Alternative way of representing comdats

[majnemer]

Summary

This is a variation on http://reviews.llvm.org/D4094

I think it is at feature parity. In particular, lib/Linker has not been updated and passes have not been audited to not modify the set of symbols in a comdat.

What it does is change the representation to not be a Value. It is intended as a way of making a discussion about these treadeoffs more concrete.

[majnemer]

It seems like we have rough consensus that COMDATs should behave more like Types than Values. Commandeering this differential.

[rnk]

+1, I don't think we need to support RAUW on comdats.

[majnemer]

• Address some review feedback.

[majnemer]

• Update GlobalOpt and GlobalDCE for Comdats.

[majnemer]

• Implement changes for lib/Linker

[• rafael]

This also needs tests for the errors. In particular

• comdat that is only a forward ref
• redefinition of a comdat.
• linker errors

And one more thing, when dropping a comdat we should drop it from the comdat symbol table.

[• rafael]

docs/LangRef.rst

732

Other set of objects/sections with that key.

"the linker" in here is a bit ambiguous. Are we talking about the system linker or lib/Linker. Since we produce relocatable objects, it is probably better to state this in terms of what we do: put the global objects in a section with that comdat.

734

Move this after the part the defines what the selection kind is maybe?

761

This paragraph is now redundant.

763

There are still some things should probably be mentioned:

• We split "IR level sections" into multiple sections in the final .o since in object files you can't have a section that is part in one comdat and part in another.
• Explain how we map mulitple sections with the same comdat to using associative comdts on COFF. Otherwise someone is sure to look at the list and think associative is missing :-)
• Alias are placed in the same comdat the aliasee computes to, if any.

include/llvm/ADT/UniqueVector.h

24–29

Cool, I had no idea we had this :-)

include/llvm/IR/GlobalValue.h

115

This looks odd. You probably want the const version to forward to the non-const:

return const_cast<GlobalValue*>(this)->getComdat();

include/llvm/Support/GCOV.h

369 ↗

(On Diff #10593)

unrelated change?

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

260

Nice cleanup. You can commit this now if you want.

863

Reminds me: we should probably reject GlobalObject with common linkage and an explicit comdat in the verifier.

lib/Linker/LinkModules.cpp

549

Not sure I follow. Is this assuming that the comdat name is related to the name of a global? That may be a common use but is not in any way required by the semantics.

565

I assume this logic matches what the COFF linker does? Can you extract it to a helper like

bool computeResultingSeelectionKind(Comdat::SelectionKind &Result, Comdat::SelectionKind &Src, Comdat::SelectionKind &Dst) ...

and put a comment saying where the merging logic comes from.

590

This is fairly arbitrary for Newest. We should probably error or actually check the bitcode timestamp, no?

602

"auto *" instead of "auto" is probably better since it makes it obvious it is a pointer.

633

Not just ExactMatch.

889

BTW, do we produce as error if we have

module1:

@a = ... comdat $c1, largest

module2

@b = ... comdat $c1, newest

that is, we have conflicting comdat selection kinds, but the comdats never show up in the corresponding global objects.

[majnemer]

• Address some review feedback.
• Update GlobalOpt and GlobalDCE for Comdats.
• Implement changes for lib/Linker
• Address the latest round of review feedback

docs/LangRef.rst

732

The objects aren't in the same section. In COFF, they are in different sections which are associated.

734

Done.

761

Done.

763

I did the last two, I don't quite understand the first one.

include/llvm/IR/GlobalValue.h

115

Done.

include/llvm/Support/GCOV.h

369 ↗

(On Diff #10593)

Done.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

260

Done in r211601.

863

Done.

lib/Linker/LinkModules.cpp

549

The COMDAT name is related to the name of the global when the selection kind isn't any.

565

Done.

590

Done.

602

Done.

633

Done.

889

There is code for this:

return emitError("Linking COMDATs named '" + ComdatName +
                 "': invalid selection kinds!");

[• rafael]

almost there I think :-)

docs/LangRef.rst

778

OK, I get it in what way the comdat name is significant. The linker will pick the section whose comdat symbol is the largest, not the largest section. Can you think of a way to document this a bit more explicitly? If not, don't worry. I assume this should be clear to anyone familiar with COFF.

It does suggest that for any selection other than any we should require a real symbol to be present. For example, we should error on

$bar = comdat largest
@foo = global i32 42, comdat $bar

saying "no matching symbol for $bar, cannot decide which one is largest".

798

What I meant about IR sections being split into multiple object sections is that in, for example,

$foo = comdat any
$bar = comdat any
@g1 = global i32 42, section "sec", comdat $foo
@g2 = global i32 42, section "sec", comdat $bar

there is only one section "sec", but in the object file we end up with two sections named sec:

[ 6] sec               PROGBITS        0000000000000000 000050 000004 00 WAG  0   0  4
[ 7] sec               PROGBITS        0000000000000000 000054 000004 00 WAG  0   0  4

One last thing that we might want to document is that CodeGen can create comdats even if no comdat is explicit in the IR. For example, given

@bar = weak global i32 42

we create a comdat and put bar in it. This is something we might want to change now that we have proper comdat support, but for now it is better to just document it.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

270

I think you can avoid the duplicated error reporting with a static helper:

const Comdat *getELFComdat(const GlobalValue &GV)

it returns null if there is no comdat and reports a fatal error it there is one with a seleciton other than any.

597

similar to the ELF helper.

762–803

It looks like for COFF we miscompile

$foo = comdat any
@bar = global i32 42, comdat $foo
@foo = global i32 42

Is that even possible to represent in COFF? Since "foo" is a comdat key, the symbol "foo" has to be in the section that has that comdat.I guess in an object file you could have two symbols named foo, but that would be hard to represent in assembly.or even at the IR for selections other than any.

Can we just error in codegen when trying to output that for COFF? It does work for ELF, so it would probably be an interesting testcase to have.

820

This (along with the other non-any issues) can probably be handled earlier in the Verifier since we know that ELF only supports any.

lib/Linker/LinkModules.cpp

551

Is the alias handling correct? They don't exist in the object file (they are just another symbol). So I assume that given

$foo = comdat largest
@foo = alias  getelementptr([2 x i32]* @bar, i32 0, i32 1)
@bar = global [2 x i32] zeroinitializer, comdat $foo

The linker will end up seeing that foo has a size of 32 bits.

Can we just error in here for now? Something along the lines of "cannot compute size of this alias, so cannot select one comdat". We can extend it afterwards for cases where we are able to compute the size, like simple geps.

[majnemer]

• Address some review feedback.
• Update GlobalOpt and GlobalDCE for Comdats.
• Implement changes for lib/Linker
• Address the latest round of review feedback
• Address latest round of feedback.

docs/LangRef.rst

778

Added a verifier check.

798

Done.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

270

Done.

597

Done.

762–803

This is possible to represent, we get this "correct":

SECTION HEADER #2
   .data name
       0 physical address
       0 virtual address
       4 size of raw data
      B4 file pointer to raw data (000000B4 to 000000B7)
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
C0300040 flags
         Initialized Data
         4 byte align
         Read Write

SECTION HEADER #4
   .data name
       0 physical address
       0 virtual address
       4 size of raw data
      B8 file pointer to raw data (000000B8 to 000000BB)
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
C0301040 flags
         Initialized Data
         COMDAT; sym= _bar
         4 byte align
         Read Write

006 00000000 SECT4  notype       Static       | .data
    Section length    4, selection    5 (pick associative Section 0x2)
009 00000000 SECT4  notype       External     | _bar
00A 00000000 SECT2  notype       External     | _foo

820

We cannot do this because we don't know if we are targeting COFF vs ELF in the Verifier.

lib/Linker/LinkModules.cpp

551

Done.

[• rafael]

Rafael Ávila de Espíndola wrote:

This (along with the other non-any issues) can probably be handled earlier in the Verifier since we know that ELF only supports any.

We cannot do this because we don't know if we are targeting COFF vs ELF in the Verifier.

We know that ELF only supports any, so we can (and should) assume COFF
for the other selectors.

Cheers,
Rafael

[majnemer]

• Require that Comdat group leaders be in their Comdat group for COFF.

[• rafael]

Please add

$foo = comdat any
@bar = global i32 42, comdat $foo
@foo = global i32 42

as an ELF test.

docs/LangRef.rst

746

The sections or the comdat symbols btw? What does link.exe do?

751

This is undocumented and it seems unused. How about removing it for now?

756

The sections or the comdat symbol must be the same size?

763

"selected if the COMDAT's key is the largest."

i.e., we have to drop the "section", no?

798

This is still missing:

What I meant about IR sections being split into multiple object sections is that in, for example,

$foo = comdat any
$bar = comdat any
@g1 = global i32 42, section "sec", comdat $foo
@g2 = global i32 42, section "sec", comdat $bar

there is only one section "sec", but in the object file we end up with two sections named sec:

[ 6] sec               PROGBITS        0000000000000000 000050 000004 00 WAG  0   0  4
[ 7] sec               PROGBITS        0000000000000000 000054 000004 00 WAG  0   0  4

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

796

needs test

800

needs test.

lib/IR/Verifier.cpp

601

nit:

C.getSelectionKind() != omdat::Any

lib/Linker/LinkModules.cpp

601

needs test.

test/MC/COFF/comdat.ll

1 ↗

(On Diff #10904)

This should probably be in codegen, no MC.

test/MC/ELF/comdat.ll

1 ↗

(On Diff #10904)

CodeGen.

test/MC/MachO/comdat.ll

1 ↗

(On Diff #10904)

CodeGen

[majnemer]

• Remove the "newest" selection kind

[majnemer]

• Address review comments.

[majnemer]

I've added a test for ELF.

docs/LangRef.rst

746

Sections, COFF doesn't know how big a symbol is.

751

Done.

756

Sections, COFF doesn't know how big a symbol is.

763

Your understanding is correct.

798

Done.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

796

Done.

800

Done.

lib/IR/Verifier.cpp

601

Done.

lib/Linker/LinkModules.cpp

601

Done.

test/MC/COFF/comdat.ll

1 ↗

(On Diff #10904)

Done.

test/MC/ELF/comdat.ll

1 ↗

(On Diff #10904)

Done.

test/MC/MachO/comdat.ll

1 ↗

(On Diff #10904)

Done.

[• rafael]

Rafael Ávila de Espíndola wrote:

The sections or the comdat symbols btw? What does link.exe do?

Sections, COFF doesn't know how big a symbol is.

OK, I must still be missing something. If the linker doesn't know how
big a symbol is, how is largest implemented? In the current
documentation we have

• largest: largest COMDAT key, i.e., the symbol.
• samesize: The size of the section is the important one.

Cheers,
Rafael

[majnemer]

• Fix associative sections on COFF
• Add a test showcasing how MS RTTI would look

[majnemer]

• Make sure that COMDATs groups whose key are an alias work

[• rafael]

There is just one more issue that I think we need to handle in the linker and we are good:

It is very important that we don't change the set of symbol in a comdat. That means that if we have code like

@foo = global i32 42, comdat $bar
@zed = alias ...

and we cannot compute the base of zed, we should produce an error in the linker since we don't know if it is the same comdat as foo or not. So far all the aliases have been computable, so this is really just to make sure we error instead of miscompile if someone writes a fancier one.

LGTM with the doc clarification and test improvements, but please split the assembly changes in its own commit.

docs/LangRef.rst

733

You should probably expand the "The global object may not have local linkage." in its own paragraph. It should probably go after the note on how this is implemented in COFF. Then we can say something like

Given how this is implemented on COFF, we know there is only one GlobalObject per section. That global object, or an alias based on it, is required to have the same name as the comdat. That is used to find the object and compute the section size during linking.

Since the comdat name has to be stable, the global value that shares its name must not have local linkage since those can be renamed.

include/llvm/MC/MCContext.h

164

The idea is that it should be possible to create two different sections with

.section foo, "dr", largest, bar
.section foo, "dr", same_size, bar

correct? That is fine, but it can be tested with assembly and should probably be an independent first patch. This part LGTM with the above test checking that we create two sections.

lib/Linker/LinkModules.cpp

557

needs test.

564

needs test.

test/CodeGen/X86/coff-comdat.ll

56

Thin can/should be private, right?

test/Linker/Inputs/comdat5.ll

13

this can/should be private.

[majnemer]

Closed by commit rL211920 (authored by @majnemer).

docs/LangRef.rst

 A global variable may be declared to reside in a target-specific
 numbered address space. For targets that support them, address spaces
 may affect how optimizations are performed and/or what target
 instructions are used to access the variable. The default address space
 is zero. The address space qualifier must precede any other attributes.
 
 LLVM allows an explicit section to be specified for globals. If the
 target supports it, it will emit globals to the section specified.
+Additionally, the global can placed in a comdat if the target has the necessary
+support.
 
 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``.

 LLVM function definitions consist of the "``define``" keyword, an
 optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
 style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
 an optional :ref:`calling convention <callingconv>`,
 an optional ``unnamed_addr`` 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
+an optional section, an optional alignment,
+an optional :ref:`comdat <langref_comdats>`,
+an optional :ref:`garbage collector name <gc>`, an optional :ref:`prefix <prefixdata>`, an opening
 curly brace, a list of basic blocks, and a closing curly brace.
 
 LLVM function declarations consist of the "``declare``" keyword, an
 optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
 style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
 an optional :ref:`calling convention <callingconv>`,
 an optional ``unnamed_addr`` attribute, a return type, an optional
 :ref:`parameter attribute <paramattrs>` for the return type, a function

 The first basic block in a function is special in two ways: it is
 immediately executed on entrance to the function, and it is not allowed
 to have predecessor basic blocks (i.e. there can not be any branches to
 the entry block of a function). Because the block can have no
 predecessors, it also cannot have any :ref:`PHI nodes <i_phi>`.
 
 LLVM allows an explicit section to be specified for functions. If the
 target supports it, it will emit functions to the section specified.
+Additionally, the function can placed in a COMDAT.
 
 An explicit alignment may be specified for a function. If not present,
 or if the alignment is set to zero, the alignment of the function is set
 by the target to whatever it feels convenient. If an explicit alignment
 is specified, the function is forced to have at least that much
 alignment. All alignments must be a power of 2.
 
