.
``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.
.. _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 an optimization hint to the code generator when generating
the caller, allowing 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.
.. _gc:
Garbage Collector Names
-----------------------
Each function may specify a garbage collector name, which is simply a
string:
.. code-block:: llvm
define void @f() gc "name" { ... }
The compiler declares the supported values of *name*. Specifying a
collector which will cause the compiler to alter its output in order to
support the named garbage collection algorithm.
.. _prefixdata:
Prefix Data
-----------
Prefix data is data associated with a function which the code generator
will emit immediately before the function body. 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. This implies that the IR symbol points to the start
of the prefix data.
To maintain the semantics of ordinary function calls, the prefix 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 prefix 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 prefix data. Obviously this
makes the format of the prefix data highly target dependent.
Prefix data is laid out as if it were an initializer for a global variable
of the prefix data's type. No padding is automatically placed between the
prefix data and the function body. If padding is required, it must be part
of the prefix data.
A trivial example of valid prefix data for the x86 architecture is ``i8 144``,
which encodes the ``nop`` instruction:
.. code-block:: llvm
define void @f() prefix i8 144 { ... }
Generally prefix data can be formed by encoding a relative branch instruction
which skips the metadata, as in this example of valid prefix data for the
x86_64 architecture, where the first two bytes encode ``jmp .+10``:
.. code-block:: llvm
%0 = type <{ i8, i8, i8* }>
define void @f() prefix %0 <{ i8 235, i8 8, i8* @md}> { ... }
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.
.. _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.
``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 which 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.
``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.
``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.
``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.
``nounwind``
This function attribute indicates that the function never returns
with an unwind or exceptional control flow. If the function does
unwind, its runtime behavior is undefined.
``optnone``
This function attribute indicates that the function is not optimized
by any optimization or code generator passes with the
exception of interprocedural optimization passes.
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.
``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 that it cannot unwind exceptions by calling
the ``C++`` exception throwing methods.
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. It cannot
unwind an exception by calling the ``C++`` exception throwing
methods.
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.
``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.
``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.
``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``.
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.
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.
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.
``uwtable``
This attribute indicates that the ABI being targeted requires that
an unwind table entry be produce 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.
.. _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.
The inline asm code is simply printed to the machine code .s file when
assembly code is generated.
.. _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.
``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. Specifying the ``<pref>`` alignment is optional. If omitted, the
preceding ``:`` should be omitted too. 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<size>:<abi>:<pref>``
This specifies the alignment for an aggregate type of a given bit
``<size>``.
``s<size>:<abi>:<pref>``
This specifies the alignment for a stack object of a given bit
``<size>``.
``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.
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. If you would like to generate IR
that does not embed this target-specific detail into the IR, then you
don't have to specify the string. This will disable some optimizations
that require precise layout information, but this also prevents those
optimizations from introducing target specificity into the IR.
.. _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 ``getelementptr`` operation is *based*
on the first 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 transformation
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 which 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
an ordering parameter that determines 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 which only reads, ``release`` for an operation which 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.
.. _singlethread:
If an atomic operation is marked ``singlethread``, it only *synchronizes
with* or participates in modification and seq\_cst total orderings with
other operations running in the same thread (for example, in signal
handlers).
.. _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>`) have the following flags that can set to enable
otherwise unsafe floating point operations
``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.
``fast``
Fast - Allow algebraically equivalent transformations that may
dramatically change results in floating point (e.g. reassociate). This
flag implies all the others.
.. _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)
.. _t_x86mmx:
X86mmx Type
"""""""""""
:Overview:
The x86mmx 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:
::
x86mmx
.. _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 or floating point type, or a pointer to
these types. 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_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
=========
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>`.
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 x86mmx.
.. _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.
**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 structure-like constant with :ref:`metadata
type <t_metadata>`. For example:
"``metadata !{ i32 0, metadata !"test" }``". Unlike other
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
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:: llvm
%A = xor undef, undef
%B = undef
%C = xor %B, %B
%D = undef
%E = icmp lt %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:: llvm
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
which cannot evoke side effects has nevertheless detected a condition
which 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 affect that any instruction which 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* @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* @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* %narrowaddr ; Returns a poison value.
%poison4 = load 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)``
Truncate a constant to another type. The bit size of CST must be
larger than the bit size of TYPE. Both types must be integers.
``zext (CST to TYPE)``
Zero extend a constant to another type. The bit size of CST must be
smaller than the bit size of TYPE. Both types must be integers.
``sext (CST to TYPE)``
Sign extend a constant to another type. The bit size of CST must be
smaller than the bit size of TYPE. Both types must be integers.
``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)``
Convert a pointer typed constant to the corresponding integer
constant. ``TYPE`` must be an integer type. ``CST`` must be of
pointer type. The ``CST`` value is zero extended, truncated, or
unchanged to make it fit in ``TYPE``.
``inttoptr (CST to TYPE)``
Convert an integer constant to a pointer constant. TYPE must be a
pointer type. CST must be of integer type. The CST value is zero
extended, truncated, or unchanged to make it fit in a pointer size.
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 (CSTPTR, IDX0, IDX1, ...)``, ``getelementptr inbounds (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 zero or more indexes, which are
required to make sense for the type of "CSTPTR".
``select (COND, VAL1, VAL2)``
Perform the :ref:`select operation <i_select>` on constants.
``icmp COND (VAL1, VAL2)``
Performs the :ref:`icmp operation <i_icmp>` on constants.
``fcmp COND (VAL1, VAL2)``
Performs 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 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. 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 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 Nodes and Metadata Strings
-----------------------------------
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.
All metadata has the ``metadata`` type and is identified in syntax by a
preceding exclamation point ('``!``').
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
!{ metadata !"test\00", i32 10}
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 = metadata !{!4, !3}
Metadata can be used as function arguments. Here ``llvm.dbg.value``
function is using two metadata arguments:
.. code-block:: llvm
call void @llvm.dbg.value(metadata !24, i64 0, metadata !25)
Metadata can be attached with 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
More information about specific metadata nodes recognized by the
optimizers and code generator is found below.
'``tbaa``' Metadata
^^^^^^^^^^^^^^^^^^^
In LLVM IR, memory does not have types, so LLVM's own type system is not
suitable for doing TBAA. Instead, metadata is added to the IR to
describe a type system of a higher level language. This can be used to
implement typical C/C++ TBAA, but it can also be used to implement
custom alias analysis behavior for other languages.
The current metadata format is very simple. TBAA metadata nodes have up
to three fields, e.g.:
.. code-block:: llvm
!0 = metadata !{ metadata !"an example type tree" }
!1 = metadata !{ metadata !"int", metadata !0 }
!2 = metadata !{ metadata !"float", metadata !0 }
!3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
The first field is an identity field. It can be any value, usually a
metadata string, which uniquely identifies the type. The most important
name in the tree is the name of the root node. Two trees with different
root node names are entirely disjoint, even if they have leaves with
common names.
The second field identifies the type's parent node in the tree, or is
null or omitted for a root node. A type is considered to alias all of
its descendants and all of its ancestors in the tree. Also, a type is
considered to alias all types in other trees, so that bitcode produced
from multiple front-ends is handled conservatively.
If the third field is present, it's an integer which if equal to 1
indicates that the type is "constant" (meaning
``pointsToConstantMemory`` should return true; see `other useful
AliasAnalysis methods <AliasAnalysis.html#OtherItfs>`_).
'``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 = metadata !{ i64 0, i64 4, metadata !1, i64 8, i64 4, metadata !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.
'``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 floating point
number representing the maximum relative error, for example:
.. code-block:: llvm
!0 = metadata !{ float 2.5 } ; maximum acceptable inaccuracy is 2.5 ULPs
'``range``' Metadata
^^^^^^^^^^^^^^^^^^^^
``range`` metadata may be attached only to loads of integer types. It
expresses the possible ranges the loaded value is in. The ranges are
represented with a flattened list of integers. The loaded value 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* %x, align 1, !range !0 ; Can only be 0 or 1
%b = load i8* %y, align 1, !range !1 ; Can only be 255 (-1), 0 or 1
%c = load i8* %z, align 1, !range !2 ; Can only be 0, 1, 3, 4 or 5
%d = load i8* %z, align 1, !range !3 ; Can only be -2, -1, 3, 4 or 5
...
!0 = metadata !{ i8 0, i8 2 }
!1 = metadata !{ i8 255, i8 2 }
!2 = metadata !{ i8 0, i8 2, i8 3, i8 6 }
!3 = metadata !{ i8 -2, i8 0, i8 3, i8 6 }
'``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 = metadata !{ metadata !0 }
!1 = metadata !{ metadata !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.vectorizer.unroll`` metadata is understood
by the loop vectorizer to indicate how many times to unroll the loop:
.. code-block:: llvm
br i1 %exitcond, label %._crit_edge, label %.lr.ph, !llvm.loop !0
...
!0 = metadata !{ metadata !0, metadata !1 }
!1 = metadata !{ metadata !"llvm.vectorizer.unroll", i32 2 }
'``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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
For a loop to be parallel, in addition to using
the ``llvm.loop`` metadata to mark the loop latch branch instruction,
also all of the memory accessing instructions in the loop body need to be
marked with the ``llvm.mem.parallel_loop_access`` metadata. If there
is at least one memory accessing instruction not marked with the metadata,
the loop must be considered a sequential loop. This causes parallel loops to be
converted to sequential loops due to optimization passes that are unaware of
the parallel semantics and that insert new memory instructions to 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:
...
%0 = load i32* %arrayidx, align 4, !llvm.mem.parallel_loop_access !0
...
store i32 %0, i32* %arrayidx4, align 4, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !0
for.end:
...
!0 = metadata !{ metadata !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:
...
inner.for.body:
...
%0 = load i32* %arrayidx, align 4, !llvm.mem.parallel_loop_access !0
...
store i32 %0, i32* %arrayidx4, align 4, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %inner.for.end, label %inner.for.body, !llvm.loop !1
inner.for.end:
...
%0 = load i32* %arrayidx, align 4, !llvm.mem.parallel_loop_access !0
...
store i32 %0, i32* %arrayidx4, align 4, !llvm.mem.parallel_loop_access !0
...
br i1 %exitcond, label %outer.for.end, label %outer.for.body, !llvm.loop !2
outer.for.end: ; preds = %for.body
...
!0 = metadata !{ metadata !1, metadata !2 } ; a list of loop identifiers
!1 = metadata !{ metadata !1 } ; an identifier for the inner loop
!2 = metadata !{ metadata !2 } ; an identifier for the outer loop
'``llvm.vectorizer``'
^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.vectorizer`` is used to control per-loop
vectorization parameters such as vectorization factor and unroll factor.
``llvm.vectorizer`` metadata should be used in conjunction with ``llvm.loop``
loop identification metadata.
'``llvm.vectorizer.unroll``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata instructs the loop vectorizer to unroll the specified
loop exactly ``N`` times.
The first operand is the string ``llvm.vectorizer.unroll`` and the second
operand is an integer specifying the unroll factor. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.vectorizer.unroll", i32 4 }
Note that setting ``llvm.vectorizer.unroll`` to 1 disables unrolling of the
loop.
If ``llvm.vectorizer.unroll`` is set to 0 then the amount of unrolling will be
determined automatically.
'``llvm.vectorizer.width``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata sets the target width of the vectorizer to ``N``. Without
this metadata, the vectorizer will choose a width automatically.
Regardless of this metadata, the vectorizer will only vectorize loops if
it believes it is valid to do so.
The first operand is the string ``llvm.vectorizer.width`` and the second
operand is an integer specifying the width. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.vectorizer.width", i32 4 }
Note that setting ``llvm.vectorizer.width`` to 1 disables vectorization of the
loop.