 If the ``unnamed_addr`` attribute is given, the address is know to not
 be significant and two identical functions can be merged.
 
 Syntax::
 
     define [linkage] [visibility] [DLLStorageClass]
            [cconv] [ret attrs]
            <ResultType> @<FunctionName> ([argument list])
-           [unnamed_addr] [fn Attrs] [section "name"] [align N]
-           [gc] [prefix Constant] { ... }
+           [unnamed_addr] [fn Attrs] [section "name"] [comdat $<ComdatName>]
+           [align N] [gc] [prefix Constant] { ... }
 
 .. _langref_aliases:
 
 Aliases
 -------
 
 Aliases, unlike function or variables, don't create any new data. They
 are just a new symbol and metadata for an existing position.

 
 * No alias in the expression can be weak as the possibility of the
   intermediate alias being overridden cannot be represented in an
   object file.
 
 * No global value in the expression can be a declaration, since that
   would require a relocation, which is not possible.
 
+.. _langref_comdats:
+
+Comdats
+-------
+
+Comdat IR provides access to COFF and ELF object file COMDAT functionality.
+
+Comdats have a name which represents the COMDAT key.  All global objects which
+specify this key will only end up in the final object file if the linker chooses
+that key over some other key.  Aliases are placed in the same COMDAT that their
+aliasee computes to, if any.  The global object may not have local linkage.
+
+Comdats have a selection kind to provide input on how the linker should
+choose between keys in two different object files.
+
+Syntax::
+
+    $<Name> = comdat SelectionKind
+
+The selection kind must be one of the following:
+
+``any``
+    The linker may choose any COMDAT key, the choice is arbitrary.
+``exactmatch``
+    The linker may choose any COMDAT key but the sections must contain the
+    same data.
+``largest``
+    The linker will choose the section containing the largest COMDAT key.
+``noduplicates``
+    The linker requires that only section with this COMDAT key exist.
+``samesize``
+    The linker may choose any COMDAT key but the sections must contain the
+    same amount of data.
+
+Note that the Mach-O platform doesn't support COMDATs and ELF only supports
+``any`` as a selection kind.
+
+Here is an example of a COMDAT group where a function will only be selected if
+the COMDAT key's section is the largest:
+
+.. code-block:: llvm
+
+   $foo = comdat largest
+   @foo = global i32 2, comdat $foo
+
+   define void @bar() comdat $foo {
+     ret void
+   }
+
+In a COFF object file, this will create a COMDAT section with selection kind
+``IMAGE_COMDAT_SELECT_LARGEST`` containing the contents of the ``@foo`` symbol
+and another COMDAT section with selection kind
+``IMAGE_COMDAT_SELECT_ASSOCIATIVE`` which is associated with the first COMDAT
+section and contains the contents of the ``@baz`` symbol.
+
+The combined use of COMDATS and section attributes may yield surprising results.
+For example:
+
+.. code-block:: llvm
+
+   $foo = comdat any
+   $bar = comdat any
+   @g1 = global i32 42, section "sec", comdat $foo
+   @g2 = global i32 42, section "sec", comdat $bar
+
+From the object file perspective, this requires the creation of two sections
+with the same name.  This is necessary because both globals belong to different
+COMDAT groups and COMDATs, at the object file level, are represented by
+sections.
+
+Note that certain IR constructs like global variables and functions may create
+COMDATs in the object file in addition to any which are specified using COMDAT
+IR.  This arises, for example, when a global variable has linkonce_odr linkage.
+
 .. _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::

include/llvm/ADT/UniqueVector.h

 
 namespace llvm {
 
 //===----------------------------------------------------------------------===//
 /// UniqueVector - This class produces a sequential ID number (base 1) for each
 /// unique entry that is added.  T is the type of entries in the vector. This
 /// class should have an implementation of operator== and of operator<.
 /// Entries can be fetched using operator[] with the entry ID.
 template<class T> class UniqueVector {
+public:
+  typedef typename std::vector<T> VectorType;
+  typedef typename VectorType::iterator iterator;
+  typedef typename VectorType::const_iterator const_iterator;
+
 private:
   // Map - Used to handle the correspondence of entry to ID.
   std::map<T, unsigned> Map;
 
   // Vector - ID ordered vector of entries. Entries can be indexed by ID - 1.
   //
-  std::vector<T> Vector;
+  VectorType Vector;
 
 public:
   /// insert - Append entry to the vector if it doesn't already exist.  Returns
   /// the entry's index + 1 to be used as a unique ID.
   unsigned insert(const T &Entry) {
     // Check if the entry is already in the map.
     unsigned &Val = Map[Entry];
 
public:
 
   /// operator[] - Returns a reference to the entry with the specified ID.
   ///
   const T &operator[](unsigned ID) const {
     assert(ID-1 < size() && "ID is 0 or out of range!");
     return Vector[ID - 1];
   }
 
+  /// \brief Return an iterator to the start of the vector.
+  iterator begin() { return Vector.begin(); }
+
+  /// \brief Return an iterator to the start of the vector.
+  const_iterator begin() const { return Vector.begin(); }
+
+  /// \brief Return an iterator to the end of the vector.
+  iterator end() { return Vector.end(); }
+
+  /// \brief Return an iterator to the end of the vector.
+  const_iterator end() const { return Vector.end(); }
+
   /// size - Returns the number of entries in the vector.
   ///
   size_t size() const { return Vector.size(); }
 
   /// empty - Returns true if the vector is empty.
   ///
   bool empty() const { return Vector.empty(); }
 

include/llvm/Bitcode/LLVMBitCodes.h

enum ModuleCodes {
     MODULE_CODE_FUNCTION    = 8,
 
     // ALIAS: [alias type, aliasee val#, linkage, visibility]
     MODULE_CODE_ALIAS       = 9,
 
     // MODULE_CODE_PURGEVALS: [numvals]
     MODULE_CODE_PURGEVALS   = 10,
 
-    MODULE_CODE_GCNAME      = 11   // GCNAME: [strchr x N]
+    MODULE_CODE_GCNAME      = 11,  // GCNAME: [strchr x N]
+    MODULE_CODE_COMDAT      = 12,  // COMDAT: [selection_kind, name]
   };
 
   /// PARAMATTR blocks have code for defining a parameter attribute set.
   enum AttributeCodes {
     // FIXME: Remove `PARAMATTR_CODE_ENTRY_OLD' in 4.0
     PARAMATTR_CODE_ENTRY_OLD  = 1, // ENTRY: [paramidx0, attr0,
                                    //         paramidx1, attr1...]
     PARAMATTR_CODE_ENTRY      = 2, // ENTRY: [paramidx0, attrgrp0,
enum AttributeKindCodes {
     ATTR_KIND_BUILTIN = 35,
     ATTR_KIND_COLD = 36,
     ATTR_KIND_OPTIMIZE_NONE = 37,
     ATTR_KIND_IN_ALLOCA = 38,
     ATTR_KIND_NON_NULL = 39,
     ATTR_KIND_JUMP_TABLE = 40
   };
 
+  enum ComdatSelectionKindCodes {
+    COMDAT_SELECTION_KIND_ANY = 1,
+    COMDAT_SELECTION_KIND_EXACT_MATCH = 2,
+    COMDAT_SELECTION_KIND_LARGEST = 3,
+    COMDAT_SELECTION_KIND_NO_DUPLICATES = 4,
+    COMDAT_SELECTION_KIND_SAME_SIZE = 5,
+  };
+
 } // End bitc namespace
 } // End llvm namespace
 
 #endif

include/llvm/IR/Comdat.h

+//===-- llvm/IR/Comdat.h - Comdat definitions -------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+/// @file
+/// This file contains the declaration of the Comdat class, which represents a
+/// single COMDAT in LLVM.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_IR_COMDAT_H
+#define LLVM_IR_COMDAT_H
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Compiler.h"
+
+namespace llvm {
+
+class raw_ostream;
+template <typename ValueTy> class StringMapEntry;
+
+// This is a Name X SelectionKind pair. The reason for having this be an
+// independent object instead of just adding the name and the SelectionKind
+// to a GlobalObject is that it is invalid to have two Comdats with the same
+// name but different SelectionKind. This structure makes that unrepresentable.
+class Comdat {
+public:
+  enum SelectionKind {
+    Any,          ///< The linker may choose any COMDAT.
+    ExactMatch,   ///< The data referenced by the COMDAT must be the same.
+    Largest,      ///< The linker will choose the largest COMDAT.
+    NoDuplicates, ///< No other Module may specify this COMDAT.
+    SameSize,     ///< The data referenced by the COMDAT must be the same size.
+  };
+
+  Comdat(Comdat &&C);
+  SelectionKind getSelectionKind() const { return SK; }
+  void setSelectionKind(SelectionKind Val) { SK = Val; }
+  StringRef getName() const;
+  void print(raw_ostream &OS) const;
+  void dump() const;
+
+private:
+  friend class Module;
+  Comdat();
+  Comdat(SelectionKind SK, StringMapEntry<Comdat> *Name);
+  Comdat(const Comdat &) LLVM_DELETED_FUNCTION;
+
+  // Points to the map in Module.
+  StringMapEntry<Comdat> *Name;
+  SelectionKind SK;
+};
+
+inline raw_ostream &operator<<(raw_ostream &OS, const Comdat &C) {
+  C.print(OS);
+  return OS;
+}
+
+} // end llvm namespace
+
+#endif

include/llvm/IR/GlobalAlias.h

public:
   void setAliasee(Constant *Aliasee);
   const Constant *getAliasee() const {
     return const_cast<GlobalAlias *>(this)->getAliasee();
   }
   Constant *getAliasee() {
     return getOperand(0);
   }
 
+  const GlobalObject *getBaseObject() const {
+    return const_cast<GlobalAlias *>(this)->getBaseObject();
+  }
+  GlobalObject *getBaseObject() {
+    return dyn_cast<GlobalObject>(getAliasee()->stripInBoundsOffsets());
+  }
+
+  const GlobalObject *getBaseObject(const DataLayout &DL, APInt &Offset) const {
+    return const_cast<GlobalAlias *>(this)->getBaseObject(DL, Offset);
+  }
+  GlobalObject *getBaseObject(const DataLayout &DL, APInt &Offset) {
+    return dyn_cast<GlobalObject>(
+        getAliasee()->stripAndAccumulateInBoundsConstantOffsets(DL, Offset));
+  }
+
   static bool isValidLinkage(LinkageTypes L) {
     return isExternalLinkage(L) || isLocalLinkage(L) ||
       isWeakLinkage(L) || isLinkOnceLinkage(L);
   }
 
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const Value *V) {
     return V->getValueID() == Value::GlobalAliasVal;

include/llvm/IR/GlobalObject.h

 #ifndef LLVM_IR_GLOBALOBJECT_H
 #define LLVM_IR_GLOBALOBJECT_H
 
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/GlobalValue.h"
 
 namespace llvm {
-
+class Comdat;
 class Module;
 
 class GlobalObject : public GlobalValue {
   GlobalObject(const GlobalObject &) LLVM_DELETED_FUNCTION;
 
 protected:
   GlobalObject(Type *Ty, ValueTy VTy, Use *Ops, unsigned NumOps,
                LinkageTypes Linkage, const Twine &Name)
-      : GlobalValue(Ty, VTy, Ops, NumOps, Linkage, Name) {
+      : GlobalValue(Ty, VTy, Ops, NumOps, Linkage, Name), ObjComdat(nullptr) {
     setGlobalValueSubClassData(0);
   }
 
   std::string Section;     // Section to emit this into, empty means default
+  Comdat *ObjComdat;
 public:
   unsigned getAlignment() const {
     return (1u << getGlobalValueSubClassData()) >> 1;
   }
   void setAlignment(unsigned Align);
 
   bool hasSection() const { return !StringRef(getSection()).empty(); }
   const char *getSection() const { return Section.c_str(); }
   void setSection(StringRef S);
 
+  bool hasComdat() const { return getComdat() != nullptr; }
+  const Comdat *getComdat() const { return ObjComdat; }
+  Comdat *getComdat() { return ObjComdat; }
+  void setComdat(Comdat *C) { ObjComdat = C; }
+
   void copyAttributesFrom(const GlobalValue *Src) override;
 
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const Value *V) {
     return V->getValueID() == Value::FunctionVal ||
            V->getValueID() == Value::GlobalVariableVal;
   }
 };
 
 } // End llvm namespace
 
 #endif

include/llvm/IR/GlobalValue.h

 #ifndef LLVM_IR_GLOBALVALUE_H
 #define LLVM_IR_GLOBALVALUE_H
 
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/DerivedTypes.h"
 
 namespace llvm {
 
+class Comdat;
 class PointerType;
 class Module;
 
 class GlobalValue : public Constant {
   GlobalValue(const GlobalValue &) LLVM_DELETED_FUNCTION;
 public:
   /// @brief An enumeration for the kinds of linkage for global values.
   enum LinkageTypes {
~GlobalValue() {
     removeDeadConstantUsers();   // remove any dead constants using this.
   }
 
   unsigned getAlignment() const;
 
   bool hasUnnamedAddr() const { return UnnamedAddr; }
   void setUnnamedAddr(bool Val) { UnnamedAddr = Val; }
 
+  bool hasComdat() const { return getComdat() != nullptr; }
+  Comdat *getComdat();
+  const Comdat *getComdat() const {
+    return const_cast<GlobalValue *>(this)->getComdat();
+  }
+
   VisibilityTypes getVisibility() const { return VisibilityTypes(Visibility); }
   bool hasDefaultVisibility() const { return Visibility == DefaultVisibility; }
   bool hasHiddenVisibility() const { return Visibility == HiddenVisibility; }
   bool hasProtectedVisibility() const {
     return Visibility == ProtectedVisibility;
   }
   void setVisibility(VisibilityTypes V) {
     assert((!hasLocalLinkage() || V == DefaultVisibility) &&

include/llvm/IR/Module.h

 /// Module.h This file contains the declarations for the Module class.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_IR_MODULE_H
 #define LLVM_IR_MODULE_H
 
 #include "llvm/ADT/iterator_range.h"
+#include "llvm/IR/Comdat.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalAlias.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/Support/CBindingWrapping.h"
 #include "llvm/Support/DataTypes.h"
 #include <system_error>
public:
   /// The type for the list of global variables.
   typedef iplist<GlobalVariable> GlobalListType;
   /// The type for the list of functions.
   typedef iplist<Function> FunctionListType;
   /// The type for the list of aliases.
   typedef iplist<GlobalAlias> AliasListType;
   /// The type for the list of named metadata.
   typedef ilist<NamedMDNode> NamedMDListType;
+  /// The type of the comdat "symbol" table.
+  typedef StringMap<Comdat> ComdatSymTabType;
 
   /// The Global Variable iterator.
   typedef GlobalListType::iterator                      global_iterator;
   /// The Global Variable constant iterator.
   typedef GlobalListType::const_iterator          const_global_iterator;
 
   /// The Function iterators.
   typedef FunctionListType::iterator                           iterator;
private:
   LLVMContext &Context;           ///< The LLVMContext from which types and
                                   ///< constants are allocated.
   GlobalListType GlobalList;      ///< The Global Variables in the module
   FunctionListType FunctionList;  ///< The Functions in the module
   AliasListType AliasList;        ///< The Aliases in the module
   NamedMDListType NamedMDList;    ///< The named metadata in the module
   std::string GlobalScopeAsm;     ///< Inline Asm at global scope.
   ValueSymbolTable *ValSymTab;    ///< Symbol table for values
+  ComdatSymTabType ComdatSymTab;  ///< Symbol table for COMDATs
   std::unique_ptr<GVMaterializer>
   Materializer;                   ///< Used to materialize GlobalValues
   std::string ModuleID;           ///< Human readable identifier for the module
   std::string TargetTriple;       ///< Platform target triple Module compiled on
   void *NamedMDSymTab;            ///< NamedMDNode names.
   // Allow lazy initialization in const method.
   mutable RandomNumberGenerator *RNG; ///< The random number generator for this module.
 
/// @{
   /// returns a new NamedMDNode if a NamedMDNode with the specified name is not
   /// found.
   NamedMDNode *getOrInsertNamedMetadata(StringRef Name);
 
   /// Remove the given NamedMDNode from this module and delete it.
   void eraseNamedMetadata(NamedMDNode *NMD);
 
 /// @}
+/// @name Comdat Accessors
+/// @{
+
+  /// Return the Comdat in the module with the specified name. It is created
+  /// if it didn't already exist.
+  Comdat *getOrInsertComdat(StringRef Name);
+
+/// @}
 /// @name Module Flags Accessors
 /// @{
 
   /// Returns the module flags in the provided vector.
   void getModuleFlagsMetadata(SmallVectorImpl<ModuleFlagEntry> &Flags) const;
 
   /// Return the corresponding value if Key appears in module flags, otherwise
   /// return null.
/// @{
   NamedMDListType        &getNamedMDList()            { return NamedMDList; }
   static ilist<NamedMDNode> Module::*getSublistAccess(NamedMDNode*) {
     return &Module::NamedMDList;
   }
   /// Get the symbol table of global variable and function identifiers
   const ValueSymbolTable &getValueSymbolTable() const { return *ValSymTab; }
   /// Get the Module's symbol table of global variable and function identifiers.
   ValueSymbolTable       &getValueSymbolTable()       { return *ValSymTab; }
+  /// Get the Module's symbol table for COMDATs (constant).
+  const ComdatSymTabType &getComdatSymbolTable() const { return ComdatSymTab; }
+  /// Get the Module's symbol table for COMDATs.
+  ComdatSymTabType &getComdatSymbolTable() { return ComdatSymTab; }
 
 /// @}
 /// @name Global Variable Iteration
 /// @{
 
   global_iterator       global_begin()       { return GlobalList.begin(); }
   const_global_iterator global_begin() const { return GlobalList.begin(); }
   global_iterator       global_end  ()       { return GlobalList.end(); }

include/llvm/Linker/Linker.h

 #ifndef LLVM_LINKER_LINKER_H
 #define LLVM_LINKER_LINKER_H
 
 #include "llvm/ADT/SmallPtrSet.h"
 #include <string>
 
 namespace llvm {
 
+class Comdat;
+class GlobalValue;
 class Module;
 class StringRef;
 class StructType;
 
 /// This class provides the core functionality of linking in LLVM. It keeps a
 /// pointer to the merged module so far. It doesn't take ownership of the
 /// module since it is assumed that the user of this class will want to do
 /// something with it after the linking.

include/llvm/MC/MCContext.h

 #include "llvm/ADT/StringMap.h"
 #include "llvm/MC/MCDwarf.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/SectionKind.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/raw_ostream.h"
 #include <map>
+#include <tuple>
 #include <vector> // FIXME: Shouldn't be needed.
 
 namespace llvm {
   class MCAsmInfo;
   class MCExpr;
   class MCSection;
   class MCSymbol;
   class MCLabel;
private:
     /// differences between temporary and non-temporary labels (primarily on
     /// Darwin).
     bool AllowTemporaryLabels;
 
     /// The Compile Unit ID that we are currently processing.
     unsigned DwarfCompileUnitID;
 
     typedef std::pair<std::string, std::string> SectionGroupPair;
+    typedef std::tuple<std::string, std::string, int> SectionGroupTriple;
 
     StringMap<const MCSectionMachO*> MachOUniquingMap;
     std::map<SectionGroupPair, const MCSectionELF *> ELFUniquingMap;
-    std::map<SectionGroupPair, const MCSectionCOFF *> COFFUniquingMap;
+    std::map<SectionGroupTriple, const MCSectionCOFF *> COFFUniquingMap;
 
     /// Do automatic reset in destructor
     bool AutoReset;
 
     MCSymbol *CreateSymbol(StringRef Name);
 
     MCSymbol *getOrCreateDirectionalLocalSymbol(unsigned LocalLabelVal,
                                                 unsigned Instance);

lib/AsmParser/LLLexer.h

private:
     void SkipLineComment();
     lltok::Kind ReadString(lltok::Kind kind);
     bool ReadVarName();
 
     lltok::Kind LexIdentifier();
     lltok::Kind LexDigitOrNegative();
     lltok::Kind LexPositive();
     lltok::Kind LexAt();
+    lltok::Kind LexDollar();
     lltok::Kind LexExclaim();
     lltok::Kind LexPercent();
     lltok::Kind LexQuote();
     lltok::Kind Lex0x();
     lltok::Kind LexHash();
 
     uint64_t atoull(const char *Buffer, const char *End);
     uint64_t HexIntToVal(const char *Buffer, const char *End);
     void HexToIntPair(const char *Buffer, const char *End, uint64_t Pair[2]);
     void FP80HexToIntPair(const char *Buff, const char *End, uint64_t Pair[2]);
   };
 } // end namespace llvm
 
 #endif

lib/AsmParser/LLLexer.cpp

lltok::Kind LLLexer::LexToken() {
   case ' ':
   case '\t':
   case '\n':
   case '\r':
     // Ignore whitespace.
     return LexToken();
   case '+': return LexPositive();
   case '@': return LexAt();
+  case '$': return LexDollar();
   case '%': return LexPercent();
   case '"': return LexQuote();
   case '.':
     if (const char *Ptr = isLabelTail(CurPtr)) {
       CurPtr = Ptr;
       StrVal.assign(TokStart, CurPtr-1);
       return lltok::LabelStr;
     }
     if (CurPtr[0] == '.' && CurPtr[1] == '.') {
       CurPtr += 2;
       return lltok::dotdotdot;
     }
     return lltok::Error;
-  case '$':
-    if (const char *Ptr = isLabelTail(CurPtr)) {
-      CurPtr = Ptr;
-      StrVal.assign(TokStart, CurPtr-1);
-      return lltok::LabelStr;
-    }
-    return lltok::Error;
   case ';':
     SkipLineComment();
     return LexToken();
   case '!': return LexExclaim();
   case '#': return LexHash();
   case '0': case '1': case '2': case '3': case '4':
   case '5': case '6': case '7': case '8': case '9':
   case '-':
if ((unsigned)Val != Val)
       Error("invalid value number (too large)!");
     UIntVal = unsigned(Val);
     return lltok::GlobalID;
   }
 
   return lltok::Error;
 }
 
+lltok::Kind LLLexer::LexDollar() {
+  if (const char *Ptr = isLabelTail(TokStart)) {
+    CurPtr = Ptr;
+    StrVal.assign(TokStart, CurPtr - 1);
+    return lltok::LabelStr;
+  }
+
+  // Handle DollarStringConstant: $\"[^\"]*\"
+  if (CurPtr[0] == '"') {
+    ++CurPtr;
+
+    while (1) {
+      int CurChar = getNextChar();
+
+      if (CurChar == EOF) {
+        Error("end of file in COMDAT variable name");
+        return lltok::Error;
+      }
+      if (CurChar == '"') {
+        StrVal.assign(TokStart + 2, CurPtr - 1);
+        UnEscapeLexed(StrVal);
+        if (StringRef(StrVal).find_first_of(0) != StringRef::npos) {
+          Error("Null bytes are not allowed in names");
+          return lltok::Error;
+        }
+        return lltok::ComdatVar;
+      }
+    }
+  }
+
+  // Handle ComdatVarName: $[-a-zA-Z$._][-a-zA-Z$._0-9]*
+  if (ReadVarName())
+    return lltok::ComdatVar;
+
+  return lltok::Error;
+}
+
 /// ReadString - Read a string until the closing quote.
 lltok::Kind LLLexer::ReadString(lltok::Kind kind) {
   const char *Start = CurPtr;
   while (1) {
     int CurChar = getNextChar();
 
     if (CurChar == EOF) {
       Error("end of file in string constant");
#define KEYWORD(STR)                                                    \
   KEYWORD(sanitize_thread);
   KEYWORD(sanitize_memory);
   KEYWORD(uwtable);
   KEYWORD(zeroext);
 
   KEYWORD(type);
   KEYWORD(opaque);
 
+  KEYWORD(comdat);
+
+  // Comdat types
+  KEYWORD(any);
+  KEYWORD(exactmatch);
+  KEYWORD(largest);
+  KEYWORD(noduplicates);
+  KEYWORD(samesize);
+
   KEYWORD(eq); KEYWORD(ne); KEYWORD(slt); KEYWORD(sgt); KEYWORD(sle);
   KEYWORD(sge); KEYWORD(ult); KEYWORD(ugt); KEYWORD(ule); KEYWORD(uge);
   KEYWORD(oeq); KEYWORD(one); KEYWORD(olt); KEYWORD(ogt); KEYWORD(ole);
   KEYWORD(oge); KEYWORD(ord); KEYWORD(uno); KEYWORD(ueq); KEYWORD(une);
 
   KEYWORD(xchg); KEYWORD(nand); KEYWORD(max); KEYWORD(min); KEYWORD(umax);
   KEYWORD(umin);
 

lib/AsmParser/LLParser.h

namespace llvm {
   class Module;
   class OpaqueType;
   class Function;
   class Value;
   class BasicBlock;
   class Instruction;
   class Constant;
   class GlobalValue;
+  class Comdat;
   class MDString;
   class MDNode;
   class StructType;
 
   /// ValID - Represents a reference of a definition of some sort with no type.
   /// There are several cases where we have to parse the value but where the
   /// type can depend on later context.  This may either be a numeric reference
   /// or a symbolic (%var) reference.  This is just a discriminated union.
private:
     std::vector<TrackingVH<MDNode> > NumberedMetadata;
     std::map<unsigned, std::pair<TrackingVH<MDNode>, LocTy> > ForwardRefMDNodes;
 
     // Global Value reference information.
     std::map<std::string, std::pair<GlobalValue*, LocTy> > ForwardRefVals;
     std::map<unsigned, std::pair<GlobalValue*, LocTy> > ForwardRefValIDs;
     std::vector<GlobalValue*> NumberedVals;
 
+    // Comdat forward reference information.
+    std::map<std::string, LocTy> ForwardRefComdats;
+
     // References to blockaddress.  The key is the function ValID, the value is
     // a list of references to blocks in that function.
     std::map<ValID, std::vector<std::pair<ValID, GlobalValue*> > >
       ForwardRefBlockAddresses;
 
     // Attribute builder reference information.
     std::map<Value*, std::vector<unsigned> > ForwardRefAttrGroups;
     std::map<unsigned, AttrBuilder> NumberedAttrBuilders;
private:
     }
 
     /// GetGlobalVal - Get a value with the specified name or ID, creating a
     /// forward reference record if needed.  This can return null if the value
     /// exists but does not have the right type.
     GlobalValue *GetGlobalVal(const std::string &N, Type *Ty, LocTy Loc);
     GlobalValue *GetGlobalVal(unsigned ID, Type *Ty, LocTy Loc);
 
+    /// Get a Comdat with the specified name, creating a forward reference
+    /// record if needed.
+    Comdat *getComdat(const std::string &N, LocTy Loc);
+
     // Helper Routines.
     bool ParseToken(lltok::Kind T, const char *ErrMsg);
     bool EatIfPresent(lltok::Kind T) {
       if (Lex.getKind() != T) return false;
       Lex.Lex();
       return true;
     }
 
private:
     bool ParseNamedGlobal();
     bool ParseGlobal(const std::string &Name, LocTy Loc, unsigned Linkage,
                      bool HasLinkage, unsigned Visibility,
                      unsigned DLLStorageClass,
                      GlobalVariable::ThreadLocalMode TLM, bool UnnamedAddr);
     bool ParseAlias(const std::string &Name, LocTy Loc, unsigned Visibility,
                     unsigned DLLStorageClass,
                     GlobalVariable::ThreadLocalMode TLM, bool UnnamedAddr);
+    bool parseComdat();
     bool ParseStandaloneMetadata();
     bool ParseNamedMetadata();
     bool ParseMDString(MDString *&Result);
     bool ParseMDNodeID(MDNode *&Result);
     bool ParseMDNodeID(MDNode *&Result, unsigned &SlotNo);
     bool ParseUnnamedAttrGrp();
     bool ParseFnAttributeValuePairs(AttrBuilder &B,
                                     std::vector<unsigned> &FwdRefAttrGrps,
private:
     bool ParseParameterList(SmallVectorImpl<ParamInfo> &ArgList,
                             PerFunctionState &PFS);
 