If ``llvm.vectorizer.width`` is set to 0 then the width will be determined
automatically.
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.
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 = metadata !{ i32 1, metadata !"foo", i32 1 }
!1 = metadata !{ i32 4, metadata !"bar", i32 37 }
!2 = metadata !{ i32 2, metadata !"qux", i32 42 }
!3 = metadata !{ i32 3, metadata !"qux",
metadata !{
metadata !"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:
::
metadata !{ metadata !"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.
Automatic Linker Flags Module Flags 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 metadata in the module flags section,
using the ``Linker Options`` key. The merge behavior for this flag is required
to be ``AppendUnique``, and the value for the key 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 = metadata !{ i32 6, metadata !"Linker Options",
metadata !{
metadata !{ metadata !"-lz" },
metadata !{ metadata !"-framework", metadata !"Cocoa" } } }
!llvm.module.flags = !{ !0 }
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 ()* }
@llvm.global_ctors = appending global [1 x %0] [%0 { i32 65535, void ()* @ctor }]
The ``@llvm.global_ctors`` array contains a list of constructor
functions and associated priorities. 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.
.. _llvmglobaldtors:
The '``llvm.global_dtors``' Global Variable
-------------------------------------------
.. code-block:: llvm
%0 = type { i32, void ()* }
@llvm.global_dtors = appending global [1 x %0] [%0 { i32 65535, void ()* @dtor }]
The ``@llvm.global_dtors`` array contains a list of destructor functions
and associated priorities. The functions referenced by this array will
be called in descending order of priority (i.e. highest first) when the
module is loaded. The order of functions with the same priority is not
defined.
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>`', 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] <ptr to function ty> <function ptr val>(<function args>) [fn attrs]
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.
#. '``ptr to function ty``': shall be the signature of the pointer to
function value being invoked. In most cases, this is a direct
function invocation, but indirect ``invoke``'s are just as possible,
branching off an arbitrary pointer to function value.
#. '``function ptr val``': An LLVM value containing a pointer to a
function to be invoked.
#. '``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. Only
'``noreturn``', '``nounwind``', '``readonly``' and '``readnone``'
attributes are valid here.
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_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:: llvm
<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:: llvm
<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:: llvm
<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:: llvm
<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:: llvm
<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:: llvm
<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 leads to 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:: llvm
<result> = udiv i32 4, %var ; yields {i32}:result = 4 / %var
'``sdiv``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = sdiv <ty> <op1>, <op2> ; yields {ty}:result
<result> = sdiv exact <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``sdiv``' instruction returns the quotient of its two operands.
Arguments:
""""""""""
The two arguments to the '``sdiv``' 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 signed integer quotient of the two operands
rounded towards zero.
Note that signed integer division and unsigned integer division are
distinct operations; for unsigned integer division, use '``udiv``'.
Division by zero leads to undefined behavior. Overflow also leads to
undefined behavior; this is a rare case, but can occur, for example, by
doing a 32-bit division of -2147483648 by -1.
If the ``exact`` keyword is present, the result value of the ``sdiv`` is
a :ref:`poison value <poisonvalues>` if the result would be rounded.
Example:
""""""""
.. code-block:: llvm
<result> = sdiv i32 4, %var ; yields {i32}:result = 4 / %var
.. _i_fdiv:
'``fdiv``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fdiv [fast-math flags]* <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``fdiv``' instruction returns the quotient of its two operands.
Arguments:
""""""""""
The two arguments to the '``fdiv``' 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 quotient 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:: llvm
<result> = fdiv float 4.0, %var ; yields {float}:result = 4.0 / %var
'``urem``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = urem <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``urem``' instruction returns the remainder from the unsigned
division of its two arguments.
Arguments:
""""""""""
The two arguments to the '``urem``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
This instruction returns the unsigned integer *remainder* of a division.
This instruction always performs an unsigned division to get the
remainder.
Note that unsigned integer remainder and signed integer remainder are
distinct operations; for signed integer remainder, use '``srem``'.
Taking the remainder of a division by zero leads to undefined behavior.
Example:
""""""""
.. code-block:: llvm
<result> = urem i32 4, %var ; yields {i32}:result = 4 % %var
'``srem``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = srem <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``srem``' instruction returns the remainder from the signed
division of its two operands. This instruction can also take
:ref:`vector <t_vector>` versions of the values in which case the elements
must be integers.
Arguments:
""""""""""
The two arguments to the '``srem``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
This instruction returns the *remainder* of a division (where the result
is either zero or has the same sign as the dividend, ``op1``), not the
*modulo* operator (where the result is either zero or has the same sign
as the divisor, ``op2``) of a value. For more information about the
difference, see `The Math
Forum <http://mathforum.org/dr.math/problems/anne.4.28.99.html>`_. For a
table of how this is implemented in various languages, please see
`Wikipedia: modulo
operation <http://en.wikipedia.org/wiki/Modulo_operation>`_.
Note that signed integer remainder and unsigned integer remainder are
distinct operations; for unsigned integer remainder, use '``urem``'.
Taking the remainder of a division by zero leads to undefined behavior.
Overflow also leads to undefined behavior; this is a rare case, but can
occur, for example, by taking the remainder of a 32-bit division of
-2147483648 by -1. (The remainder doesn't actually overflow, but this
rule lets srem be implemented using instructions that return both the
result of the division and the remainder.)
Example:
""""""""
.. code-block:: llvm
<result> = srem i32 4, %var ; yields {i32}:result = 4 % %var
.. _i_frem:
'``frem``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = frem [fast-math flags]* <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``frem``' instruction returns the remainder from the division of
its two operands.
Arguments:
""""""""""
The two arguments to the '``frem``' instruction must be :ref:`floating
point <t_floating>` or :ref:`vector <t_vector>` of floating point values.
Both arguments must have identical types.
Semantics:
""""""""""
This instruction returns the *remainder* of a division. The remainder
has the same sign as the dividend. 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:: llvm
<result> = frem float 4.0, %var ; yields {float}:result = 4.0 % %var
.. _bitwiseops:
Bitwise Binary Operations
-------------------------
Bitwise binary operators are used to do various forms of bit-twiddling
in a program. They are generally very efficient instructions and can
commonly be strength reduced from other instructions. They require two
operands of the same type, execute an operation on them, and produce a
single value. The resulting value is the same type as its operands.
'``shl``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = shl <ty> <op1>, <op2> ; yields {ty}:result
<result> = shl nuw <ty> <op1>, <op2> ; yields {ty}:result
<result> = shl nsw <ty> <op1>, <op2> ; yields {ty}:result
<result> = shl nuw nsw <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``shl``' instruction returns the first operand shifted to the left
a specified number of bits.
Arguments:
""""""""""
Both arguments to the '``shl``' instruction must be the same
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer type.
'``op2``' is treated as an unsigned value.
Semantics:
""""""""""
The value produced is ``op1`` \* 2\ :sup:`op2` mod 2\ :sup:`n`,
where ``n`` is the width of the result. If ``op2`` is (statically or
dynamically) negative or equal to or larger than the number of bits in
``op1``, the result is undefined. If the arguments are vectors, each
vector element of ``op1`` is shifted by the corresponding shift amount
in ``op2``.
If the ``nuw`` keyword is present, then the shift produces a :ref:`poison
value <poisonvalues>` if it shifts out any non-zero bits. If the
``nsw`` keyword is present, then the shift produces a :ref:`poison
value <poisonvalues>` if it shifts out any bits that disagree with the
resultant sign bit. As such, NUW/NSW have the same semantics as they
would if the shift were expressed as a mul instruction with the same
nsw/nuw bits in (mul %op1, (shl 1, %op2)).
Example:
""""""""
.. code-block:: llvm
<result> = shl i32 4, %var ; yields {i32}: 4 << %var
<result> = shl i32 4, 2 ; yields {i32}: 16
<result> = shl i32 1, 10 ; yields {i32}: 1024
<result> = shl i32 1, 32 ; undefined
<result> = shl <2 x i32> < i32 1, i32 1>, < i32 1, i32 2> ; yields: result=<2 x i32> < i32 2, i32 4>
'``lshr``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = lshr <ty> <op1>, <op2> ; yields {ty}:result
<result> = lshr exact <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``lshr``' instruction (logical shift right) returns the first
operand shifted to the right a specified number of bits with zero fill.
Arguments:
""""""""""
Both arguments to the '``lshr``' instruction must be the same
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer type.
'``op2``' is treated as an unsigned value.
Semantics:
""""""""""
This instruction always performs a logical shift right operation. The
most significant bits of the result will be filled with zero bits after
the shift. If ``op2`` is (statically or dynamically) equal to or larger
than the number of bits in ``op1``, the result is undefined. If the
arguments are vectors, each vector element of ``op1`` is shifted by the
corresponding shift amount in ``op2``.
If the ``exact`` keyword is present, the result value of the ``lshr`` is
a :ref:`poison value <poisonvalues>` if any of the bits shifted out are
non-zero.
Example:
""""""""
.. code-block:: llvm
<result> = lshr i32 4, 1 ; yields {i32}:result = 2
<result> = lshr i32 4, 2 ; yields {i32}:result = 1
<result> = lshr i8 4, 3 ; yields {i8}:result = 0
<result> = lshr i8 -2, 1 ; yields {i8}:result = 0x7F
<result> = lshr i32 1, 32 ; undefined
<result> = lshr <2 x i32> < i32 -2, i32 4>, < i32 1, i32 2> ; yields: result=<2 x i32> < i32 0x7FFFFFFF, i32 1>
'``ashr``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = ashr <ty> <op1>, <op2> ; yields {ty}:result
<result> = ashr exact <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``ashr``' instruction (arithmetic shift right) returns the first
operand shifted to the right a specified number of bits with sign
extension.
Arguments:
""""""""""
Both arguments to the '``ashr``' instruction must be the same
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer type.
'``op2``' is treated as an unsigned value.
Semantics:
""""""""""
This instruction always performs an arithmetic shift right operation,
The most significant bits of the result will be filled with the sign bit
of ``op1``. If ``op2`` is (statically or dynamically) equal to or larger
than the number of bits in ``op1``, the result is undefined. If the
arguments are vectors, each vector element of ``op1`` is shifted by the
corresponding shift amount in ``op2``.
If the ``exact`` keyword is present, the result value of the ``ashr`` is
a :ref:`poison value <poisonvalues>` if any of the bits shifted out are
non-zero.
Example:
""""""""
.. code-block:: llvm
<result> = ashr i32 4, 1 ; yields {i32}:result = 2
<result> = ashr i32 4, 2 ; yields {i32}:result = 1
<result> = ashr i8 4, 3 ; yields {i8}:result = 0
<result> = ashr i8 -2, 1 ; yields {i8}:result = -1
<result> = ashr i32 1, 32 ; undefined
<result> = ashr <2 x i32> < i32 -2, i32 4>, < i32 1, i32 3> ; yields: result=<2 x i32> < i32 -1, i32 0>
'``and``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = and <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``and``' instruction returns the bitwise logical and of its two
operands.
Arguments:
""""""""""
The two arguments to the '``and``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The truth table used for the '``and``' instruction is:
+-----+-----+-----+
| In0 | In1 | Out |
+-----+-----+-----+
| 0 | 0 | 0 |
+-----+-----+-----+
| 0 | 1 | 0 |
+-----+-----+-----+
| 1 | 0 | 0 |
+-----+-----+-----+
| 1 | 1 | 1 |
+-----+-----+-----+
Example:
""""""""
.. code-block:: llvm
<result> = and i32 4, %var ; yields {i32}:result = 4 & %var
<result> = and i32 15, 40 ; yields {i32}:result = 8
<result> = and i32 4, 8 ; yields {i32}:result = 0
'``or``' Instruction
^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = or <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``or``' instruction returns the bitwise logical inclusive or of its
two operands.
Arguments:
""""""""""
The two arguments to the '``or``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The truth table used for the '``or``' instruction is:
+-----+-----+-----+
| In0 | In1 | Out |
+-----+-----+-----+
| 0 | 0 | 0 |
+-----+-----+-----+
| 0 | 1 | 1 |
+-----+-----+-----+
| 1 | 0 | 1 |
+-----+-----+-----+
| 1 | 1 | 1 |
+-----+-----+-----+
Example:
""""""""
::
<result> = or i32 4, %var ; yields {i32}:result = 4 | %var
<result> = or i32 15, 40 ; yields {i32}:result = 47
<result> = or i32 4, 8 ; yields {i32}:result = 12
'``xor``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = xor <ty> <op1>, <op2> ; yields {ty}:result
Overview:
"""""""""
The '``xor``' instruction returns the bitwise logical exclusive or of
its two operands. The ``xor`` is used to implement the "one's
complement" operation, which is the "~" operator in C.
Arguments:
""""""""""
The two arguments to the '``xor``' instruction must be
:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. Both
arguments must have identical types.
Semantics:
""""""""""
The truth table used for the '``xor``' instruction is:
+-----+-----+-----+
| In0 | In1 | Out |
+-----+-----+-----+
| 0 | 0 | 0 |
+-----+-----+-----+
| 0 | 1 | 1 |
+-----+-----+-----+
| 1 | 0 | 1 |
+-----+-----+-----+
| 1 | 1 | 0 |
+-----+-----+-----+
Example:
""""""""
.. code-block:: llvm
<result> = xor i32 4, %var ; yields {i32}:result = 4 ^ %var
<result> = xor i32 15, 40 ; yields {i32}:result = 39
<result> = xor i32 4, 8 ; yields {i32}:result = 12
<result> = xor i32 %V, -1 ; yields {i32}:result = ~%V
Vector Operations
-----------------
LLVM supports several instructions to represent vector operations in a
target-independent manner. These instructions cover the element-access
and vector-specific operations needed to process vectors effectively.
While LLVM does directly support these vector operations, many
sophisticated algorithms will want to use target-specific intrinsics to
take full advantage of a specific target.
.. _i_extractelement:
'``extractelement``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = extractelement <n x <ty>> <val>, i32 <idx> ; yields <ty>
Overview:
"""""""""
The '``extractelement``' instruction extracts a single scalar element
from a vector at a specified index.
Arguments:
""""""""""
The first operand of an '``extractelement``' instruction is a value of
:ref:`vector <t_vector>` type. The second operand is an index indicating
the position from which to extract the element. The index may be a
variable.
Semantics:
""""""""""
The result is a scalar of the same type as the element type of ``val``.
Its value is the value at position ``idx`` of ``val``. If ``idx``
exceeds the length of ``val``, the results are undefined.
Example:
""""""""
.. code-block:: llvm
<result> = extractelement <4 x i32> %vec, i32 0 ; yields i32
.. _i_insertelement:
'``insertelement``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = insertelement <n x <ty>> <val>, <ty> <elt>, i32 <idx> ; yields <n x <ty>>
Overview:
"""""""""
The '``insertelement``' instruction inserts a scalar element into a
vector at a specified index.
Arguments:
""""""""""
The first operand of an '``insertelement``' instruction is a value of
:ref:`vector <t_vector>` type. The second operand is a scalar value whose
type must equal the element type of the first operand. The third operand
is an index indicating the position at which to insert the value. The
index may be a variable.
Semantics:
""""""""""
The result is a vector of the same type as ``val``. Its element values
are those of ``val`` except at position ``idx``, where it gets the value
``elt``. If ``idx`` exceeds the length of ``val``, the results are
undefined.
Example:
""""""""
.. code-block:: llvm
<result> = insertelement <4 x i32> %vec, i32 1, i32 0 ; yields <4 x i32>
.. _i_shufflevector:
'``shufflevector``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = shufflevector <n x <ty>> <v1>, <n x <ty>> <v2>, <m x i32> <mask> ; yields <m x <ty>>
Overview:
"""""""""
The '``shufflevector``' instruction constructs a permutation of elements
from two input vectors, returning a vector with the same element type as
the input and length that is the same as the shuffle mask.
Arguments:
""""""""""
The first two operands of a '``shufflevector``' instruction are vectors
with the same type. The third argument is a shuffle mask whose element
type is always 'i32'. The result of the instruction is a vector whose
length is the same as the shuffle mask and whose element type is the
same as the element type of the first two operands.
The shuffle mask operand is required to be a constant vector with either
constant integer or undef values.
Semantics:
""""""""""
The elements of the two input vectors are numbered from left to right
across both of the vectors. The shuffle mask operand specifies, for each
element of the result vector, which element of the two input vectors the
result element gets. The element selector may be undef (meaning "don't
care") and the second operand may be undef if performing a shuffle from
only one vector.
Example:
""""""""
.. code-block:: llvm
<result> = shufflevector <4 x i32> %v1, <4 x i32> %v2,
<4 x i32> <i32 0, i32 4, i32 1, i32 5> ; yields <4 x i32>
<result> = shufflevector <4 x i32> %v1, <4 x i32> undef,
<4 x i32> <i32 0, i32 1, i32 2, i32 3> ; yields <4 x i32> - Identity shuffle.
<result> = shufflevector <8 x i32> %v1, <8 x i32> undef,
<4 x i32> <i32 0, i32 1, i32 2, i32 3> ; yields <4 x i32>
<result> = shufflevector <4 x i32> %v1, <4 x i32> %v2,
<8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7 > ; yields <8 x i32>
Aggregate Operations
--------------------
LLVM supports several instructions for working with
:ref:`aggregate <t_aggregate>` values.
.. _i_extractvalue:
'``extractvalue``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = extractvalue <aggregate type> <val>, <idx>{, <idx>}*
Overview:
"""""""""
The '``extractvalue``' instruction extracts the value of a member field
from an :ref:`aggregate <t_aggregate>` value.
Arguments:
""""""""""
The first operand of an '``extractvalue``' instruction is a value of
:ref:`struct <t_struct>` or :ref:`array <t_array>` type. The operands are
constant indices to specify which value to extract in a similar manner
as indices in a '``getelementptr``' instruction.
The major differences to ``getelementptr`` indexing are:
- Since the value being indexed is not a pointer, the first index is
omitted and assumed to be zero.
- At least one index must be specified.
- Not only struct indices but also array indices must be in bounds.
Semantics:
""""""""""
The result is the value at the position in the aggregate specified by
the index operands.
Example:
""""""""
.. code-block:: llvm
<result> = extractvalue {i32, float} %agg, 0 ; yields i32
.. _i_insertvalue:
'``insertvalue``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = insertvalue <aggregate type> <val>, <ty> <elt>, <idx>{, <idx>}* ; yields <aggregate type>
Overview:
"""""""""
The '``insertvalue``' instruction inserts a value into a member field in
an :ref:`aggregate <t_aggregate>` value.
Arguments:
""""""""""
The first operand of an '``insertvalue``' instruction is a value of
:ref:`struct <t_struct>` or :ref:`array <t_array>` type. The second operand is
a first-class value to insert. The following operands are constant
indices indicating the position at which to insert the value in a
similar manner as indices in a '``extractvalue``' instruction. The value
to insert must have the same type as the value identified by the
indices.
Semantics:
""""""""""
The result is an aggregate of the same type as ``val``. Its value is
that of ``val`` except that the value at the position specified by the
indices is that of ``elt``.
Example:
""""""""
.. code-block:: llvm
%agg1 = insertvalue {i32, float} undef, i32 1, 0 ; yields {i32 1, float undef}
%agg2 = insertvalue {i32, float} %agg1, float %val, 1 ; yields {i32 1, float %val}
%agg3 = insertvalue {i32, {float}} %agg1, float %val, 1, 0 ; yields {i32 1, float %val}
.. _memoryops:
Memory Access and Addressing Operations
---------------------------------------
A key design point of an SSA-based representation is how it represents
memory. In LLVM, no memory locations are in SSA form, which makes things
very simple. This section describes how to read, write, and allocate
memory in LLVM.
.. _i_alloca:
'``alloca``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = alloca <type>[, <ty> <NumElements>][, align <alignment>] ; yields {type*}:result
Overview:
"""""""""
The '``alloca``' instruction allocates memory on the stack frame of the
currently executing function, to be automatically released when this
function returns to its caller. The object is always allocated in the
generic address space (address space zero).
Arguments:
""""""""""
The '``alloca``' instruction allocates ``sizeof(<type>)*NumElements``
bytes of memory on the runtime stack, returning a pointer of the
appropriate type to the program. If "NumElements" is specified, it is
the number of elements allocated, otherwise "NumElements" is defaulted
to be one. If a constant alignment is specified, the value result of the
allocation is guaranteed to be aligned to at least that boundary. If not
specified, or if zero, the target can choose to align the allocation on
any convenient boundary compatible with the type.
'``type``' may be any sized type.
Semantics:
""""""""""
Memory is allocated; a pointer is returned. The operation is undefined
if there is insufficient stack space for the allocation. '``alloca``'d
memory is automatically released when the function returns. The
'``alloca``' instruction is commonly used to represent automatic
variables that must have an address available. When the function returns
(either with the ``ret`` or ``resume`` instructions), the memory is
reclaimed. Allocating zero bytes is legal, but the result is undefined.
The order in which memory is allocated (ie., which way the stack grows)
is not specified.
Example:
""""""""
.. code-block:: llvm
%ptr = alloca i32 ; yields {i32*}:ptr
%ptr = alloca i32, i32 4 ; yields {i32*}:ptr
%ptr = alloca i32, i32 4, align 1024 ; yields {i32*}:ptr
%ptr = alloca i32, align 1024 ; yields {i32*}:ptr
.. _i_load:
'``load``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = load [volatile] <ty>* <pointer>[, align <alignment>][, !nontemporal !<index>][, !invariant.load !<index>]
<result> = load atomic [volatile] <ty>* <pointer> [singlethread] <ordering>, align <alignment>
!<index> = !{ i32 1 }
Overview:
"""""""""
The '``load``' instruction is used to read from memory.
Arguments:
""""""""""
The argument to the ``load`` instruction specifies the memory address
from which to load. The pointer must point to a :ref:`first
class <t_firstclass>` type. If the ``load`` is marked as ``volatile``,
then the optimizer is not allowed to modify the number or order of
execution of this ``load`` with other :ref:`volatile
operations <volatile>`.
If the ``load`` is marked as ``atomic``, it takes an extra
:ref:`ordering <ordering>` and optional ``singlethread`` argument. The
``release`` and ``acq_rel`` orderings are not valid on ``load``
instructions. Atomic loads produce :ref:`defined <memmodel>` results
when they may see multiple atomic stores. The type of the pointee must
be an integer type whose bit width is a power of two greater than or
equal to eight and less than or equal to a target-specific size limit.
``align`` must be explicitly specified on atomic loads, and the load has
undefined behavior if the alignment is not set to a value which is at
least the size in bytes of the pointee. ``!nontemporal`` does not have
any defined semantics for atomic loads.
The optional constant ``align`` argument specifies the alignment of the
operation (that is, the alignment of the memory address). A value of 0
or an omitted ``align`` argument means that the operation has the ABI
alignment for the target. It is the responsibility of the code emitter
to ensure that the alignment information is correct. Overestimating the
alignment results in undefined behavior. Underestimating the alignment
may produce less efficient code. An alignment of 1 is always safe.