     // Constant Parsing.
     bool ParseValID(ValID &ID, PerFunctionState *PFS = nullptr);
     bool ParseGlobalValue(Type *Ty, Constant *&V);
     bool ParseGlobalTypeAndValue(Constant *&V);
     bool ParseGlobalValueVector(SmallVectorImpl<Constant*> &Elts);
+    bool parseOptionalComdat(Comdat *&C);
     bool ParseMetadataListValue(ValID &ID, PerFunctionState *PFS);
     bool ParseMetadataValue(ValID &ID, PerFunctionState *PFS);
     bool ParseMDNodeVector(SmallVectorImpl<Value*> &, PerFunctionState *PFS);
     bool ParseInstructionMetadata(Instruction *Inst, PerFunctionState *PFS);
 
     // Function Parsing.
     struct ArgInfo {
       LocTy Loc;

lib/AsmParser/LLParser.cpp

if (NumberedTypes[i].second.isValid())
                    "use of undefined type '%" + Twine(i) + "'");
 
   for (StringMap<std::pair<Type*, LocTy> >::iterator I =
        NamedTypes.begin(), E = NamedTypes.end(); I != E; ++I)
     if (I->second.second.isValid())
       return Error(I->second.second,
                    "use of undefined type named '" + I->getKey() + "'");
 
+  if (!ForwardRefComdats.empty())
+    return Error(ForwardRefComdats.begin()->second,
+                 "use of undefined comdat '$" +
+                     ForwardRefComdats.begin()->first + "'");
+
   if (!ForwardRefVals.empty())
     return Error(ForwardRefVals.begin()->second.second,
                  "use of undefined value '@" + ForwardRefVals.begin()->first +
                  "'");
 
   if (!ForwardRefValIDs.empty())
     return Error(ForwardRefValIDs.begin()->second.second,
                  "use of undefined value '@" +
while (1) {
     case lltok::kw_define:  if (ParseDefine()) return true; break;
     case lltok::kw_module:  if (ParseModuleAsm()) return true; break;
     case lltok::kw_target:  if (ParseTargetDefinition()) return true; break;
     case lltok::kw_deplibs: if (ParseDepLibs()) return true; break;
     case lltok::LocalVarID: if (ParseUnnamedType()) return true; break;
     case lltok::LocalVar:   if (ParseNamedType()) return true; break;
     case lltok::GlobalID:   if (ParseUnnamedGlobal()) return true; break;
     case lltok::GlobalVar:  if (ParseNamedGlobal()) return true; break;
+    case lltok::ComdatVar:  if (parseComdat()) return true; break;
     case lltok::exclaim:    if (ParseStandaloneMetadata()) return true; break;
     case lltok::MetadataVar:if (ParseNamedMetadata()) return true; break;
 
     // The Global variable production with no name can have many different
     // optional leading prefixes, the production is:
     // GlobalVar ::= OptionalLinkage OptionalVisibility OptionalDLLStorageClass
     //               OptionalThreadLocal OptionalAddrSpace OptionalUnNammedAddr
     //               ('constant'|'global') ...
bool LLParser::ParseNamedGlobal() {
 
   if (HasLinkage || Lex.getKind() != lltok::kw_alias)
     return ParseGlobal(Name, NameLoc, Linkage, HasLinkage, Visibility,
                        DLLStorageClass, TLM, UnnamedAddr);
   return ParseAlias(Name, NameLoc, Visibility, DLLStorageClass, TLM,
                     UnnamedAddr);
 }
 
+bool LLParser::parseComdat() {
+  assert(Lex.getKind() == lltok::ComdatVar);
+  std::string Name = Lex.getStrVal();
+  LocTy NameLoc = Lex.getLoc();
+  Lex.Lex();
+
+  if (ParseToken(lltok::equal, "expected '=' here"))
+    return true;
+
+  if (ParseToken(lltok::kw_comdat, "expected comdat keyword"))
+    return TokError("expected comdat type");
+
+  Comdat::SelectionKind SK;
+  switch (Lex.getKind()) {
+  default:
+    return TokError("unknown selection kind");
+  case lltok::kw_any:
+    SK = Comdat::Any;
+    break;
+  case lltok::kw_exactmatch:
+    SK = Comdat::ExactMatch;
+    break;
+  case lltok::kw_largest:
+    SK = Comdat::Largest;
+    break;
+  case lltok::kw_noduplicates:
+    SK = Comdat::NoDuplicates;
+    break;
+  case lltok::kw_samesize:
+    SK = Comdat::SameSize;
+    break;
+  }
+  Lex.Lex();
+
+  // See if the comdat was forward referenced, if so, use the comdat.
+  Module::ComdatSymTabType &ComdatSymTab = M->getComdatSymbolTable();
+  Module::ComdatSymTabType::iterator I = ComdatSymTab.find(Name);
+  if (I != ComdatSymTab.end() && !ForwardRefComdats.erase(Name))
+    return Error(NameLoc, "redefinition of comdat '$" + Name + "'");
+
+  Comdat *C;
+  if (I != ComdatSymTab.end())
+    C = &I->second;
+  else
+    C = M->getOrInsertComdat(Name);
+  C->setSelectionKind(SK);
+
+  return false;
+}
+
 // MDString:
 //   ::= '!' STRINGCONSTANT
 bool LLParser::ParseMDString(MDString *&Result) {
   std::string Str;
   if (ParseStringConstant(Str)) return true;
   llvm::UpgradeMDStringConstant(Str);
   Result = MDString::get(Context, Str);
   return false;
if (Lex.getKind() == lltok::kw_section) {
       GV->setSection(Lex.getStrVal());
       if (ParseToken(lltok::StringConstant, "expected global section string"))
         return true;
     } else if (Lex.getKind() == lltok::kw_align) {
       unsigned Alignment;
       if (ParseOptionalAlignment(Alignment)) return true;
       GV->setAlignment(Alignment);
     } else {
-      TokError("unknown global variable property!");
+      Comdat *C;
+      if (parseOptionalComdat(C))
+        return true;
+      if (C)
+        GV->setComdat(C);
+      else
+        return TokError("unknown global variable property!");
     }
   }
 
   return false;
 }
 
 /// ParseUnnamedAttrGrp
 ///   ::= 'attributes' AttrGrpID '=' '{' AttrValPair+ '}'
FwdVal = new GlobalVariable(*M, PTy->getElementType(), false,
                                 GlobalValue::ExternalWeakLinkage, nullptr, "");
 
   ForwardRefValIDs[ID] = std::make_pair(FwdVal, Loc);
   return FwdVal;
 }
 
 
 //===----------------------------------------------------------------------===//
+// Comdat Reference/Resolution Routines.
+//===----------------------------------------------------------------------===//
+
+Comdat *LLParser::getComdat(const std::string &Name, LocTy Loc) {
+  // Look this name up in the comdat symbol table.
+  Module::ComdatSymTabType &ComdatSymTab = M->getComdatSymbolTable();
+  Module::ComdatSymTabType::iterator I = ComdatSymTab.find(Name);
+  if (I != ComdatSymTab.end())
+    return &I->second;
+
+  // Otherwise, create a new forward reference for this value and remember it.
+  Comdat *C = M->getOrInsertComdat(Name);
+  ForwardRefComdats[Name] = Loc;
+  return C;
+}
+
+
+//===----------------------------------------------------------------------===//
 // Helper Routines.
 //===----------------------------------------------------------------------===//
 
 /// ParseToken - If the current token has the specified kind, eat it and return
 /// success.  Otherwise, emit the specified error and return failure.
 bool LLParser::ParseToken(lltok::Kind T, const char *ErrMsg) {
   if (Lex.getKind() != T)
     return TokError(ErrMsg);

 }
 
 bool LLParser::ParseGlobalTypeAndValue(Constant *&V) {
   Type *Ty = nullptr;
   return ParseType(Ty) ||
          ParseGlobalValue(Ty, V);
 }
 
+bool LLParser::parseOptionalComdat(Comdat *&C) {
+  C = nullptr;
+  if (!EatIfPresent(lltok::kw_comdat))
+    return false;
+  if (Lex.getKind() != lltok::ComdatVar)
+    return TokError("expected comdat variable");
+  LocTy Loc = Lex.getLoc();
+  StringRef Name = Lex.getStrVal();
+  C = getComdat(Name, Loc);
+  Lex.Lex();
+  return false;
+}
+
 /// ParseGlobalValueVector
 ///   ::= /*empty*/
 ///   ::= TypeAndValue (',' TypeAndValue)*
 bool LLParser::ParseGlobalValueVector(SmallVectorImpl<Constant*> &Elts) {
   // Empty list.
   if (Lex.getKind() == lltok::rbrace ||
       Lex.getKind() == lltok::rsquare ||
       Lex.getKind() == lltok::greater ||
bool LLParser::ParseFunctionHeader(Function *&Fn, bool isDefine) {
   std::vector<unsigned> FwdRefAttrGrps;
   LocTy BuiltinLoc;
   std::string Section;
   unsigned Alignment;
   std::string GC;
   bool UnnamedAddr;
   LocTy UnnamedAddrLoc;
   Constant *Prefix = nullptr;
+  Comdat *C;
 
   if (ParseArgumentList(ArgList, isVarArg) ||
       ParseOptionalToken(lltok::kw_unnamed_addr, UnnamedAddr,
                          &UnnamedAddrLoc) ||
       ParseFnAttributeValuePairs(FuncAttrs, FwdRefAttrGrps, false,
                                  BuiltinLoc) ||
       (EatIfPresent(lltok::kw_section) &&
        ParseStringConstant(Section)) ||
+      parseOptionalComdat(C) ||
       ParseOptionalAlignment(Alignment) ||
       (EatIfPresent(lltok::kw_gc) &&
        ParseStringConstant(GC)) ||
       (EatIfPresent(lltok::kw_prefix) &&
        ParseGlobalTypeAndValue(Prefix)))
     return true;
 
   if (FuncAttrs.contains(Attribute::Builtin))
bool LLParser::ParseFunctionHeader(Function *&Fn, bool isDefine) {
   Fn->setLinkage((GlobalValue::LinkageTypes)Linkage);
   Fn->setVisibility((GlobalValue::VisibilityTypes)Visibility);
   Fn->setDLLStorageClass((GlobalValue::DLLStorageClassTypes)DLLStorageClass);
   Fn->setCallingConv(CC);
   Fn->setAttributes(PAL);
   Fn->setUnnamedAddr(UnnamedAddr);
   Fn->setAlignment(Alignment);
   Fn->setSection(Section);
+  Fn->setComdat(C);
   if (!GC.empty()) Fn->setGC(GC.c_str());
   Fn->setPrefixData(Prefix);
   ForwardRefAttrGroups[Fn] = FwdRefAttrGrps;
 
   // Add all of the arguments we parsed to the function.
   Function::arg_iterator ArgIt = Fn->arg_begin();
   for (unsigned i = 0, e = ArgList.size(); i != e; ++i, ++ArgIt) {
     // If the argument has a name, insert it into the argument symbol table.

lib/AsmParser/LLToken.h

enum Kind {
     kw_sanitize_thread,
     kw_sanitize_memory,
     kw_uwtable,
     kw_zeroext,
 
     kw_type,
     kw_opaque,
 
+    kw_comdat,
+
+    // Comdat types
+    kw_any,
+    kw_exactmatch,
+    kw_largest,
+    kw_noduplicates,
+    kw_samesize,
+
     kw_eq, kw_ne, kw_slt, kw_sgt, kw_sle, kw_sge, kw_ult, kw_ugt, kw_ule,
     kw_uge, kw_oeq, kw_one, kw_olt, kw_ogt, kw_ole, kw_oge, kw_ord, kw_uno,
     kw_ueq, kw_une,
 
     // atomicrmw operations that aren't also instruction keywords.
     kw_xchg, kw_nand, kw_max, kw_min, kw_umax, kw_umin,
 
     // Instruction Opcodes (Opcode in UIntVal).
enum Kind {
     // Unsigned Valued tokens (UIntVal).
     GlobalID,          // @42
     LocalVarID,        // %42
     AttrGrpID,         // #42
 
     // String valued tokens (StrVal).
     LabelStr,          // foo:
     GlobalVar,         // @foo @"foo"
+    ComdatVar,         // $foo
     LocalVar,          // %foo %"foo"
     MetadataVar,       // !foo
     StringConstant,    // "foo"
 
     // Type valued tokens (TyVal).
     Type,
 
     APFloat,  // APFloatVal
     APSInt // APSInt
   };
 } // end namespace lltok
 } // end namespace llvm
 
 #endif

lib/Bitcode/Reader/BitcodeReader.h

 #include "llvm/IR/GVMaterializer.h"
 #include "llvm/IR/OperandTraits.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/ValueHandle.h"
 #include <system_error>
 #include <vector>
 
 namespace llvm {
+  class Comdat;
   class MemoryBuffer;
   class LLVMContext;
 
 //===----------------------------------------------------------------------===//
 //                          BitcodeReaderValueList Class
 //===----------------------------------------------------------------------===//
 
 class BitcodeReaderValueList {
class BitcodeReader : public GVMaterializer {
   BitstreamCursor Stream;
   DataStreamer *LazyStreamer;
   uint64_t NextUnreadBit;
   bool SeenValueSymbolTable;
 
   std::vector<Type*> TypeList;
   BitcodeReaderValueList ValueList;
   BitcodeReaderMDValueList MDValueList;
+  std::vector<Comdat *> ComdatList;
   SmallVector<Instruction *, 64> InstructionList;
   SmallVector<SmallVector<uint64_t, 64>, 64> UseListRecords;
 
   std::vector<std::pair<GlobalVariable*, unsigned> > GlobalInits;
   std::vector<std::pair<GlobalAlias*, unsigned> > AliasInits;
   std::vector<std::pair<Function*, unsigned> > FunctionPrefixes;
 