The optional ``!nontemporal`` metadata must reference a single
metadata name ``<index>`` corresponding to a metadata node with one
``i32`` entry of value 1. The existence of the ``!nontemporal``
metadata on the instruction tells the optimizer and code generator
that this load is not expected to be reused in the cache. The code
generator may select special instructions to save cache bandwidth, such
as the ``MOVNT`` instruction on x86.
The optional ``!invariant.load`` metadata must reference a single
metadata name ``<index>`` corresponding to a metadata node with no
entries. The existence of the ``!invariant.load`` metadata on the
instruction tells the optimizer and code generator that this load
address points to memory which does not change value during program
execution. The optimizer may then move this load around, for example, by
hoisting it out of loops using loop invariant code motion.
Semantics:
""""""""""
The location of memory pointed to is loaded. If the value being loaded
is of scalar type then the number of bytes read does not exceed the
minimum number of bytes needed to hold all bits of the type. For
example, loading an ``i24`` reads at most three bytes. When loading a
value of a type like ``i20`` with a size that is not an integral number
of bytes, the result is undefined if the value was not originally
written using a store of the same type.
Examples:
"""""""""
.. code-block:: llvm
%ptr = alloca i32 ; yields {i32*}:ptr
store i32 3, i32* %ptr ; yields {void}
%val = load i32* %ptr ; yields {i32}:val = i32 3
.. _i_store:
'``store``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
store [volatile] <ty> <value>, <ty>* <pointer>[, align <alignment>][, !nontemporal !<index>] ; yields {void}
store atomic [volatile] <ty> <value>, <ty>* <pointer> [singlethread] <ordering>, align <alignment> ; yields {void}
Overview:
"""""""""
The '``store``' instruction is used to write to memory.
Arguments:
""""""""""
There are two arguments to the ``store`` instruction: a value to store
and an address at which to store it. The type of the ``<pointer>``
operand must be a pointer to the :ref:`first class <t_firstclass>` type of
the ``<value>`` operand. If the ``store`` is marked as ``volatile``,
then the optimizer is not allowed to modify the number or order of
execution of this ``store`` with other :ref:`volatile
operations <volatile>`.
If the ``store`` is marked as ``atomic``, it takes an extra
:ref:`ordering <ordering>` and optional ``singlethread`` argument. The
``acquire`` and ``acq_rel`` orderings aren't valid on ``store``
instructions. Atomic loads produce :ref:`defined <memmodel>` results
when they may see multiple atomic stores. The type of the pointee must
be an integer type whose bit width is a power of two greater than or
equal to eight and less than or equal to a target-specific size limit.
``align`` must be explicitly specified on atomic stores, and the store
has undefined behavior if the alignment is not set to a value which is
at least the size in bytes of the pointee. ``!nontemporal`` does not
have any defined semantics for atomic stores.
The optional constant ``align`` argument specifies the alignment of the
operation (that is, the alignment of the memory address). A value of 0
or an omitted ``align`` argument means that the operation has the ABI
alignment for the target. It is the responsibility of the code emitter
to ensure that the alignment information is correct. Overestimating the
alignment results in undefined behavior. Underestimating the
alignment may produce less efficient code. An alignment of 1 is always
safe.
The optional ``!nontemporal`` metadata must reference a single metadata
name ``<index>`` corresponding to a metadata node with one ``i32`` entry of
value 1. The existence of the ``!nontemporal`` metadata on the instruction
tells the optimizer and code generator that this load is not expected to
be reused in the cache. The code generator may select special
instructions to save cache bandwidth, such as the MOVNT instruction on
x86.
Semantics:
""""""""""
The contents of memory are updated to contain ``<value>`` at the
location specified by the ``<pointer>`` operand. If ``<value>`` is
of scalar type then the number of bytes written does not exceed the
minimum number of bytes needed to hold all bits of the type. For
example, storing an ``i24`` writes at most three bytes. When writing a
value of a type like ``i20`` with a size that is not an integral number
of bytes, it is unspecified what happens to the extra bits that do not
belong to the type, but they will typically be overwritten.
Example:
""""""""
.. code-block:: llvm
%ptr = alloca i32 ; yields {i32*}:ptr
store i32 3, i32* %ptr ; yields {void}
%val = load i32* %ptr ; yields {i32}:val = i32 3
.. _i_fence:
'``fence``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
fence [singlethread] <ordering> ; yields {void}
Overview:
"""""""""
The '``fence``' instruction is used to introduce happens-before edges
between operations.
Arguments:
""""""""""
'``fence``' instructions take an :ref:`ordering <ordering>` argument which
defines what *synchronizes-with* edges they add. They can only be given
``acquire``, ``release``, ``acq_rel``, and ``seq_cst`` orderings.
Semantics:
""""""""""
A fence A which has (at least) ``release`` ordering semantics
*synchronizes with* a fence B with (at least) ``acquire`` ordering
semantics if and only if there exist atomic operations X and Y, both
operating on some atomic object M, such that A is sequenced before X, X
modifies M (either directly or through some side effect of a sequence
headed by X), Y is sequenced before B, and Y observes M. This provides a
*happens-before* dependency between A and B. Rather than an explicit
``fence``, one (but not both) of the atomic operations X or Y might
provide a ``release`` or ``acquire`` (resp.) ordering constraint and
still *synchronize-with* the explicit ``fence`` and establish the
*happens-before* edge.
A ``fence`` which has ``seq_cst`` ordering, in addition to having both
``acquire`` and ``release`` semantics specified above, participates in
the global program order of other ``seq_cst`` operations and/or fences.
The optional ":ref:`singlethread <singlethread>`" argument specifies
that the fence only synchronizes with other fences in the same thread.
(This is useful for interacting with signal handlers.)
Example:
""""""""
.. code-block:: llvm
fence acquire ; yields {void}
fence singlethread seq_cst ; yields {void}
.. _i_cmpxchg:
'``cmpxchg``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
cmpxchg [volatile] <ty>* <pointer>, <ty> <cmp>, <ty> <new> [singlethread] <ordering> ; yields {ty}
Overview:
"""""""""
The '``cmpxchg``' instruction is used to atomically modify memory. It
loads a value in memory and compares it to a given value. If they are
equal, it stores a new value into the memory.
Arguments:
""""""""""
There are three arguments to the '``cmpxchg``' instruction: an address
to operate on, a value to compare to the value currently be at that
address, and a new value to place at that address if the compared values
are equal. The type of '<cmp>' must be an integer type whose bit width
is a power of two greater than or equal to eight and less than or equal
to a target-specific size limit. '<cmp>' and '<new>' must have the same
type, and the type of '<pointer>' must be a pointer to that type. If the
``cmpxchg`` is marked as ``volatile``, then the optimizer is not allowed
to modify the number or order of execution of this ``cmpxchg`` with
other :ref:`volatile operations <volatile>`.
The :ref:`ordering <ordering>` argument specifies how this ``cmpxchg``
synchronizes with other atomic operations.
The optional "``singlethread``" argument declares that the ``cmpxchg``
is only atomic with respect to code (usually signal handlers) running in
the same thread as the ``cmpxchg``. Otherwise the cmpxchg is atomic with
respect to all other code in the system.
The pointer passed into cmpxchg must have alignment greater than or
equal to the size in memory of the operand.
Semantics:
""""""""""
The contents of memory at the location specified by the '``<pointer>``'
operand is read and compared to '``<cmp>``'; if the read value is the
equal, '``<new>``' is written. The original value at the location is
returned.
A successful ``cmpxchg`` is a read-modify-write instruction for the purpose
of identifying release sequences. A failed ``cmpxchg`` is equivalent to an
atomic load with an ordering parameter determined by dropping any
``release`` part of the ``cmpxchg``'s ordering.
Example:
""""""""
.. code-block:: llvm
entry:
%orig = atomic load i32* %ptr unordered ; yields {i32}
br label %loop
loop:
%cmp = phi i32 [ %orig, %entry ], [%old, %loop]
%squared = mul i32 %cmp, %cmp
%old = cmpxchg i32* %ptr, i32 %cmp, i32 %squared ; yields {i32}
%success = icmp eq i32 %cmp, %old
br i1 %success, label %done, label %loop
done:
...
.. _i_atomicrmw:
'``atomicrmw``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
atomicrmw [volatile] <operation> <ty>* <pointer>, <ty> <value> [singlethread] <ordering> ; yields {ty}
Overview:
"""""""""
The '``atomicrmw``' instruction is used to atomically modify memory.
Arguments:
""""""""""
There are three arguments to the '``atomicrmw``' instruction: an
operation to apply, an address whose value to modify, an argument to the
operation. The operation must be one of the following keywords:
- xchg
- add
- sub
- and
- nand
- or
- xor
- max
- min
- umax
- umin
The type of '<value>' must be an integer type whose bit width is a power
of two greater than or equal to eight and less than or equal to a
target-specific size limit. The type of the '``<pointer>``' operand must
be a pointer to that type. If the ``atomicrmw`` is marked as
``volatile``, then the optimizer is not allowed to modify the number or
order of execution of this ``atomicrmw`` with other :ref:`volatile
operations <volatile>`.
Semantics:
""""""""""
The contents of memory at the location specified by the '``<pointer>``'
operand are atomically read, modified, and written back. The original
value at the location is returned. The modification is specified by the
operation argument:
- xchg: ``*ptr = val``
- add: ``*ptr = *ptr + val``
- sub: ``*ptr = *ptr - val``
- and: ``*ptr = *ptr & val``
- nand: ``*ptr = ~(*ptr & val)``
- or: ``*ptr = *ptr | val``
- xor: ``*ptr = *ptr ^ val``
- max: ``*ptr = *ptr > val ? *ptr : val`` (using a signed comparison)
- min: ``*ptr = *ptr < val ? *ptr : val`` (using a signed comparison)
- umax: ``*ptr = *ptr > val ? *ptr : val`` (using an unsigned
comparison)
- umin: ``*ptr = *ptr < val ? *ptr : val`` (using an unsigned
comparison)
Example:
""""""""
.. code-block:: llvm
%old = atomicrmw add i32* %ptr, i32 1 acquire ; yields {i32}
.. _i_getelementptr:
'``getelementptr``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = getelementptr <pty>* <ptrval>{, <ty> <idx>}*
<result> = getelementptr inbounds <pty>* <ptrval>{, <ty> <idx>}*
<result> = getelementptr <ptr vector> ptrval, <vector index type> idx
Overview:
"""""""""
The '``getelementptr``' instruction is used to get the address of a
subelement of an :ref:`aggregate <t_aggregate>` data structure. It performs
address calculation only and does not access memory.
Arguments:
""""""""""
The first argument is always a pointer or a vector of pointers, and
forms the basis of the calculation. The remaining arguments are indices
that indicate which of the elements of the aggregate object are indexed.
The interpretation of each index is dependent on the type being indexed
into. The first index always indexes the pointer value given as the
first argument, the second index indexes a value of the type pointed to
(not necessarily the value directly pointed to, since the first index
can be non-zero), etc. The first type indexed into must be a pointer
value, subsequent types can be arrays, vectors, and structs. Note that
subsequent types being indexed into can never be pointers, since that
would require loading the pointer before continuing calculation.
The type of each index argument depends on the type it is indexing into.
When indexing into a (optionally packed) structure, only ``i32`` integer
**constants** are allowed (when using a vector of indices they must all
be the **same** ``i32`` integer constant). When indexing into an array,
pointer or vector, integers of any width are allowed, and they are not
required to be constant. These integers are treated as signed values
where relevant.
For example, let's consider a C code fragment and how it gets compiled
to LLVM:
.. code-block:: c
struct RT {
char A;
int B[10][20];
char C;
};
struct ST {
int X;
double Y;
struct RT Z;
};
int *foo(struct ST *s) {
return &s[1].Z.B[5][13];
}
The LLVM code generated by Clang is:
.. code-block:: llvm
%struct.RT = type { i8, [10 x [20 x i32]], i8 }
%struct.ST = type { i32, double, %struct.RT }
define i32* @foo(%struct.ST* %s) nounwind uwtable readnone optsize ssp {
entry:
%arrayidx = getelementptr inbounds %struct.ST* %s, i64 1, i32 2, i32 1, i64 5, i64 13
ret i32* %arrayidx
}
Semantics:
""""""""""
In the example above, the first index is indexing into the
'``%struct.ST*``' type, which is a pointer, yielding a '``%struct.ST``'
= '``{ i32, double, %struct.RT }``' type, a structure. The second index
indexes into the third element of the structure, yielding a
'``%struct.RT``' = '``{ i8 , [10 x [20 x i32]], i8 }``' type, another
structure. The third index indexes into the second element of the
structure, yielding a '``[10 x [20 x i32]]``' type, an array. The two
dimensions of the array are subscripted into, yielding an '``i32``'
type. The '``getelementptr``' instruction returns a pointer to this
element, thus computing a value of '``i32*``' type.
Note that it is perfectly legal to index partially through a structure,
returning a pointer to an inner element. Because of this, the LLVM code
for the given testcase is equivalent to:
.. code-block:: llvm
define i32* @foo(%struct.ST* %s) {
%t1 = getelementptr %struct.ST* %s, i32 1 ; yields %struct.ST*:%t1
%t2 = getelementptr %struct.ST* %t1, i32 0, i32 2 ; yields %struct.RT*:%t2
%t3 = getelementptr %struct.RT* %t2, i32 0, i32 1 ; yields [10 x [20 x i32]]*:%t3
%t4 = getelementptr [10 x [20 x i32]]* %t3, i32 0, i32 5 ; yields [20 x i32]*:%t4
%t5 = getelementptr [20 x i32]* %t4, i32 0, i32 13 ; yields i32*:%t5
ret i32* %t5
}
If the ``inbounds`` keyword is present, the result value of the
``getelementptr`` is a :ref:`poison value <poisonvalues>` if the base
pointer is not an *in bounds* address of an allocated object, or if any
of the addresses that would be formed by successive addition of the
offsets implied by the indices to the base address with infinitely
precise signed arithmetic are not an *in bounds* address of that
allocated object. The *in bounds* addresses for an allocated object are
all the addresses that point into the object, plus the address one byte
past the end. In cases where the base is a vector of pointers the
``inbounds`` keyword applies to each of the computations element-wise.
If the ``inbounds`` keyword is not present, the offsets are added to the
base address with silently-wrapping two's complement arithmetic. If the
offsets have a different width from the pointer, they are sign-extended
or truncated to the width of the pointer. The result value of the
``getelementptr`` may be outside the object pointed to by the base
pointer. The result value may not necessarily be used to access memory
though, even if it happens to point into allocated storage. See the
:ref:`Pointer Aliasing Rules <pointeraliasing>` section for more
information.
The getelementptr instruction is often confusing. For some more insight
into how it works, see :doc:`the getelementptr FAQ <GetElementPtr>`.
Example:
""""""""
.. code-block:: llvm
; yields [12 x i8]*:aptr
%aptr = getelementptr {i32, [12 x i8]}* %saptr, i64 0, i32 1
; yields i8*:vptr
%vptr = getelementptr {i32, <2 x i8>}* %svptr, i64 0, i32 1, i32 1
; yields i8*:eptr
%eptr = getelementptr [12 x i8]* %aptr, i64 0, i32 1
; yields i32*:iptr
%iptr = getelementptr [10 x i32]* @arr, i16 0, i16 0
In cases where the pointer argument is a vector of pointers, each index
must be a vector with the same number of elements. For example:
.. code-block:: llvm
%A = getelementptr <4 x i8*> %ptrs, <4 x i64> %offsets,
Conversion Operations
---------------------
The instructions in this category are the conversion instructions
(casting) which all take a single operand and a type. They perform
various bit conversions on the operand.
'``trunc .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = trunc <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``trunc``' instruction truncates its operand to the type ``ty2``.
Arguments:
""""""""""
The '``trunc``' instruction takes a value to trunc, and a type to trunc
it to. Both types must be of :ref:`integer <t_integer>` types, or vectors
of the same number of integers. The bit size of the ``value`` must be
larger than the bit size of the destination type, ``ty2``. Equal sized
types are not allowed.
Semantics:
""""""""""
The '``trunc``' instruction truncates the high order bits in ``value``
and converts the remaining bits to ``ty2``. Since the source size must
be larger than the destination size, ``trunc`` cannot be a *no-op cast*.
It will always truncate bits.
Example:
""""""""
.. code-block:: llvm
%X = trunc i32 257 to i8 ; yields i8:1
%Y = trunc i32 123 to i1 ; yields i1:true
%Z = trunc i32 122 to i1 ; yields i1:false
%W = trunc <2 x i16> <i16 8, i16 7> to <2 x i8> ; yields <i8 8, i8 7>
'``zext .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = zext <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``zext``' instruction zero extends its operand to type ``ty2``.
Arguments:
""""""""""
The '``zext``' instruction takes a value to cast, and a type to cast it
to. Both types must be of :ref:`integer <t_integer>` types, or vectors of
the same number of integers. The bit size of the ``value`` must be
smaller than the bit size of the destination type, ``ty2``.
Semantics:
""""""""""
The ``zext`` fills the high order bits of the ``value`` with zero bits
until it reaches the size of the destination type, ``ty2``.
When zero extending from i1, the result will always be either 0 or 1.
Example:
""""""""
.. code-block:: llvm
%X = zext i32 257 to i64 ; yields i64:257
%Y = zext i1 true to i32 ; yields i32:1
%Z = zext <2 x i16> <i16 8, i16 7> to <2 x i32> ; yields <i32 8, i32 7>
'``sext .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = sext <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``sext``' sign extends ``value`` to the type ``ty2``.
Arguments:
""""""""""
The '``sext``' instruction takes a value to cast, and a type to cast it
to. Both types must be of :ref:`integer <t_integer>` types, or vectors of
the same number of integers. The bit size of the ``value`` must be
smaller than the bit size of the destination type, ``ty2``.
Semantics:
""""""""""
The '``sext``' instruction performs a sign extension by copying the sign
bit (highest order bit) of the ``value`` until it reaches the bit size
of the type ``ty2``.
When sign extending from i1, the extension always results in -1 or 0.
Example:
""""""""
.. code-block:: llvm
%X = sext i8 -1 to i16 ; yields i16 :65535
%Y = sext i1 true to i32 ; yields i32:-1
%Z = sext <2 x i16> <i16 8, i16 7> to <2 x i32> ; yields <i32 8, i32 7>
'``fptrunc .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fptrunc <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``fptrunc``' instruction truncates ``value`` to type ``ty2``.
Arguments:
""""""""""
The '``fptrunc``' instruction takes a :ref:`floating point <t_floating>`
value to cast and a :ref:`floating point <t_floating>` type to cast it to.
The size of ``value`` must be larger than the size of ``ty2``. This
implies that ``fptrunc`` cannot be used to make a *no-op cast*.
Semantics:
""""""""""
The '``fptrunc``' instruction truncates a ``value`` from a larger
:ref:`floating point <t_floating>` type to a smaller :ref:`floating
point <t_floating>` type. If the value cannot fit within the
destination type, ``ty2``, then the results are undefined.
Example:
""""""""
.. code-block:: llvm
%X = fptrunc double 123.0 to float ; yields float:123.0
%Y = fptrunc double 1.0E+300 to float ; yields undefined
'``fpext .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fpext <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``fpext``' extends a floating point ``value`` to a larger floating
point value.
Arguments:
""""""""""
The '``fpext``' instruction takes a :ref:`floating point <t_floating>`
``value`` to cast, and a :ref:`floating point <t_floating>` type to cast it
to. The source type must be smaller than the destination type.
Semantics:
""""""""""
The '``fpext``' instruction extends the ``value`` from a smaller
:ref:`floating point <t_floating>` type to a larger :ref:`floating
point <t_floating>` type. The ``fpext`` cannot be used to make a
*no-op cast* because it always changes bits. Use ``bitcast`` to make a
*no-op cast* for a floating point cast.
Example:
""""""""
.. code-block:: llvm
%X = fpext float 3.125 to double ; yields double:3.125000e+00
%Y = fpext double %X to fp128 ; yields fp128:0xL00000000000000004000900000000000
'``fptoui .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fptoui <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``fptoui``' converts a floating point ``value`` to its unsigned
integer equivalent of type ``ty2``.
Arguments:
""""""""""
The '``fptoui``' instruction takes a value to cast, which must be a
scalar or vector :ref:`floating point <t_floating>` value, and a type to
cast it to ``ty2``, which must be an :ref:`integer <t_integer>` type. If
``ty`` is a vector floating point type, ``ty2`` must be a vector integer
type with the same number of elements as ``ty``
Semantics:
""""""""""
The '``fptoui``' instruction converts its :ref:`floating
point <t_floating>` operand into the nearest (rounding towards zero)
unsigned integer value. If the value cannot fit in ``ty2``, the results
are undefined.
Example:
""""""""
.. code-block:: llvm
%X = fptoui double 123.0 to i32 ; yields i32:123
%Y = fptoui float 1.0E+300 to i1 ; yields undefined:1
%Z = fptoui float 1.04E+17 to i8 ; yields undefined:1
'``fptosi .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fptosi <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``fptosi``' instruction converts :ref:`floating point <t_floating>`
``value`` to type ``ty2``.
Arguments:
""""""""""
The '``fptosi``' instruction takes a value to cast, which must be a
scalar or vector :ref:`floating point <t_floating>` value, and a type to
cast it to ``ty2``, which must be an :ref:`integer <t_integer>` type. If
``ty`` is a vector floating point type, ``ty2`` must be a vector integer
type with the same number of elements as ``ty``
Semantics:
""""""""""
The '``fptosi``' instruction converts its :ref:`floating
point <t_floating>` operand into the nearest (rounding towards zero)
signed integer value. If the value cannot fit in ``ty2``, the results
are undefined.
Example:
""""""""
.. code-block:: llvm
%X = fptosi double -123.0 to i32 ; yields i32:-123
%Y = fptosi float 1.0E-247 to i1 ; yields undefined:1
%Z = fptosi float 1.04E+17 to i8 ; yields undefined:1
'``uitofp .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = uitofp <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``uitofp``' instruction regards ``value`` as an unsigned integer
and converts that value to the ``ty2`` type.
Arguments:
""""""""""
The '``uitofp``' instruction takes a value to cast, which must be a
scalar or vector :ref:`integer <t_integer>` value, and a type to cast it to
``ty2``, which must be an :ref:`floating point <t_floating>` type. If
``ty`` is a vector integer type, ``ty2`` must be a vector floating point
type with the same number of elements as ``ty``
Semantics:
""""""""""
The '``uitofp``' instruction interprets its operand as an unsigned
integer quantity and converts it to the corresponding floating point
value. If the value cannot fit in the floating point value, the results
are undefined.
Example:
""""""""
.. code-block:: llvm
%X = uitofp i32 257 to float ; yields float:257.0
%Y = uitofp i8 -1 to double ; yields double:255.0
'``sitofp .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = sitofp <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``sitofp``' instruction regards ``value`` as a signed integer and
converts that value to the ``ty2`` type.