   SmallVector<Instruction*, 64> InstsWithTBAATag;

lib/Bitcode/Reader/BitcodeReader.cpp

void BitcodeReader::materializeForwardReferencedFunctions() {
   }
 }
 
 void BitcodeReader::FreeState() {
   Buffer = nullptr;
   std::vector<Type*>().swap(TypeList);
   ValueList.clear();
   MDValueList.clear();
+  std::vector<Comdat *>().swap(ComdatList);
 
   std::vector<AttributeSet>().swap(MAttributes);
   std::vector<BasicBlock*>().swap(FunctionBBs);
   std::vector<Function*>().swap(FunctionsWithBodies);
   DeferredFunctionInfo.clear();
   MDKindMap.clear();
 
   assert(BlockAddrFwdRefs.empty() && "Unresolved blockaddress fwd references");

 static SynchronizationScope GetDecodedSynchScope(unsigned Val) {
   switch (Val) {
   case bitc::SYNCHSCOPE_SINGLETHREAD: return SingleThread;
   default: // Map unknown scopes to cross-thread.
   case bitc::SYNCHSCOPE_CROSSTHREAD: return CrossThread;
   }
 }
 
+static Comdat::SelectionKind getDecodedComdatSelectionKind(unsigned Val) {
+  switch (Val) {
+  default: // Map unknown selection kinds to any.
+  case bitc::COMDAT_SELECTION_KIND_ANY:
+    return Comdat::Any;
+  case bitc::COMDAT_SELECTION_KIND_EXACT_MATCH:
+    return Comdat::ExactMatch;
+  case bitc::COMDAT_SELECTION_KIND_LARGEST:
+    return Comdat::Largest;
+  case bitc::COMDAT_SELECTION_KIND_NO_DUPLICATES:
+    return Comdat::NoDuplicates;
+  case bitc::COMDAT_SELECTION_KIND_SAME_SIZE:
+    return Comdat::SameSize;
+  }
+}
+
 static void UpgradeDLLImportExportLinkage(llvm::GlobalValue *GV, unsigned Val) {
   switch (Val) {
   case 5: GV->setDLLStorageClass(GlobalValue::DLLImportStorageClass); break;
   case 6: GV->setDLLStorageClass(GlobalValue::DLLExportStorageClass); break;
   }
 }
 
 namespace llvm {
while (1) {
     }
     case bitc::MODULE_CODE_GCNAME: {  // SECTIONNAME: [strchr x N]
       std::string S;
       if (ConvertToString(Record, 0, S))
         return Error(InvalidRecord);
       GCTable.push_back(S);
       break;
     }
+    case bitc::MODULE_CODE_COMDAT: { // COMDAT: [selection_kind, name]
+      if (Record.size() < 2)
+        return Error(InvalidRecord);
+      Comdat::SelectionKind SK = getDecodedComdatSelectionKind(Record[0]);
+      unsigned ComdatNameSize = Record[1];
+      std::string ComdatName;
+      ComdatName.reserve(ComdatNameSize);
+      for (unsigned i = 0; i != ComdatNameSize; ++i)
+        ComdatName += (char)Record[2 + i];
+      Comdat *C = TheModule->getOrInsertComdat(ComdatName);
+      C->setSelectionKind(SK);
+      ComdatList.push_back(C);
+      break;
+    }
     // GLOBALVAR: [pointer type, isconst, initid,
     //             linkage, alignment, section, visibility, threadlocal,
     //             unnamed_addr, dllstorageclass]
     case bitc::MODULE_CODE_GLOBALVAR: {
       if (Record.size() < 6)
         return Error(InvalidRecord);
       Type *Ty = getTypeByID(Record[0]);
       if (!Ty)
case bitc::MODULE_CODE_GLOBALVAR: {
       else
         UpgradeDLLImportExportLinkage(NewGV, Record[3]);
 
       ValueList.push_back(NewGV);
 
       // Remember which value to use for the global initializer.
       if (unsigned InitID = Record[2])
         GlobalInits.push_back(std::make_pair(NewGV, InitID-1));
+
+      if (Record.size() > 11)
+        if (unsigned ComdatID = Record[11]) {
+          assert(ComdatID <= ComdatList.size());
+          NewGV->setComdat(ComdatList[ComdatID - 1]);
+        }
       break;
     }
     // FUNCTION:  [type, callingconv, isproto, linkage, paramattr,
     //             alignment, section, visibility, gc, unnamed_addr,
     //             dllstorageclass]
     case bitc::MODULE_CODE_FUNCTION: {
       if (Record.size() < 8)
         return Error(InvalidRecord);
case bitc::MODULE_CODE_FUNCTION: {
       if (Record.size() > 10 && Record[10] != 0)
         FunctionPrefixes.push_back(std::make_pair(Func, Record[10]-1));
 
       if (Record.size() > 11)
         Func->setDLLStorageClass(GetDecodedDLLStorageClass(Record[11]));
       else
         UpgradeDLLImportExportLinkage(Func, Record[3]);
 
+      if (Record.size() > 12)
+        if (unsigned ComdatID = Record[12]) {
+          assert(ComdatID <= ComdatList.size());
+          Func->setComdat(ComdatList[ComdatID - 1]);
+        }
+
       ValueList.push_back(Func);
 
       // If this is a function with a body, remember the prototype we are
       // creating now, so that we can match up the body with them later.
       if (!isProto) {
         FunctionsWithBodies.push_back(Func);
         if (LazyStreamer) DeferredFunctionInfo[Func] = 0;
       }

lib/Bitcode/Writer/BitcodeWriter.cpp

switch (GV.getThreadLocalMode()) {
     case GlobalVariable::GeneralDynamicTLSModel: return 1;
     case GlobalVariable::LocalDynamicTLSModel:   return 2;
     case GlobalVariable::InitialExecTLSModel:    return 3;
     case GlobalVariable::LocalExecTLSModel:      return 4;
   }
   llvm_unreachable("Invalid TLS model");
 }
 
+static unsigned getEncodedComdatSelectionKind(const Comdat &C) {
+  switch (C.getSelectionKind()) {
+  case Comdat::Any:
+    return bitc::COMDAT_SELECTION_KIND_ANY;
+  case Comdat::ExactMatch:
+    return bitc::COMDAT_SELECTION_KIND_EXACT_MATCH;
+  case Comdat::Largest:
+    return bitc::COMDAT_SELECTION_KIND_LARGEST;
+  case Comdat::NoDuplicates:
+    return bitc::COMDAT_SELECTION_KIND_NO_DUPLICATES;
+  case Comdat::SameSize:
+    return bitc::COMDAT_SELECTION_KIND_SAME_SIZE;
+  }
+  llvm_unreachable("Invalid selection kind");
+}
+
+static void writeComdats(const ValueEnumerator &VE, BitstreamWriter &Stream) {
+  SmallVector<uint8_t, 64> Vals;
+  for (const Comdat *C : VE.getComdats()) {
+    // COMDAT: [selection_kind, name]
+    Vals.push_back(getEncodedComdatSelectionKind(*C));
+    Vals.push_back(C->getName().size());
+    for (char Chr : C->getName())
+      Vals.push_back((unsigned char)Chr);
+    Stream.EmitRecord(bitc::MODULE_CODE_COMDAT, Vals, /*AbbrevToUse=*/0);
+    Vals.clear();
+  }
+}
+
 // Emit top-level description of module, including target triple, inline asm,
 // descriptors for global variables, and function prototype info.
 static void WriteModuleInfo(const Module *M, const ValueEnumerator &VE,
                             BitstreamWriter &Stream) {
   // Emit various pieces of data attached to a module.
   if (!M->getTargetTriple().empty())
     WriteStringRecord(bitc::MODULE_CODE_TRIPLE, M->getTargetTriple(),
                       0/*TODO*/, Stream);
for (const GlobalVariable &GV : M->globals()) {
     Vals.push_back(GV.isDeclaration() ? 0 :
                    (VE.getValueID(GV.getInitializer()) + 1));
     Vals.push_back(getEncodedLinkage(GV));
     Vals.push_back(Log2_32(GV.getAlignment())+1);
     Vals.push_back(GV.hasSection() ? SectionMap[GV.getSection()] : 0);
     if (GV.isThreadLocal() ||
         GV.getVisibility() != GlobalValue::DefaultVisibility ||
         GV.hasUnnamedAddr() || GV.isExternallyInitialized() ||
-        GV.getDLLStorageClass() != GlobalValue::DefaultStorageClass) {
+        GV.getDLLStorageClass() != GlobalValue::DefaultStorageClass ||
+        GV.hasComdat()) {
       Vals.push_back(getEncodedVisibility(GV));
       Vals.push_back(getEncodedThreadLocalMode(GV));
       Vals.push_back(GV.hasUnnamedAddr());
       Vals.push_back(GV.isExternallyInitialized());
       Vals.push_back(getEncodedDLLStorageClass(GV));
+      Vals.push_back(GV.hasComdat() ? VE.getComdatID(GV.getComdat()) : 0);
     } else {
       AbbrevToUse = SimpleGVarAbbrev;
     }
 
     Stream.EmitRecord(bitc::MODULE_CODE_GLOBALVAR, Vals, AbbrevToUse);
     Vals.clear();
   }
 
for (const Function &F : *M) {
     Vals.push_back(Log2_32(F.getAlignment())+1);
     Vals.push_back(F.hasSection() ? SectionMap[F.getSection()] : 0);
     Vals.push_back(getEncodedVisibility(F));
     Vals.push_back(F.hasGC() ? GCMap[F.getGC()] : 0);
     Vals.push_back(F.hasUnnamedAddr());
     Vals.push_back(F.hasPrefixData() ? (VE.getValueID(F.getPrefixData()) + 1)
                                       : 0);
     Vals.push_back(getEncodedDLLStorageClass(F));
+    Vals.push_back(F.hasComdat() ? VE.getComdatID(F.getComdat()) : 0);
 
     unsigned AbbrevToUse = 0;
     Stream.EmitRecord(bitc::MODULE_CODE_FUNCTION, Vals, AbbrevToUse);
     Vals.clear();
   }
 
   // Emit the alias information.
   for (const GlobalAlias &A : M->aliases()) {
static void WriteModule(const Module *M, BitstreamWriter &Stream) {
   WriteAttributeGroupTable(VE, Stream);
 
   // Emit information about parameter attributes.
   WriteAttributeTable(VE, Stream);
 
   // Emit information describing all of the types in the module.
   WriteTypeTable(VE, Stream);
 
+  writeComdats(VE, Stream);
+
   // Emit top-level description of module, including target triple, inline asm,
   // descriptors for global variables, and function prototype info.
   WriteModuleInfo(M, VE, Stream);
 
   // Emit constants.
   WriteModuleConstants(VE, Stream);
 
   // Emit metadata.

lib/Bitcode/Writer/ValueEnumerator.h

 //
 //===----------------------------------------------------------------------===//
 
 #ifndef VALUE_ENUMERATOR_H
 #define VALUE_ENUMERATOR_H
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/UniqueVector.h"
 #include "llvm/IR/Attributes.h"
 #include <vector>
 
 namespace llvm {
 
 class Type;
 class Value;
 class Instruction;
 class BasicBlock;
+class Comdat;
 class Function;
 class Module;
 class MDNode;
 class NamedMDNode;
 class AttributeSet;
 class ValueSymbolTable;
 class MDSymbolTable;
 class raw_ostream;
 
 class ValueEnumerator {
 public:
   typedef std::vector<Type*> TypeList;
 
   // For each value, we remember its Value* and occurrence frequency.
   typedef std::vector<std::pair<const Value*, unsigned> > ValueList;
 private:
   typedef DenseMap<Type*, unsigned> TypeMapType;
   TypeMapType TypeMap;
   TypeList Types;
 
   typedef DenseMap<const Value*, unsigned> ValueMapType;
   ValueMapType ValueMap;
   ValueList Values;
+
+  typedef UniqueVector<const Comdat *> ComdatSetType;
+  ComdatSetType Comdats;
+
   ValueList MDValues;
   SmallVector<const MDNode *, 8> FunctionLocalMDs;
   ValueMapType MDValueMap;
 
   typedef DenseMap<AttributeSet, unsigned> AttributeGroupMapType;
   AttributeGroupMapType AttributeGroupMap;
   std::vector<AttributeSet> AttributeGroups;
 
public:
   }
   const std::vector<AttributeSet> &getAttributes() const {
     return Attribute;
   }
   const std::vector<AttributeSet> &getAttributeGroups() const {
     return AttributeGroups;
   }
 
+  const ComdatSetType &getComdats() const { return Comdats; }
+  unsigned getComdatID(const Comdat *C) const;
+
   /// getGlobalBasicBlockID - This returns the function-specific ID for the
   /// specified basic block.  This is relatively expensive information, so it
   /// should only be used by rare constructs such as address-of-label.
   unsigned getGlobalBasicBlockID(const BasicBlock *BB) const;
 
   /// incorporateFunction/purgeFunction - If you'd like to deal with a function,
   /// use these two methods to get its data into the ValueEnumerator!
   ///

lib/Bitcode/Writer/ValueEnumerator.cpp

 }
 
 unsigned ValueEnumerator::getInstructionID(const Instruction *Inst) const {
   InstructionMapType::const_iterator I = InstructionMap.find(Inst);
   assert(I != InstructionMap.end() && "Instruction is not mapped!");
   return I->second;
 }
 
+unsigned ValueEnumerator::getComdatID(const Comdat *C) const {
+  unsigned ComdatID = Comdats.idFor(C);
+  assert(ComdatID && "Comdat not found!");
+  return ComdatID;
+}
+
 void ValueEnumerator::setInstructionID(const Instruction *I) {
   InstructionMap[I] = InstructionCount++;
 }
 
 unsigned ValueEnumerator::getValueID(const Value *V) const {
   if (isa<MDNode>(V) || isa<MDString>(V)) {
     ValueMapType::const_iterator I = MDValueMap.find(V);
     assert(I != MDValueMap.end() && "Value not in slotcalculator!");
void ValueEnumerator::EnumerateValue(const Value *V) {
   // Check to see if it's already in!
   unsigned &ValueID = ValueMap[V];
   if (ValueID) {
     // Increment use count.
     Values[ValueID-1].second++;
     return;
   }
 