Arguments:
""""""""""
The '``sitofp``' instruction takes a value to cast, which must be a
scalar or vector :ref:`integer <t_integer>` value, and a type to cast it to
``ty2``, which must be an :ref:`floating point <t_floating>` type. If
``ty`` is a vector integer type, ``ty2`` must be a vector floating point
type with the same number of elements as ``ty``
Semantics:
""""""""""
The '``sitofp``' instruction interprets its operand as a signed integer
quantity and converts it to the corresponding floating point value. If
the value cannot fit in the floating point value, the results are
undefined.
Example:
""""""""
.. code-block:: llvm
%X = sitofp i32 257 to float ; yields float:257.0
%Y = sitofp i8 -1 to double ; yields double:-1.0
.. _i_ptrtoint:
'``ptrtoint .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = ptrtoint <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``ptrtoint``' instruction converts the pointer or a vector of
pointers ``value`` to the integer (or vector of integers) type ``ty2``.
Arguments:
""""""""""
The '``ptrtoint``' instruction takes a ``value`` to cast, which must be
a a value of type :ref:`pointer <t_pointer>` or a vector of pointers, and a
type to cast it to ``ty2``, which must be an :ref:`integer <t_integer>` or
a vector of integers type.
Semantics:
""""""""""
The '``ptrtoint``' instruction converts ``value`` to integer type
``ty2`` by interpreting the pointer value as an integer and either
truncating or zero extending that value to the size of the integer type.
If ``value`` is smaller than ``ty2`` then a zero extension is done. If
``value`` is larger than ``ty2`` then a truncation is done. If they are
the same size, then nothing is done (*no-op cast*) other than a type
change.
Example:
""""""""
.. code-block:: llvm
%X = ptrtoint i32* %P to i8 ; yields truncation on 32-bit architecture
%Y = ptrtoint i32* %P to i64 ; yields zero extension on 32-bit architecture
%Z = ptrtoint <4 x i32*> %P to <4 x i64>; yields vector zero extension for a vector of addresses on 32-bit architecture
.. _i_inttoptr:
'``inttoptr .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = inttoptr <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``inttoptr``' instruction converts an integer ``value`` to a
pointer type, ``ty2``.
Arguments:
""""""""""
The '``inttoptr``' instruction takes an :ref:`integer <t_integer>` value to
cast, and a type to cast it to, which must be a :ref:`pointer <t_pointer>`
type.
Semantics:
""""""""""
The '``inttoptr``' instruction converts ``value`` to type ``ty2`` by
applying either a zero extension or a truncation depending on the size
of the integer ``value``. If ``value`` is larger than the size of a
pointer then a truncation is done. If ``value`` is smaller than the size
of a pointer then a zero extension is done. If they are the same size,
nothing is done (*no-op cast*).
Example:
""""""""
.. code-block:: llvm
%X = inttoptr i32 255 to i32* ; yields zero extension on 64-bit architecture
%Y = inttoptr i32 255 to i32* ; yields no-op on 32-bit architecture
%Z = inttoptr i64 0 to i32* ; yields truncation on 32-bit architecture
%Z = inttoptr <4 x i32> %G to <4 x i8*>; yields truncation of vector G to four pointers
.. _i_bitcast:
'``bitcast .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = bitcast <ty> <value> to <ty2> ; yields ty2
Overview:
"""""""""
The '``bitcast``' instruction converts ``value`` to type ``ty2`` without
changing any bits.
Arguments:
""""""""""
The '``bitcast``' instruction takes a value to cast, which must be a
non-aggregate first class value, and a type to cast it to, which must
also be a non-aggregate :ref:`first class <t_firstclass>` type. The
bit sizes of ``value`` and the destination type, ``ty2``, must be
identical. If the source type is a pointer, the destination type must
also be a pointer of the same size. This instruction supports bitwise
conversion of vectors to integers and to vectors of other types (as
long as they have the same size).
Semantics:
""""""""""
The '``bitcast``' instruction converts ``value`` to type ``ty2``. It
is always a *no-op cast* because no bits change with this
conversion. The conversion is done as if the ``value`` had been stored
to memory and read back as type ``ty2``. Pointer (or vector of
pointers) types may only be converted to other pointer (or vector of
pointers) types with the same address space through this instruction.
To convert pointers to other types, use the :ref:`inttoptr <i_inttoptr>`
or :ref:`ptrtoint <i_ptrtoint>` instructions first.
Example:
""""""""
.. code-block:: llvm
%X = bitcast i8 255 to i8 ; yields i8 :-1
%Y = bitcast i32* %x to sint* ; yields sint*:%x
%Z = bitcast <2 x int> %V to i64; ; yields i64: %V
%Z = bitcast <2 x i32*> %V to <2 x i64*> ; yields <2 x i64*>
.. _i_addrspacecast:
'``addrspacecast .. to``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = addrspacecast <pty> <ptrval> to <pty2> ; yields pty2
Overview:
"""""""""
The '``addrspacecast``' instruction converts ``ptrval`` from ``pty`` in
address space ``n`` to type ``pty2`` in address space ``m``.
Arguments:
""""""""""
The '``addrspacecast``' instruction takes a pointer or vector of pointer value
to cast and a pointer type to cast it to, which must have a different
address space.
Semantics:
""""""""""
The '``addrspacecast``' instruction converts the pointer value
``ptrval`` to type ``pty2``. It can be a *no-op cast* or a complex
value modification, depending on the target and the address space
pair. Pointer conversions within the same address space must be
performed with the ``bitcast`` instruction. Note that if the address space
conversion is legal then both result and operand refer to the same memory
location.
Example:
""""""""
.. code-block:: llvm
%X = addrspacecast i32* %x to i32 addrspace(1)* ; yields i32 addrspace(1)*:%x
%Y = addrspacecast i32 addrspace(1)* %y to i64 addrspace(2)* ; yields i64 addrspace(2)*:%y
%Z = addrspacecast <4 x i32*> %z to <4 x float addrspace(3)*> ; yields <4 x float addrspace(3)*>:%z
.. _otherops:
Other Operations
----------------
The instructions in this category are the "miscellaneous" instructions,
which defy better classification.
.. _i_icmp:
'``icmp``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = icmp <cond> <ty> <op1>, <op2> ; yields {i1} or {<N x i1>}:result
Overview:
"""""""""
The '``icmp``' instruction returns a boolean value or a vector of
boolean values based on comparison of its two integer, integer vector,
pointer, or pointer vector operands.
Arguments:
""""""""""
The '``icmp``' instruction takes three operands. The first operand is
the condition code indicating the kind of comparison to perform. It is
not a value, just a keyword. The possible condition code are:
#. ``eq``: equal
#. ``ne``: not equal
#. ``ugt``: unsigned greater than
#. ``uge``: unsigned greater or equal
#. ``ult``: unsigned less than
#. ``ule``: unsigned less or equal
#. ``sgt``: signed greater than
#. ``sge``: signed greater or equal
#. ``slt``: signed less than
#. ``sle``: signed less or equal
The remaining two arguments must be :ref:`integer <t_integer>` or
:ref:`pointer <t_pointer>` or integer :ref:`vector <t_vector>` typed. They
must also be identical types.
Semantics:
""""""""""
The '``icmp``' compares ``op1`` and ``op2`` according to the condition
code given as ``cond``. The comparison performed always yields either an
:ref:`i1 <t_integer>` or vector of ``i1`` result, as follows:
#. ``eq``: yields ``true`` if the operands are equal, ``false``
otherwise. No sign interpretation is necessary or performed.
#. ``ne``: yields ``true`` if the operands are unequal, ``false``
otherwise. No sign interpretation is necessary or performed.
#. ``ugt``: interprets the operands as unsigned values and yields
``true`` if ``op1`` is greater than ``op2``.
#. ``uge``: interprets the operands as unsigned values and yields
``true`` if ``op1`` is greater than or equal to ``op2``.
#. ``ult``: interprets the operands as unsigned values and yields
``true`` if ``op1`` is less than ``op2``.
#. ``ule``: interprets the operands as unsigned values and yields
``true`` if ``op1`` is less than or equal to ``op2``.
#. ``sgt``: interprets the operands as signed values and yields ``true``
if ``op1`` is greater than ``op2``.
#. ``sge``: interprets the operands as signed values and yields ``true``
if ``op1`` is greater than or equal to ``op2``.
#. ``slt``: interprets the operands as signed values and yields ``true``
if ``op1`` is less than ``op2``.
#. ``sle``: interprets the operands as signed values and yields ``true``
if ``op1`` is less than or equal to ``op2``.
If the operands are :ref:`pointer <t_pointer>` typed, the pointer values
are compared as if they were integers.
If the operands are integer vectors, then they are compared element by
element. The result is an ``i1`` vector with the same number of elements
as the values being compared. Otherwise, the result is an ``i1``.
Example:
""""""""
.. code-block:: llvm
<result> = icmp eq i32 4, 5 ; yields: result=false
<result> = icmp ne float* %X, %X ; yields: result=false
<result> = icmp ult i16 4, 5 ; yields: result=true
<result> = icmp sgt i16 4, 5 ; yields: result=false
<result> = icmp ule i16 -4, 5 ; yields: result=false
<result> = icmp sge i16 4, 5 ; yields: result=false
Note that the code generator does not yet support vector types with the
``icmp`` instruction.
.. _i_fcmp:
'``fcmp``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = fcmp <cond> <ty> <op1>, <op2> ; yields {i1} or {<N x i1>}:result
Overview:
"""""""""
The '``fcmp``' instruction returns a boolean value or vector of boolean
values based on comparison of its operands.
If the operands are floating point scalars, then the result type is a
boolean (:ref:`i1 <t_integer>`).
If the operands are floating point vectors, then the result type is a
vector of boolean with the same number of elements as the operands being
compared.
Arguments:
""""""""""
The '``fcmp``' instruction takes three operands. The first operand is
the condition code indicating the kind of comparison to perform. It is
not a value, just a keyword. The possible condition code are:
#. ``false``: no comparison, always returns false
#. ``oeq``: ordered and equal
#. ``ogt``: ordered and greater than
#. ``oge``: ordered and greater than or equal
#. ``olt``: ordered and less than
#. ``ole``: ordered and less than or equal
#. ``one``: ordered and not equal
#. ``ord``: ordered (no nans)
#. ``ueq``: unordered or equal
#. ``ugt``: unordered or greater than
#. ``uge``: unordered or greater than or equal
#. ``ult``: unordered or less than
#. ``ule``: unordered or less than or equal
#. ``une``: unordered or not equal
#. ``uno``: unordered (either nans)
#. ``true``: no comparison, always returns true
*Ordered* means that neither operand is a QNAN while *unordered* means
that either operand may be a QNAN.
Each of ``val1`` and ``val2`` arguments must be either a :ref:`floating
point <t_floating>` type or a :ref:`vector <t_vector>` of floating point
type. They must have identical types.
Semantics:
""""""""""
The '``fcmp``' instruction compares ``op1`` and ``op2`` according to the
condition code given as ``cond``. If the operands are vectors, then the
vectors are compared element by element. Each comparison performed
always yields an :ref:`i1 <t_integer>` result, as follows:
#. ``false``: always yields ``false``, regardless of operands.
#. ``oeq``: yields ``true`` if both operands are not a QNAN and ``op1``
is equal to ``op2``.
#. ``ogt``: yields ``true`` if both operands are not a QNAN and ``op1``
is greater than ``op2``.
#. ``oge``: yields ``true`` if both operands are not a QNAN and ``op1``
is greater than or equal to ``op2``.
#. ``olt``: yields ``true`` if both operands are not a QNAN and ``op1``
is less than ``op2``.