+  if (auto *GO = dyn_cast<GlobalObject>(V))
+    if (const Comdat *C = GO->getComdat())
+      Comdats.insert(C);
+
   // Enumerate the type of this value.
   EnumerateType(V->getType());
 
   if (const Constant *C = dyn_cast<Constant>(V)) {
     if (isa<GlobalValue>(C)) {
       // Initializers for globals are handled explicitly elsewhere.
     } else if (C->getNumOperands()) {
       // If a constant has operands, enumerate them.  This makes sure that if a

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

if (K.isMergeableCString() || K.isMergeableConst4() ||
     Flags |= ELF::SHF_MERGE;
 
   if (K.isMergeableCString())
     Flags |= ELF::SHF_STRINGS;
 
   return Flags;
 }
 
+static const Comdat *getELFComdat(const GlobalValue *GV) {
+  const Comdat *C = GV->getComdat();
+  if (!C)
+    return nullptr;
+
+  if (C->getSelectionKind() != Comdat::Any)
+    report_fatal_error("ELF COMDATs only support SelectionKind::Any, '" +
+                       C->getName() + "' cannot be lowered.");
+
+  return C;
+}
+
 const MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
     const GlobalValue *GV, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM) const {
   StringRef SectionName = GV->getSection();
 
   // Infer section flags from the section name if we can.
   Kind = getELFKindForNamedSection(SectionName, Kind);
 
+  StringRef Group = "";
+  unsigned Flags = getELFSectionFlags(Kind);
+  if (const Comdat *C = getELFComdat(GV)) {
+    Group = C->getName();
+    Flags |= ELF::SHF_GROUP;
+  }
   return getContext().getELFSection(SectionName,
-                                    getELFSectionType(SectionName, Kind),
-                                    getELFSectionFlags(Kind), Kind);
+                                    getELFSectionType(SectionName, Kind), Flags,
+                                    Kind, /*EntrySize=*/0, Group);
 }
 
 /// getSectionPrefixForGlobal - Return the section prefix name used by options
 /// FunctionsSections and DataSections.
 static StringRef getSectionPrefixForGlobal(SectionKind Kind) {
   if (Kind.isText())                 return ".text.";
   if (Kind.isReadOnly())             return ".rodata.";
   if (Kind.isBSS())                  return ".bss.";
 
   if (Kind.isThreadData())           return ".tdata.";
   if (Kind.isThreadBSS())            return ".tbss.";
 
   if (Kind.isDataNoRel())            return ".data.";
   if (Kind.isDataRelLocal())         return ".data.rel.local.";
   if (Kind.isDataRel())              return ".data.rel.";
   if (Kind.isReadOnlyWithRelLocal()) return ".data.rel.ro.local.";
 
   assert(Kind.isReadOnlyWithRel() && "Unknown section kind");
   return ".data.rel.ro.";
 }
 
-
 const MCSection *TargetLoweringObjectFileELF::
 SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
                        Mangler &Mang, const TargetMachine &TM) const {
   // If we have -ffunction-section or -fdata-section then we should emit the
   // global value to a uniqued section specifically for it.
   bool EmitUniquedSection;
   if (Kind.isText())
     EmitUniquedSection = TM.getFunctionSections();
   else
     EmitUniquedSection = TM.getDataSections();
 
   // If this global is linkonce/weak and the target handles this by emitting it
   // into a 'uniqued' section name, create and return the section now.
-  if ((GV->isWeakForLinker() || EmitUniquedSection) &&
+  if ((GV->isWeakForLinker() || EmitUniquedSection || GV->hasComdat()) &&
       !Kind.isCommon()) {
     StringRef Prefix = getSectionPrefixForGlobal(Kind);
 
     SmallString<128> Name(Prefix);
     TM.getNameWithPrefix(Name, GV, Mang, true);
 
     StringRef Group = "";
     unsigned Flags = getELFSectionFlags(Kind);
-    if (GV->isWeakForLinker()) {
+    if (GV->isWeakForLinker() || GV->hasComdat()) {
+      if (const Comdat *C = getELFComdat(GV))
+        Group = C->getName();
+      else
         Group = Name.substr(Prefix.size());
       Flags |= ELF::SHF_GROUP;
     }
 
     return getContext().getELFSection(Name.str(),
                                       getELFSectionType(Name.str(), Kind),
                                       Flags, Kind, 0, Group);
   }
 
emitModuleFlags(MCStreamer &Streamer,
   Streamer.SwitchSection(S);
   Streamer.EmitLabel(getContext().
                      GetOrCreateSymbol(StringRef("L_OBJC_IMAGE_INFO")));
   Streamer.EmitIntValue(VersionVal, 4);
   Streamer.EmitIntValue(ImageInfoFlags, 4);
   Streamer.AddBlankLine();
 }
 
+static void checkMachOComdat(const GlobalValue *GV) {
+  const Comdat *C = GV->getComdat();
+  if (!C)
+    return;
+
+  report_fatal_error("MachO doesn't support COMDATs, '" + C->getName() +
+                     "' cannot be lowered.");
+}
+
 const MCSection *TargetLoweringObjectFileMachO::getExplicitSectionGlobal(
     const GlobalValue *GV, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM) const {
   // Parse the section specifier and create it if valid.
   StringRef Segment, Section;
   unsigned TAA = 0, StubSize = 0;
   bool TAAParsed;
+
+  checkMachOComdat(GV);
+
   std::string ErrorCode =
     MCSectionMachO::ParseSectionSpecifier(GV->getSection(), Segment, Section,
                                           TAA, TAAParsed, StubSize);
   if (!ErrorCode.empty()) {
     // If invalid, report the error with report_fatal_error.
     report_fatal_error("Global variable '" + GV->getName() +
                        "' has an invalid section specifier '" +
                        GV->getSection() + "': " + ErrorCode + ".");
bool TargetLoweringObjectFileMachO::isSectionAtomizableBySymbols(
     case MachO::S_INTERPOSING:
       return false;
     }
 }
 
 const MCSection *TargetLoweringObjectFileMachO::
 SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
                        Mangler &Mang, const TargetMachine &TM) const {
+  checkMachOComdat(GV);
 
   // Handle thread local data.
   if (Kind.isThreadBSS()) return TLSBSSSection;
   if (Kind.isThreadData()) return TLSDataSection;
 
   if (Kind.isText())
     return GV->isWeakForLinker() ? TextCoalSection : TextSection;
 
   // If this is weak/linkonce, put this in a coalescable section, either in text
   // or data depending on if it is writable.
   if (GV->isWeakForLinker()) {
getCOFFSectionFlags(SectionKind K) {
   else if (K.isWriteable())
     Flags |=
       COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
       COFF::IMAGE_SCN_MEM_READ |
       COFF::IMAGE_SCN_MEM_WRITE;
 
   return Flags;
 }
 
+static int getSelectionForCOFF(const GlobalValue *GV) {
+  if (const Comdat *C = GV->getComdat()) {
+    if (C->getName() == GV->getName()) {
+      switch (C->getSelectionKind()) {
+      case Comdat::Any:
+        return COFF::IMAGE_COMDAT_SELECT_ANY;
+      case Comdat::ExactMatch:
+        return COFF::IMAGE_COMDAT_SELECT_EXACT_MATCH;
+      case Comdat::Largest:
+        return COFF::IMAGE_COMDAT_SELECT_LARGEST;
+      case Comdat::NoDuplicates:
+        return COFF::IMAGE_COMDAT_SELECT_NODUPLICATES;
+      case Comdat::SameSize:
+        return COFF::IMAGE_COMDAT_SELECT_SAME_SIZE;
+      }
+    } else {
+      return COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE;
+    }
+  } else if (GV->isWeakForLinker()) {
+    return COFF::IMAGE_COMDAT_SELECT_ANY;
+  }
+  return 0;
+}
+
+const GlobalValue *getComdatGVForCOFF(const GlobalValue *GV) {
+  const Comdat *C = GV->getComdat();
+  assert(C && "expected GV to have a Comdat!");
+
+  StringRef ComdatGVName = C->getName();
+  const GlobalValue *ComdatGV = GV->getParent()->getNamedValue(ComdatGVName);
+  if (!ComdatGV)
+    report_fatal_error("Associative COMDAT symbol '" + ComdatGVName +
+                       "' does not exist.");
+
+  if (ComdatGV->getComdat() != C)
+    report_fatal_error("Associative COMDAT symbol '" + ComdatGVName +
+                       "' is not a key for it's COMDAT.");
+
+  return ComdatGV;
+}
+
 const MCSection *TargetLoweringObjectFileCOFF::getExplicitSectionGlobal(
     const GlobalValue *GV, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM) const {
   int Selection = 0;
   unsigned Characteristics = getCOFFSectionFlags(Kind);
   StringRef Name = GV->getSection();
   StringRef COMDATSymName = "";
-  if (GV->isWeakForLinker()) {
-    Selection = COFF::IMAGE_COMDAT_SELECT_ANY;
-    Characteristics |= COFF::IMAGE_SCN_LNK_COMDAT;
-    MCSymbol *Sym = TM.getSymbol(GV, Mang);
+  if ((GV->isWeakForLinker() || GV->hasComdat()) && !Kind.isCommon()) {
+    Selection = getSelectionForCOFF(GV);
+    if (Selection == COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE ||
+        !GV->hasPrivateLinkage()) {
+      const GlobalValue *ComdatGV;
+      if (Selection == COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE)
+        ComdatGV = getComdatGVForCOFF(GV);
+      else
+        ComdatGV = GV;
+      MCSymbol *Sym = TM.getSymbol(ComdatGV, Mang);
       COMDATSymName = Sym->getName();
+      Characteristics |= COFF::IMAGE_SCN_LNK_COMDAT;
+    } else {
+      Selection = 0;
+    }
   }
   return getContext().getCOFFSection(Name,
                                      Characteristics,
                                      Kind,
                                      COMDATSymName,
                                      Selection);
 }
 
if (Kind.isText())
     EmitUniquedSection = TM.getFunctionSections();
   else
     EmitUniquedSection = TM.getDataSections();
 
   // If this global is linkonce/weak and the target handles this by emitting it
   // into a 'uniqued' section name, create and return the section now.
   // Section names depend on the name of the symbol which is not feasible if the
   // symbol has private linkage.
-  if ((GV->isWeakForLinker() || EmitUniquedSection) &&
-      !GV->hasPrivateLinkage() && !Kind.isCommon()) {
+  if ((GV->isWeakForLinker() || EmitUniquedSection || GV->hasComdat()) &&
+      !Kind.isCommon()) {
     const char *Name = getCOFFSectionNameForUniqueGlobal(Kind);
     unsigned Characteristics = getCOFFSectionFlags(Kind);
 
     Characteristics |= COFF::IMAGE_SCN_LNK_COMDAT;
-    MCSymbol *Sym = TM.getSymbol(GV, Mang);
-    return getContext().getCOFFSection(
-        Name, Characteristics, Kind, Sym->getName(),
-        GV->isWeakForLinker() ? COFF::IMAGE_COMDAT_SELECT_ANY
-                              : COFF::IMAGE_COMDAT_SELECT_NODUPLICATES);
+    int Selection = getSelectionForCOFF(GV);
+    if (!Selection)
+      Selection = COFF::IMAGE_COMDAT_SELECT_NODUPLICATES;
+    if (Selection == COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE ||
+        !GV->hasPrivateLinkage()) {
+      const GlobalValue *ComdatGV;
+      if (Selection == COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE)
+        ComdatGV = getComdatGVForCOFF(GV);
+      else
+        ComdatGV = GV;
+      MCSymbol *Sym = TM.getSymbol(ComdatGV, Mang);
+      StringRef COMDATSymName = Sym->getName();
+      return getContext().getCOFFSection(Name, Characteristics, Kind,
+                                         COMDATSymName, Selection);
+    }
   }
 
   if (Kind.isText())
     return TextSection;
 
   if (Kind.isThreadLocal())
     return TLSDataSection;
 

lib/IR/AsmWriter.h

 // LLVM IR and various helper classes that are used in printing.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_IR_ASSEMBLYWRITER_H
 #define LLVM_IR_ASSEMBLYWRITER_H
 
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SetVector.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/TypeFinder.h"
 #include "llvm/Support/FormattedStream.h"
 
 namespace llvm {
 
 class BasicBlock;
 class Function;
 class GlobalValue;
+class Comdat;
 class Module;
 class NamedMDNode;
 class Value;
 class SlotTracker;
 
 /// Create a new SlotTracker for a Module 
 SlotTracker *createSlotTracker(const Module *M);
 
protected:
   formatted_raw_ostream &Out;
   const Module *TheModule;
 
 private:
   std::unique_ptr<SlotTracker> ModuleSlotTracker;
   SlotTracker &Machine;
   TypePrinting TypePrinter;
   AssemblyAnnotationWriter *AnnotationWriter;
+  SetVector<const Comdat *> Comdats;
 
 public:
   /// Construct an AssemblyWriter with an external SlotTracker
   AssemblyWriter(formatted_raw_ostream &o, SlotTracker &Mac,
                  const Module *M, AssemblyAnnotationWriter *AAW);
 
   /// Construct an AssemblyWriter with an internally allocated SlotTracker
   AssemblyWriter(formatted_raw_ostream &o, const Module *M,
public:
 
   void writeAllMDNodes();
   void writeMDNode(unsigned Slot, const MDNode *Node);
   void writeAllAttributeGroups();
 
   void printTypeIdentities();
   void printGlobal(const GlobalVariable *GV);
   void printAlias(const GlobalAlias *GV);
+  void printComdat(const Comdat *C);
   void printFunction(const Function *F);
   void printArgument(const Argument *FA, AttributeSet Attrs, unsigned Idx);
   void printBasicBlock(const BasicBlock *BB);
   void printInstructionLine(const Instruction &I);
   void printInstruction(const Instruction &I);
 
 private:
   void init();

lib/IR/AsmWriter.cpp

if (isprint(C) && C != '\\' && C != '"')
       Out << C;
     else
       Out << '\\' << hexdigit(C >> 4) << hexdigit(C & 0x0F);
   }
 }
 
 enum PrefixType {
   GlobalPrefix,
+  ComdatPrefix,
   LabelPrefix,
   LocalPrefix,
   NoPrefix
 };
 
 /// PrintLLVMName - Turn the specified name into an 'LLVM name', which is either
 /// prefixed with % (if the string only contains simple characters) or is
 /// surrounded with ""'s (if it has special chars in it).  Print it out.
 static void PrintLLVMName(raw_ostream &OS, StringRef Name, PrefixType Prefix) {
   assert(!Name.empty() && "Cannot get empty name!");
   switch (Prefix) {
   case NoPrefix: break;
   case GlobalPrefix: OS << '@'; break;
+  case ComdatPrefix: OS << '$'; break;
   case LabelPrefix:  break;
   case LocalPrefix:  OS << '%'; break;
   }
 
   // Scan the name to see if it needs quotes first.
   bool NeedsQuotes = isdigit(static_cast<unsigned char>(Name[0]));
   if (!NeedsQuotes) {
     for (unsigned i = 0, e = Name.size(); i != e; ++i) {
static void WriteAsOperandInternal(raw_ostream &Out, const Value *V,
 
   if (Slot != -1)
     Out << Prefix << Slot;
   else
     Out << "<badref>";
 }
 
 void AssemblyWriter::init() {
-  if (TheModule)
-    TypePrinter.incorporateTypes(*TheModule);
+  if (!TheModule)
+    return;
+  for (const Function &F : *TheModule)
+    if (const Comdat *C = F.getComdat())
+      Comdats.insert(C);
+  for (const GlobalVariable &GV : TheModule->globals())
+    if (const Comdat *C = GV.getComdat())
+      Comdats.insert(C);
 }
 
 
 AssemblyWriter::AssemblyWriter(formatted_raw_ostream &o, SlotTracker &Mac,
                                const Module *M,
                                AssemblyAnnotationWriter *AAW)
   : Out(o), TheModule(M), Machine(Mac), AnnotationWriter(AAW) {
   init();
if (!rest.empty()) {
       Out << "module asm \"";
       PrintEscapedString(rest, Out);
       Out << "\"\n";
     }
   }
 
   printTypeIdentities();
 
+  // Output all comdats.
+  if (!Comdats.empty())
+    Out << '\n';
+  for (const Comdat *C : Comdats) {
+    printComdat(C);
+    if (C != Comdats.back())
+      Out << '\n';
+  }
+
   // Output all globals.
   if (!M->global_empty()) Out << '\n';
   for (Module::const_global_iterator I = M->global_begin(), E = M->global_end();
        I != E; ++I) {
     printGlobal(I); Out << '\n';
   }
 
   // Output all aliases.
if (GV->hasInitializer()) {
     writeOperand(GV->getInitializer(), false);
   }
 
   if (GV->hasSection()) {
     Out << ", section \"";
     PrintEscapedString(GV->getSection(), Out);
     Out << '"';
   }
+  if (GV->hasComdat()) {
+    Out << ", comdat ";
+    PrintLLVMName(Out, GV->getComdat()->getName(), ComdatPrefix);
+  }
   if (GV->getAlignment())
     Out << ", align " << GV->getAlignment();
 
   printInfoComment(*GV);
 }
 
 void AssemblyWriter::printAlias(const GlobalAlias *GA) {
   if (GA->isMaterializable())
void AssemblyWriter::printAlias(const GlobalAlias *GA) {
 
   if (!Aliasee) {
     TypePrinter.print(GA->getType(), Out);
     Out << " <<NULL ALIASEE>>";
   } else {
     writeOperand(Aliasee, !isa<ConstantExpr>(Aliasee));
   }
 
+  if (GA->hasComdat()) {
+    Out << ", comdat ";
+    PrintLLVMName(Out, GA->getComdat()->getName(), ComdatPrefix);
+  }
+
   printInfoComment(*GA);
   Out << '\n';
 }
 
+void AssemblyWriter::printComdat(const Comdat *C) {
+  C->print(Out);
+}
+
 void AssemblyWriter::printTypeIdentities() {
   if (TypePrinter.NumberedTypes.empty() &&
       TypePrinter.NamedTypes.empty())
     return;
 
   Out << '\n';
 
   // We know all the numbers that each type is used and we know that it is a
if (F->hasUnnamedAddr())
     Out << " unnamed_addr";
   if (Attrs.hasAttributes(AttributeSet::FunctionIndex))
     Out << " #" << Machine.getAttributeGroupSlot(Attrs.getFnAttributes());
   if (F->hasSection()) {
     Out << " section \"";
     PrintEscapedString(F->getSection(), Out);
     Out << '"';
   }
+  if (F->hasComdat()) {
+    Out << " comdat ";
+    PrintLLVMName(Out, F->getComdat()->getName(), ComdatPrefix);
+  }
   if (F->getAlignment())
     Out << " align " << F->getAlignment();
   if (F->hasGC())
     Out << " gc \"" << F->getGC() << '"';
   if (F->hasPrefixData()) {
     Out << " prefix ";
     writeOperand(F->getPrefixData(), true);
   }

 
 void NamedMDNode::print(raw_ostream &ROS) const {
   SlotTracker SlotTable(getParent());
   formatted_raw_ostream OS(ROS);
   AssemblyWriter W(OS, SlotTable, getParent(), nullptr);
   W.printNamedMDNode(this);
 }
 
+void Comdat::print(raw_ostream &ROS) const {
+  PrintLLVMName(ROS, getName(), ComdatPrefix);
+  ROS << " = comdat ";
+
+  switch (getSelectionKind()) {
+  case Comdat::Any:
+    ROS << "any";
+    break;
+  case Comdat::ExactMatch:
+    ROS << "exactmatch";
+    break;
+  case Comdat::Largest:
+    ROS << "largest";
+    break;
+  case Comdat::NoDuplicates:
+    ROS << "noduplicates";
+    break;
+  case Comdat::SameSize:
+    ROS << "samesize";
+    break;
+  }
+
+  ROS << '\n';
+}
+
 void Type::print(raw_ostream &OS) const {
   TypePrinting TP;
   TP.print(const_cast<Type*>(this), OS);
 
   // If the type is a named struct type, print the body as well.
   if (StructType *STy = dyn_cast<StructType>(const_cast<Type*>(this)))
     if (!STy->isLiteral()) {
       OS << " = type ";

 void Value::dump() const { print(dbgs()); dbgs() << '\n'; }
 
 // Type::dump - allow easy printing of Types from the debugger.
 void Type::dump() const { print(dbgs()); }
 
 // Module::dump() - Allow printing of Modules from the debugger.
 void Module::dump() const { print(dbgs(), nullptr); }
 
+// \brief Allow printing of Comdats from the debugger.
+void Comdat::dump() const { print(dbgs()); }
+
 // NamedMDNode::dump() - Allow printing of NamedMDNodes from the debugger.
 void NamedMDNode::dump() const { print(dbgs()); }

lib/IR/CMakeLists.txt

 add_llvm_library(LLVMCore
   AsmWriter.cpp
   Attributes.cpp
   AutoUpgrade.cpp
   BasicBlock.cpp
+  Comdat.cpp
   ConstantFold.cpp
   ConstantRange.cpp
   Constants.cpp
   Core.cpp
   DIBuilder.cpp
   DataLayout.cpp
   DebugInfo.cpp
   DebugLoc.cpp

lib/IR/Comdat.cpp

+//===-- Comdat.cpp - Implement Metadata classes --------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the Comdat class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/Comdat.h"
+#include "llvm/ADT/StringMap.h"
+using namespace llvm;
+
+Comdat::Comdat(SelectionKind SK, StringMapEntry<Comdat> *Name)
+    : Name(Name), SK(SK) {}
+
+Comdat::Comdat(Comdat &&C) : Name(C.Name), SK(C.SK) {}
+
+Comdat::Comdat() : Name(nullptr), SK(Comdat::Any) {}
+
+StringRef Comdat::getName() const { return Name->first(); }

lib/IR/Globals.cpp

 /// copyAttributesFrom - copy all additional attributes (those not needed to
 /// create a GlobalValue) from the GlobalValue Src to this one.
 void GlobalValue::copyAttributesFrom(const GlobalValue *Src) {
   setVisibility(Src->getVisibility());
   setUnnamedAddr(Src->hasUnnamedAddr());
   setDLLStorageClass(Src->getDLLStorageClass());
 }
 
-static const GlobalObject *getBaseObject(const Constant &C) {
-  // FIXME: We should probably return a base + offset pair for non-zero GEPs.
-  return dyn_cast<GlobalObject>(C.stripPointerCasts());
-}
-
 unsigned GlobalValue::getAlignment() const {
   if (auto *GA = dyn_cast<GlobalAlias>(this)) {
     // In general we cannot compute this at the IR level, but we try.
-    if (const GlobalObject *GO = getBaseObject(*GA->getAliasee()))
+    if (const GlobalObject *GO = GA->getBaseObject())
       return GO->getAlignment();
 
     // FIXME: we should also be able to handle:
     // Alias = Global + Offset
     // Alias = Absolute
     return 0;
   }
   return cast<GlobalObject>(this)->getAlignment();
void GlobalObject::copyAttributesFrom(const GlobalValue *Src) {
   GlobalValue::copyAttributesFrom(GV);
   setAlignment(GV->getAlignment());
   setSection(GV->getSection());
 }
 
 const char *GlobalValue::getSection() const {
   if (auto *GA = dyn_cast<GlobalAlias>(this)) {
     // In general we cannot compute this at the IR level, but we try.
-    if (const GlobalObject *GO = getBaseObject(*GA->getAliasee()))
+    if (const GlobalObject *GO = GA->getBaseObject())
       return GO->getSection();
     return "";
   }
   return cast<GlobalObject>(this)->getSection();
 }
 
+Comdat *GlobalValue::getComdat() {
+  if (auto *GA = dyn_cast<GlobalAlias>(this)) {
+    // In general we cannot compute this at the IR level, but we try.
+    if (const GlobalObject *GO = GA->getBaseObject())
+      return const_cast<GlobalObject *>(GO)->getComdat();
+    return nullptr;
+  }
+  return cast<GlobalObject>(this)->getComdat();
+}
+
 void GlobalObject::setSection(StringRef S) { Section = S; }
 
 bool GlobalValue::isDeclaration() const {
   // Globals are definitions if they have an initializer.
   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(this))
     return GV->getNumOperands() == 0;
 
   // Functions are definitions if they have a body.

lib/IR/Module.cpp

 }
 
 unsigned Module::getDwarfVersion() const {
   Value *Val = getModuleFlag("Dwarf Version");
   if (!Val)
     return dwarf::DWARF_VERSION;
   return cast<ConstantInt>(Val)->getZExtValue();
 }
+
+Comdat *Module::getOrInsertComdat(StringRef Name) {
+  Comdat C;
+  StringMapEntry<Comdat> &Entry =
+      ComdatSymTab.GetOrCreateValue(Name, std::move(C));
+  Entry.second.Name = &Entry;
+  return &Entry.second;
+}

lib/IR/Verifier.cpp

struct VerifierSupport {
   }
 
   void WriteType(Type *T) {
     if (!T)
       return;
     OS << ' ' << *T;
   }
 
+  void WriteComdat(const Comdat *C) {
+    if (!C)
+      return;
+    OS << *C;
+  }
+
   // CheckFailed - A check failed, so print out the condition and the message
   // that failed.  This provides a nice place to put a breakpoint if you want
   // to see why something is not correct.
   void CheckFailed(const Twine &Message, const Value *V1 = nullptr,
                    const Value *V2 = nullptr, const Value *V3 = nullptr,
                    const Value *V4 = nullptr) {
     OS << Message.str() << "\n";
     WriteValue(V1);
struct VerifierSupport {
   void CheckFailed(const Twine &Message, Type *T1, Type *T2 = nullptr,
                    Type *T3 = nullptr) {
     OS << Message.str() << "\n";
     WriteType(T1);
     WriteType(T2);
     WriteType(T3);
     Broken = true;
   }
+
+  void CheckFailed(const Twine &Message, const Comdat *C) {
+    OS << Message.str() << "\n";
+    WriteComdat(C);
+    Broken = true;
+  }
 };
 class Verifier : public InstVisitor<Verifier>, VerifierSupport {
   friend class InstVisitor<Verifier>;
 
   LLVMContext *Context;
   const DataLayout *DL;
   DominatorTree DT;
 
for (Module::const_alias_iterator I = M.alias_begin(), E = M.alias_end();
          I != E; ++I)
       visitGlobalAlias(*I);
 
     for (Module::const_named_metadata_iterator I = M.named_metadata_begin(),
                                                E = M.named_metadata_end();
          I != E; ++I)
       visitNamedMDNode(*I);
 