#. ``ole``: yields ``true`` if both operands are not a QNAN and ``op1``
is less than or equal to ``op2``.
#. ``one``: yields ``true`` if both operands are not a QNAN and ``op1``
is not equal to ``op2``.
#. ``ord``: yields ``true`` if both operands are not a QNAN.
#. ``ueq``: yields ``true`` if either operand is a QNAN or ``op1`` is
equal to ``op2``.
#. ``ugt``: yields ``true`` if either operand is a QNAN or ``op1`` is
greater than ``op2``.
#. ``uge``: yields ``true`` if either operand is a QNAN or ``op1`` is
greater than or equal to ``op2``.
#. ``ult``: yields ``true`` if either operand is a QNAN or ``op1`` is
less than ``op2``.
#. ``ule``: yields ``true`` if either operand is a QNAN or ``op1`` is
less than or equal to ``op2``.
#. ``une``: yields ``true`` if either operand is a QNAN or ``op1`` is
not equal to ``op2``.
#. ``uno``: yields ``true`` if either operand is a QNAN.
#. ``true``: always yields ``true``, regardless of operands.
Example:
""""""""
.. code-block:: llvm
<result> = fcmp oeq float 4.0, 5.0 ; yields: result=false
<result> = fcmp one float 4.0, 5.0 ; yields: result=true
<result> = fcmp olt float 4.0, 5.0 ; yields: result=true
<result> = fcmp ueq double 1.0, 2.0 ; yields: result=false
Note that the code generator does not yet support vector types with the
``fcmp`` instruction.
.. _i_phi:
'``phi``' Instruction
^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = phi <ty> [ <val0>, <label0>], ...
Overview:
"""""""""
The '``phi``' instruction is used to implement the φ node in the SSA
graph representing the function.
Arguments:
""""""""""
The type of the incoming values is specified with the first type field.
After this, the '``phi``' instruction takes a list of pairs as
arguments, with one pair for each predecessor basic block of the current
block. Only values of :ref:`first class <t_firstclass>` type may be used as
the value arguments to the PHI node. Only labels may be used as the
label arguments.
There must be no non-phi instructions between the start of a basic block
and the PHI instructions: i.e. PHI instructions must be first in a basic
block.
For the purposes of the SSA form, the use of each incoming value is
deemed to occur on the edge from the corresponding predecessor block to
the current block (but after any definition of an '``invoke``'
instruction's return value on the same edge).
Semantics:
""""""""""
At runtime, the '``phi``' instruction logically takes on the value
specified by the pair corresponding to the predecessor basic block that
executed just prior to the current block.
Example:
""""""""
.. code-block:: llvm
Loop: ; Infinite loop that counts from 0 on up...
%indvar = phi i32 [ 0, %LoopHeader ], [ %nextindvar, %Loop ]
%nextindvar = add i32 %indvar, 1
br label %Loop
.. _i_select:
'``select``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = select selty <cond>, <ty> <val1>, <ty> <val2> ; yields ty
selty is either i1 or {<N x i1>}
Overview:
"""""""""
The '``select``' instruction is used to choose one value based on a
condition, without branching.
Arguments:
""""""""""
The '``select``' instruction requires an 'i1' value or a vector of 'i1'
values indicating the condition, and two values of the same :ref:`first
class <t_firstclass>` type. If the val1/val2 are vectors and the
condition is a scalar, then entire vectors are selected, not individual
elements.
Semantics:
""""""""""
If the condition is an i1 and it evaluates to 1, the instruction returns
the first value argument; otherwise, it returns the second value
argument.
If the condition is a vector of i1, then the value arguments must be
vectors of the same size, and the selection is done element by element.
Example:
""""""""
.. code-block:: llvm
%X = select i1 true, i8 17, i8 42 ; yields i8:17
.. _i_call:
'``call``' Instruction
^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<result> = [tail] call [cconv] [ret attrs] <ty> [<fnty>*] <fnptrval>(<function args>) [fn attrs]
Overview:
"""""""""
The '``call``' instruction represents a simple function call.
Arguments:
""""""""""
This instruction requires several arguments:
#. The optional "tail" marker indicates that the callee function does
not access any allocas or varargs in the caller. Note that calls may
be marked "tail" even if they do not occur before a
:ref:`ret <i_ret>` instruction. If the "tail" marker is present, the
function call is eligible for tail call optimization, but `might not
in fact be optimized into a jump <CodeGenerator.html#tailcallopt>`_.
The code generator may optimize calls marked "tail" with either 1)
automatic `sibling call
optimization <CodeGenerator.html#sibcallopt>`_ when the caller and
callee have matching signatures, or 2) forced tail call optimization
when the following extra requirements are met:
- Caller and callee both have the calling convention ``fastcc``.
- The call is in tail position (ret immediately follows call and ret
uses value of call or is void).
- Option ``-tailcallopt`` is enabled, or
``llvm::GuaranteedTailCallOpt`` is ``true``.
- `Platform specific constraints are
met. <CodeGenerator.html#tailcallopt>`_
#. 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
calling convention of the call must match the calling convention of
the target function, or else the behavior is undefined.
#. 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 pointer to function value
being invoked. The argument types must match the types implied by
this signature. This type can be omitted if the function is not
varargs and if the function type does not return a pointer to a
function.
#. '``fnptrval``': An LLVM value containing a pointer to a function to
be invoked. In most cases, this is a direct function invocation, but
indirect ``call``'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.
#. The optional :ref:`function attributes <fnattrs>` list. Only
'``noreturn``', '``nounwind``', '``readonly``' and '``readnone``'
attributes are valid here.
Semantics:
""""""""""
The '``call``' instruction is used to cause control flow to transfer to
a specified function, with its incoming arguments bound to the specified
values. Upon a '``ret``' instruction in the called function, control
flow continues with the instruction after the function call, and the
return value of the function is bound to the result argument.
Example:
""""""""
.. code-block:: llvm
%retval = call i32 @test(i32 %argc)
call i32 (i8*, ...)* @printf(i8* %msg, i32 12, i8 42) ; yields i32
%X = tail call i32 @foo() ; yields i32
%Y = tail call fastcc i32 @foo() ; yields i32
call void %foo(i8 97 signext)
%struct.A = type { i32, i8 }
%r = call %struct.A @foo() ; yields { 32, i8 }
%gr = extractvalue %struct.A %r, 0 ; yields i32
%gr1 = extractvalue %struct.A %r, 1 ; yields i8
%Z = call void @foo() noreturn ; indicates that %foo never returns normally
%ZZ = call zeroext i32 @bar() ; Return value is %zero extended
llvm treats calls to some functions with names and arguments that match
the standard C99 library as being the C99 library functions, and may
perform optimizations or generate code for them under that assumption.
This is something we'd like to change in the future to provide better
support for freestanding environments and non-C-based languages.
.. _i_va_arg:
'``va_arg``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<resultval> = va_arg <va_list*> <arglist>, <argty>
Overview:
"""""""""
The '``va_arg``' instruction is used to access arguments passed through
the "variable argument" area of a function call. It is used to implement
the ``va_arg`` macro in C.
Arguments:
""""""""""
This instruction takes a ``va_list*`` value and the type of the
argument. It returns a value of the specified argument type and
increments the ``va_list`` to point to the next argument. The actual
type of ``va_list`` is target specific.
Semantics:
""""""""""
The '``va_arg``' instruction loads an argument of the specified type
from the specified ``va_list`` and causes the ``va_list`` to point to
the next argument. For more information, see the variable argument
handling :ref:`Intrinsic Functions <int_varargs>`.
It is legal for this instruction to be called in a function which does
not take a variable number of arguments, for example, the ``vfprintf``
function.
``va_arg`` is an LLVM instruction instead of an :ref:`intrinsic
function <intrinsics>` because it takes a type as an argument.
Example:
""""""""
See the :ref:`variable argument processing <int_varargs>` section.
Note that the code generator does not yet fully support va\_arg on many
targets. Also, it does not currently support va\_arg with aggregate
types on any target.
.. _i_landingpad:
'``landingpad``' Instruction
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
<resultval> = landingpad <resultty> personality <type> <pers_fn> <clause>+
<resultval> = landingpad <resultty> personality <type> <pers_fn> cleanup <clause>*
<clause> := catch <type> <value>
<clause> := filter <array constant type> <array constant>
Overview:
"""""""""
The '``landingpad``' instruction is used by `LLVM's exception handling
system <ExceptionHandling.html#overview>`_ to specify that a basic block
is a landing pad --- one where the exception lands, and corresponds to the
code found in the ``catch`` portion of a ``try``/``catch`` sequence. It
defines values supplied by the personality function (``pers_fn``) upon
re-entry to the function. The ``resultval`` has the type ``resultty``.
Arguments:
""""""""""
This instruction takes a ``pers_fn`` value. This is the personality
function associated with the unwinding mechanism. The optional
``cleanup`` flag indicates that the landing pad block is a cleanup.
A ``clause`` begins with the clause type --- ``catch`` or ``filter`` --- and
contains the global variable representing the "type" that may be caught
or filtered respectively. Unlike the ``catch`` clause, the ``filter``
clause takes an array constant as its argument. Use
"``[0 x i8**] undef``" for a filter which cannot throw. The
'``landingpad``' instruction must contain *at least* one ``clause`` or
the ``cleanup`` flag.
Semantics:
""""""""""
The '``landingpad``' instruction defines the values which are set by the
personality function (``pers_fn``) upon re-entry to the function, and
therefore the "result type" of the ``landingpad`` instruction. As with
calling conventions, how the personality function results are
represented in LLVM IR is target specific.
The clauses are applied in order from top to bottom. If two
``landingpad`` instructions are merged together through inlining, the
clauses from the calling function are appended to the list of clauses.
When the call stack is being unwound due to an exception being thrown,
the exception is compared against each ``clause`` in turn. If it doesn't
match any of the clauses, and the ``cleanup`` flag is not set, then
unwinding continues further up the call stack.
The ``landingpad`` instruction has several restrictions:
- A landing pad block is a basic block which is the unwind destination
of an '``invoke``' instruction.
- A landing pad block must have a '``landingpad``' instruction as its
first non-PHI instruction.
- There can be only one '``landingpad``' instruction within the landing
pad block.
- A basic block that is not a landing pad block may not include a
'``landingpad``' instruction.
- All '``landingpad``' instructions in a function must have the same
personality function.
Example:
""""""""
.. code-block:: llvm
;; A landing pad which can catch an integer.
%res = landingpad { i8*, i32 } personality i32 (...)* @__gxx_personality_v0
catch i8** @_ZTIi
;; A landing pad that is a cleanup.
%res = landingpad { i8*, i32 } personality i32 (...)* @__gxx_personality_v0
cleanup
;; A landing pad which can catch an integer and can only throw a double.
%res = landingpad { i8*, i32 } personality i32 (...)* @__gxx_personality_v0
catch i8** @_ZTIi
filter [1 x i8**] [@_ZTId]
.. _intrinsics:
Intrinsic Functions
===================
LLVM supports the notion of an "intrinsic function". These functions
have well known names and semantics and are required to follow certain
restrictions. Overall, these intrinsics represent an extension mechanism
for the LLVM language that does not require changing all of the
transformations in LLVM when adding to the language (or the bitcode
reader/writer, the parser, etc...).
Intrinsic function names must all start with an "``llvm.``" prefix. This
prefix is reserved in LLVM for intrinsic names; thus, function names may
not begin with this prefix. Intrinsic functions must always be external
functions: you cannot define the body of intrinsic functions. Intrinsic
functions may only be used in call or invoke instructions: it is illegal
to take the address of an intrinsic function. Additionally, because
intrinsic functions are part of the LLVM language, it is required if any
are added that they be documented here.
Some intrinsic functions can be overloaded, i.e., the intrinsic
represents a family of functions that perform the same operation but on
different data types. Because LLVM can represent over 8 million
different integer types, overloading is used commonly to allow an
intrinsic function to operate on any integer type. One or more of the
argument types or the result type can be overloaded to accept any
integer type. Argument types may also be defined as exactly matching a
previous argument's type or the result type. This allows an intrinsic
function which accepts multiple arguments, but needs all of them to be
of the same type, to only be overloaded with respect to a single
argument or the result.
Overloaded intrinsics will have the names of its overloaded argument
types encoded into its function name, each preceded by a period. Only
those types which are overloaded result in a name suffix. Arguments
whose type is matched against another type do not. For example, the
``llvm.ctpop`` function can take an integer of any width and returns an
integer of exactly the same integer width. This leads to a family of
functions such as ``i8 @llvm.ctpop.i8(i8 %val)`` and
``i29 @llvm.ctpop.i29(i29 %val)``. Only one type, the return type, is
overloaded, and only one type suffix is required. Because the argument's
type is matched against the return type, it does not require its own
name suffix.
To learn how to add an intrinsic function, please see the `Extending
LLVM Guide <ExtendingLLVM.html>`_.
.. _int_varargs:
Variable Argument Handling Intrinsics
-------------------------------------
Variable argument support is defined in LLVM with the
:ref:`va_arg <i_va_arg>` instruction and these three intrinsic
functions. These functions are related to the similarly named macros
defined in the ``<stdarg.h>`` header file.
All of these functions operate on arguments that use a target-specific
value type "``va_list``". The LLVM assembly language reference manual
does not define what this type is, so all transformations should be
prepared to handle these functions regardless of the type used.
This example shows how the :ref:`va_arg <i_va_arg>` instruction and the
variable argument handling intrinsic functions are used.
.. code-block:: llvm
define i32 @test(i32 %X, ...) {
; Initialize variable argument processing
%ap = alloca i8*
%ap2 = bitcast i8** %ap to i8*
call void @llvm.va_start(i8* %ap2)
; Read a single integer argument
%tmp = va_arg i8** %ap, i32
; Demonstrate usage of llvm.va_copy and llvm.va_end
%aq = alloca i8*
%aq2 = bitcast i8** %aq to i8*
call void @llvm.va_copy(i8* %aq2, i8* %ap2)
call void @llvm.va_end(i8* %aq2)
; Stop processing of arguments.
call void @llvm.va_end(i8* %ap2)
ret i32 %tmp
}
declare void @llvm.va_start(i8*)
declare void @llvm.va_copy(i8*, i8*)
declare void @llvm.va_end(i8*)
.. _int_va_start:
'``llvm.va_start``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.va_start(i8* <arglist>)
Overview:
"""""""""
The '``llvm.va_start``' intrinsic initializes ``*<arglist>`` for
subsequent use by ``va_arg``.
Arguments:
""""""""""
The argument is a pointer to a ``va_list`` element to initialize.
Semantics:
""""""""""
The '``llvm.va_start``' intrinsic works just like the ``va_start`` macro
available in C. In a target-dependent way, it initializes the
``va_list`` element to which the argument points, so that the next call
to ``va_arg`` will produce the first variable argument passed to the
function. Unlike the C ``va_start`` macro, this intrinsic does not need
to know the last argument of the function as the compiler can figure
that out.
'``llvm.va_end``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.va_end(i8* <arglist>)
Overview:
"""""""""
The '``llvm.va_end``' intrinsic destroys ``*<arglist>``, which has been
initialized previously with ``llvm.va_start`` or ``llvm.va_copy``.
Arguments:
""""""""""
The argument is a pointer to a ``va_list`` to destroy.
Semantics:
""""""""""
The '``llvm.va_end``' intrinsic works just like the ``va_end`` macro
available in C. In a target-dependent way, it destroys the ``va_list``
element to which the argument points. Calls to
:ref:`llvm.va_start <int_va_start>` and
:ref:`llvm.va_copy <int_va_copy>` must be matched exactly with calls to
``llvm.va_end``.
.. _int_va_copy:
'``llvm.va_copy``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.va_copy(i8* <destarglist>, i8* <srcarglist>)
Overview:
"""""""""
The '``llvm.va_copy``' intrinsic copies the current argument position
from the source argument list to the destination argument list.
Arguments:
""""""""""
The first argument is a pointer to a ``va_list`` element to initialize.
The second argument is a pointer to a ``va_list`` element to copy from.
Semantics:
""""""""""
The '``llvm.va_copy``' intrinsic works just like the ``va_copy`` macro
available in C. In a target-dependent way, it copies the source
``va_list`` element into the destination ``va_list`` element. This
intrinsic is necessary because the `` llvm.va_start`` intrinsic may be
arbitrarily complex and require, for example, memory allocation.
Accurate Garbage Collection Intrinsics
--------------------------------------
LLVM support for `Accurate Garbage Collection <GarbageCollection.html>`_
(GC) requires the implementation and generation of these intrinsics.
These intrinsics allow identification of :ref:`GC roots on the
stack <int_gcroot>`, as well as garbage collector implementations that
require :ref:`read <int_gcread>` and :ref:`write <int_gcwrite>` barriers.
Front-ends for type-safe garbage collected languages should generate
these intrinsics to make use of the LLVM garbage collectors. For more
details, see `Accurate Garbage Collection with
LLVM <GarbageCollection.html>`_.
The garbage collection intrinsics only operate on objects in the generic
address space (address space zero).
.. _int_gcroot:
'``llvm.gcroot``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
Overview:
"""""""""
The '``llvm.gcroot``' intrinsic declares the existence of a GC root to
the code generator, and allows some metadata to be associated with it.
Arguments:
""""""""""
The first argument specifies the address of a stack object that contains
the root pointer. The second pointer (which must be either a constant or
a global value address) contains the meta-data to be associated with the
root.
Semantics:
""""""""""
At runtime, a call to this intrinsic stores a null pointer into the
"ptrloc" location. At compile-time, the code generator generates
information to allow the runtime to find the pointer at GC safe points.
The '``llvm.gcroot``' intrinsic may only be used in a function which
:ref:`specifies a GC algorithm <gc>`.
.. _int_gcread:
'``llvm.gcread``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare i8* @llvm.gcread(i8* %ObjPtr, i8** %Ptr)
Overview:
"""""""""
The '``llvm.gcread``' intrinsic identifies reads of references from heap
locations, allowing garbage collector implementations that require read
barriers.
Arguments:
""""""""""
The second argument is the address to read from, which should be an
address allocated from the garbage collector. The first object is a
pointer to the start of the referenced object, if needed by the language
runtime (otherwise null).
Semantics:
""""""""""
The '``llvm.gcread``' intrinsic has the same semantics as a load
instruction, but may be replaced with substantially more complex code by
the garbage collector runtime, as needed. The '``llvm.gcread``'
intrinsic may only be used in a function which :ref:`specifies a GC
algorithm <gc>`.
.. _int_gcwrite:
'``llvm.gcwrite``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.gcwrite(i8* %P1, i8* %Obj, i8** %P2)
Overview:
"""""""""
The '``llvm.gcwrite``' intrinsic identifies writes of references to heap
locations, allowing garbage collector implementations that require write
barriers (such as generational or reference counting collectors).
Arguments:
""""""""""
The first argument is the reference to store, the second is the start of
the object to store it to, and the third is the address of the field of
Obj to store to. If the runtime does not require a pointer to the
object, Obj may be null.
Semantics:
""""""""""
The '``llvm.gcwrite``' intrinsic has the same semantics as a store
instruction, but may be replaced with substantially more complex code by
the garbage collector runtime, as needed. The '``llvm.gcwrite``'
intrinsic may only be used in a function which :ref:`specifies a GC
algorithm <gc>`.
Code Generator Intrinsics
-------------------------
These intrinsics are provided by LLVM to expose special features that
may only be implemented with code generator support.
'``llvm.returnaddress``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare i8 *@llvm.returnaddress(i32 <level>)
Overview:
"""""""""
The '``llvm.returnaddress``' intrinsic attempts to compute a
target-specific value indicating the return address of the current
function or one of its callers.
Arguments:
""""""""""
The argument to this intrinsic indicates which function to return the
address for. Zero indicates the calling function, one indicates its
caller, etc. The argument is **required** to be a constant integer
value.
Semantics:
""""""""""
The '``llvm.returnaddress``' intrinsic either returns a pointer
indicating the return address of the specified call frame, or zero if it
cannot be identified. The value returned by this intrinsic is likely to
be incorrect or 0 for arguments other than zero, so it should only be
used for debugging purposes.
Note that calling this intrinsic does not prevent function inlining or
other aggressive transformations, so the value returned may not be that
of the obvious source-language caller.
'``llvm.frameaddress``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare i8* @llvm.frameaddress(i32 <level>)
Overview:
"""""""""
The '``llvm.frameaddress``' intrinsic attempts to return the
target-specific frame pointer value for the specified stack frame.
Arguments:
""""""""""
The argument to this intrinsic indicates which function to return the
frame pointer for. Zero indicates the calling function, one indicates
its caller, etc. The argument is **required** to be a constant integer
value.
Semantics:
""""""""""
The '``llvm.frameaddress``' intrinsic either returns a pointer
indicating the frame address of the specified call frame, or zero if it
cannot be identified. The value returned by this intrinsic is likely to
be incorrect or 0 for arguments other than zero, so it should only be
used for debugging purposes.
Note that calling this intrinsic does not prevent function inlining or
other aggressive transformations, so the value returned may not be that
of the obvious source-language caller.
.. _int_stacksave:
'``llvm.stacksave``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare i8* @llvm.stacksave()
Overview:
"""""""""
The '``llvm.stacksave``' intrinsic is used to remember the current state
of the function stack, for use with
:ref:`llvm.stackrestore <int_stackrestore>`. This is useful for
implementing language features like scoped automatic variable sized
arrays in C99.
Semantics:
""""""""""
This intrinsic returns a opaque pointer value that can be passed to
:ref:`llvm.stackrestore <int_stackrestore>`. When an
``llvm.stackrestore`` intrinsic is executed with a value saved from
``llvm.stacksave``, it effectively restores the state of the stack to
the state it was in when the ``llvm.stacksave`` intrinsic executed. In
practice, this pops any :ref:`alloca <i_alloca>` blocks from the stack that
were allocated after the ``llvm.stacksave`` was executed.
.. _int_stackrestore:
'``llvm.stackrestore``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.stackrestore(i8* %ptr)
Overview:
"""""""""
The '``llvm.stackrestore``' intrinsic is used to restore the state of
the function stack to the state it was in when the corresponding
:ref:`llvm.stacksave <int_stacksave>` intrinsic executed. This is
useful for implementing language features like scoped automatic variable
sized arrays in C99.
Semantics:
""""""""""
See the description for :ref:`llvm.stacksave <int_stacksave>`.
'``llvm.prefetch``' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Syntax:
"""""""
::
declare void @llvm.prefetch(i8* <address>, i32 <rw>, i32 <locality>, i32 <cache type>)
Overview:
"""""""""
The '``llvm.prefetch``' intrinsic is a hint to the code generator to
insert a prefetch instruction if supported; otherwise, it is a noop.
Prefetches have no effect on the behavior of the program but can change
its performance characteristics.
Arguments:
""""""""""
``address`` is the address to be prefetched, ``rw`` is the specifier
determining if the fetch should be for a read (0) or write (1), and
``locality`` is a temporal locality specifier ranging from (0) - no
locality, to (3) - extremely local keep in cache. The ``cache type``