+    for (const StringMapEntry<Comdat> &SMEC : M.getComdatSymbolTable())
+      visitComdat(SMEC.getValue());
+
     visitModuleFlags(M);
     visitModuleIdents(M);
 
     return !Broken;
   }
 
 private:
   // Verification methods...
   void visitGlobalValue(const GlobalValue &GV);
   void visitGlobalVariable(const GlobalVariable &GV);
   void visitGlobalAlias(const GlobalAlias &GA);
   void visitAliaseeSubExpr(const GlobalAlias &A, const Constant &C);
   void visitAliaseeSubExpr(SmallPtrSet<const GlobalAlias *, 4> &Visited,
                            const GlobalAlias &A, const Constant &C);
   void visitNamedMDNode(const NamedMDNode &NMD);
   void visitMDNode(MDNode &MD, Function *F);
+  void visitComdat(const Comdat &C);
   void visitModuleIdents(const Module &M);
   void visitModuleFlags(const Module &M);
   void visitModuleFlag(const MDNode *Op,
                        DenseMap<const MDString *, const MDNode *> &SeenIDs,
                        SmallVectorImpl<const MDNode *> &Requirements);
   void visitFunction(const Function &F);
   void visitBasicBlock(BasicBlock &BB);
 
if (GV.hasInitializer()) {
 
     // If the global has common linkage, it must have a zero initializer and
     // cannot be constant.
     if (GV.hasCommonLinkage()) {
       Assert1(GV.getInitializer()->isNullValue(),
               "'common' global must have a zero initializer!", &GV);
       Assert1(!GV.isConstant(), "'common' global may not be marked constant!",
               &GV);
+      Assert1(!GV.hasComdat(), "'common' global may not be in a Comdat!", &GV);
     }
   } else {
     Assert1(GV.hasExternalLinkage() || GV.hasExternalWeakLinkage(),
             "invalid linkage type for global declaration", &GV);
   }
 
   if (GV.hasName() && (GV.getName() == "llvm.global_ctors" ||
                        GV.getName() == "llvm.global_dtors")) {
else if (Argument *A = dyn_cast<Argument>(Op))
       ActualF = A->getParent();
     assert(ActualF && "Unimplemented function local metadata case!");
 
     Assert2(ActualF == F, "function-local metadata used in wrong function",
             &MD, Op);
   }
 }
 
+void Verifier::visitComdat(const Comdat &C) {
+  // All Comdat::SelectionKind values other than Comdat::Any require a
+  // GlobalValue with the same name as the Comdat.
+  const GlobalValue *GV = M->getNamedValue(C.getName());
+  if (C.getSelectionKind() != Comdat::Any)
+    Assert1(GV,
+            "comdat selection kind requires a global value with the same name",
+            &C);
+  // The Module is invalid if the GlobalValue has local linkage.  Allowing
+  // otherwise opens us up to seeing the underling global value get renamed if
+  // collisions occur.
+  if (GV)
+    Assert1(!GV->hasLocalLinkage(), "comdat global value has local linkage",
+            GV);
+}
+
 void Verifier::visitModuleIdents(const Module &M) {
   const NamedMDNode *Idents = M.getNamedMetadata("llvm.ident");
   if (!Idents) 
     return;
   
   // llvm.ident takes a list of metadata entry. Each entry has only one string.
   // Scan each llvm.ident entry and make sure that this requirement is met.
   for (unsigned i = 0, e = Idents->getNumOperands(); i != e; ++i) {

lib/Linker/LinkModules.cpp

namespace {
   private:
     /// emitError - Helper method for setting a message and returning an error
     /// code.
     bool emitError(const Twine &Message) {
       ErrorMsg = Message.str();
       return true;
     }
 
+    bool getComdatLeader(Module *M, StringRef ComdatName,
+                         const GlobalVariable *&GVar);
+    bool computeResultingSelectionKind(StringRef ComdatName,
+                                       Comdat::SelectionKind Src,
+                                       Comdat::SelectionKind Dst,
+                                       Comdat::SelectionKind &Result,
+                                       bool &LinkFromSrc);
+    std::map<const Comdat *, std::pair<Comdat::SelectionKind, bool>>
+        ComdatsChosen;
+    bool getComdatResult(const Comdat *SrcC, Comdat::SelectionKind &SK,
+                         bool &LinkFromSrc);
+
     /// getLinkageResult - This analyzes the two global values and determines
     /// what the result will look like in the destination module.
     bool getLinkageResult(GlobalValue *Dest, const GlobalValue *Src,
                           GlobalValue::LinkageTypes &LT,
                           GlobalValue::VisibilityTypes &Vis,
                           bool &LinkFromSrc);
 
     /// getLinkedToGlobal - Given a global in the source module, return the
Value *ValueMaterializerTy::materializeValueFor(Value *V) {
   Function *DF = Function::Create(TypeMap.get(SF->getFunctionType()),
                                   SF->getLinkage(), SF->getName(), DstM);
   copyGVAttributes(DF, SF);
 
   LazilyLinkFunctions.push_back(SF);
   return DF;
 }
 
+bool ModuleLinker::getComdatLeader(Module *M, StringRef ComdatName,
+                                   const GlobalVariable *&GVar) {
+  const GlobalValue *GVal = M->getNamedValue(ComdatName);
+  if (const auto *GA = dyn_cast_or_null<GlobalAlias>(GVal)) {
+    GVal = GA->getBaseObject();
+    if (!GVal)
+      // We cannot resolve the size of the aliasee yet.
+      return emitError("Linking COMDATs named '" + ComdatName +
+                       "': COMDAT key involves incomputable alias size.");
+  }
+
+  GVar = dyn_cast_or_null<GlobalVariable>(GVal);
+  if (!GVar)
+    return emitError(
+        "Linking COMDATs named '" + ComdatName +
+        "': GlobalVariable required for data dependent selection!");
+
+  return false;
+}
+
+bool ModuleLinker::computeResultingSelectionKind(StringRef ComdatName,
+                                                 Comdat::SelectionKind Src,
+                                                 Comdat::SelectionKind Dst,
+                                                 Comdat::SelectionKind &Result,
+                                                 bool &LinkFromSrc) {
+  // The ability to mix Comdat::SelectionKind::Any with
+  // Comdat::SelectionKind::Largest is a behavior that comes from COFF.
+  bool DstAnyOrLargest = Dst == Comdat::SelectionKind::Any ||
+                         Dst == Comdat::SelectionKind::Largest;
+  bool SrcAnyOrLargest = Src == Comdat::SelectionKind::Any ||
+                         Src == Comdat::SelectionKind::Largest;
+  if (DstAnyOrLargest && SrcAnyOrLargest) {
+    if (Dst == Comdat::SelectionKind::Largest ||
+        Src == Comdat::SelectionKind::Largest)
+      Result = Comdat::SelectionKind::Largest;
+    else
+      Result = Comdat::SelectionKind::Any;
+  } else if (Src == Dst) {
+    Result = Dst;
+  } else {
+    return emitError("Linking COMDATs named '" + ComdatName +
+                     "': invalid selection kinds!");
+  }
+
+  switch (Result) {
+  case Comdat::SelectionKind::Any:
+    // Go with Dst.
+    LinkFromSrc = false;
+    break;
+  case Comdat::SelectionKind::NoDuplicates:
+    return emitError("Linking COMDATs named '" + ComdatName +
+                     "': noduplicates has been violated!");
+  case Comdat::SelectionKind::ExactMatch:
+  case Comdat::SelectionKind::Largest:
+  case Comdat::SelectionKind::SameSize: {
+    const GlobalVariable *DstGV;
+    const GlobalVariable *SrcGV;
+    if (getComdatLeader(DstM, ComdatName, DstGV) ||
+        getComdatLeader(SrcM, ComdatName, SrcGV))
+      return true;
+
+    const DataLayout *DstDL = DstM->getDataLayout();
+    const DataLayout *SrcDL = SrcM->getDataLayout();
+    if (!DstDL || !SrcDL) {
+      return emitError(
+          "Linking COMDATs named '" + ComdatName +
+          "': can't do size dependent selection without DataLayout!");
+    }
+    uint64_t DstSize =
+        DstDL->getTypeAllocSize(DstGV->getType()->getPointerElementType());
+    uint64_t SrcSize =
+        SrcDL->getTypeAllocSize(SrcGV->getType()->getPointerElementType());
+    if (Result == Comdat::SelectionKind::ExactMatch) {
+      if (SrcGV->getInitializer() != DstGV->getInitializer())
+        return emitError("Linking COMDATs named '" + ComdatName +
+                         "': ExactMatch violated!");
+      LinkFromSrc = false;
+    } else if (Result == Comdat::SelectionKind::Largest) {
+      LinkFromSrc = SrcSize > DstSize;
+    } else if (Result == Comdat::SelectionKind::SameSize) {
+      if (SrcSize != DstSize)
+        return emitError("Linking COMDATs named '" + ComdatName +
+                         "': SameSize violated!");
+      LinkFromSrc = false;
+    } else {
+      llvm_unreachable("unknown selection kind");
+    }
+    break;
+  }
+  }
+
+  return false;
+}
+
+bool ModuleLinker::getComdatResult(const Comdat *SrcC,
+                                   Comdat::SelectionKind &Result,
+                                   bool &LinkFromSrc) {
+  StringRef ComdatName = SrcC->getName();
+  Module::ComdatSymTabType &ComdatSymTab = DstM->getComdatSymbolTable();
+  Module::ComdatSymTabType::iterator DstCI = ComdatSymTab.find(ComdatName);
+  if (DstCI != ComdatSymTab.end()) {
+    const Comdat *DstC = &DstCI->second;
+    Comdat::SelectionKind SSK = SrcC->getSelectionKind();
+    Comdat::SelectionKind DSK = DstC->getSelectionKind();
+    if (computeResultingSelectionKind(ComdatName, SSK, DSK, Result, LinkFromSrc))
+      return true;
+  }
+  return false;
+}
 
 /// getLinkageResult - This analyzes the two global values and determines what
 /// the result will look like in the destination module.  In particular, it
 /// computes the resultant linkage type and visibility, computes whether the
 /// global in the source should be copied over to the destination (replacing
 /// the existing one), and computes whether this linkage is an error or not.
 bool ModuleLinker::getLinkageResult(GlobalValue *Dest, const GlobalValue *Src,
                                     GlobalValue::LinkageTypes &LT,

 
 /// linkGlobalProto - Loop through the global variables in the src module and
 /// merge them into the dest module.
 bool ModuleLinker::linkGlobalProto(GlobalVariable *SGV) {
   GlobalValue *DGV = getLinkedToGlobal(SGV);
   llvm::Optional<GlobalValue::VisibilityTypes> NewVisibility;
   bool HasUnnamedAddr = SGV->hasUnnamedAddr();
 
+  bool LinkFromSrc = false;
+  Comdat *DC = nullptr;
+  if (const Comdat *SC = SGV->getComdat()) {
+    Comdat::SelectionKind SK;
+    std::tie(SK, LinkFromSrc) = ComdatsChosen[SC];
+    DC = DstM->getOrInsertComdat(SC->getName());
+    DC->setSelectionKind(SK);
+  }
+
   if (DGV) {
+    if (!DC) {
       // Concatenation of appending linkage variables is magic and handled later.
       if (DGV->hasAppendingLinkage() || SGV->hasAppendingLinkage())
         return linkAppendingVarProto(cast<GlobalVariable>(DGV), SGV);
 
       // Determine whether linkage of these two globals follows the source
       // module's definition or the destination module's definition.
       GlobalValue::LinkageTypes NewLinkage = GlobalValue::InternalLinkage;
       GlobalValue::VisibilityTypes NV;
-    bool LinkFromSrc = false;
       if (getLinkageResult(DGV, SGV, NewLinkage, NV, LinkFromSrc))
         return true;
       NewVisibility = NV;
       HasUnnamedAddr = HasUnnamedAddr && DGV->hasUnnamedAddr();
 
       // If we're not linking from the source, then keep the definition that we
       // have.
       if (!LinkFromSrc) {
         // Special case for const propagation.
         if (GlobalVariable *DGVar = dyn_cast<GlobalVariable>(DGV))
-        if (DGVar->isDeclaration() && SGV->isConstant() && !DGVar->isConstant())
+          if (DGVar->isDeclaration() && SGV->isConstant() &&
+              !DGVar->isConstant())
             DGVar->setConstant(true);
 
         // Set calculated linkage, visibility and unnamed_addr.
         DGV->setLinkage(NewLinkage);
         DGV->setVisibility(*NewVisibility);
         DGV->setUnnamedAddr(HasUnnamedAddr);
+      }
+    }
 
+    if (!LinkFromSrc) {
       // Make sure to remember this mapping.
       ValueMap[SGV] = ConstantExpr::getBitCast(DGV,TypeMap.get(SGV->getType()));
 
       // Track the source global so that we don't attempt to copy it over when
       // processing global initializers.
       DoNotLinkFromSource.insert(SGV);
 
       return false;
     }
   }
 
+  // If the Comdat this variable was inside of wasn't selected, skip it.
+  if (DC && !DGV && !LinkFromSrc) {
+    DoNotLinkFromSource.insert(SGV);
+    return false;
+  }
+
   // No linking to be performed or linking from the source: simply create an
   // identical version of the symbol over in the dest module... the
   // initializer will be filled in later by LinkGlobalInits.
   GlobalVariable *NewDGV =
     new GlobalVariable(*DstM, TypeMap.get(SGV->getType()->getElementType()),
                        SGV->isConstant(), SGV->getLinkage(), /*init*/nullptr,
                        SGV->getName(), /*insertbefore*/nullptr,
                        SGV->getThreadLocalMode(),
                        SGV->getType()->getAddressSpace());
   // Propagate alignment, visibility and section info.
   copyGVAttributes(NewDGV, SGV);
   if (NewVisibility)
     NewDGV->setVisibility(*NewVisibility);
   NewDGV->setUnnamedAddr(HasUnnamedAddr);
 
+  if (DC)
+    NewDGV->setComdat(DC);
+
   if (DGV) {
     DGV->replaceAllUsesWith(ConstantExpr::getBitCast(NewDGV, DGV->getType()));
     DGV->eraseFromParent();
   }
 
   // Make sure to remember this mapping.
   ValueMap[SGV] = NewDGV;
   return false;
 }
 
 /// linkFunctionProto - Link the function in the source module into the
 /// destination module if needed, setting up mapping information.
 bool ModuleLinker::linkFunctionProto(Function *SF) {
   GlobalValue *DGV = getLinkedToGlobal(SF);
   llvm::Optional<GlobalValue::VisibilityTypes> NewVisibility;
   bool HasUnnamedAddr = SF->hasUnnamedAddr();
 
+  bool LinkFromSrc = false;
+  Comdat *DC = nullptr;
+  if (const Comdat *SC = SF->getComdat()) {
+    Comdat::SelectionKind SK;
+    std::tie(SK, LinkFromSrc) = ComdatsChosen[SC];
+    DC = DstM->getOrInsertComdat(SC->getName());
+    DC->setSelectionKind(SK);
+  }
+
   if (DGV) {
+    if (!DC) {
       GlobalValue::LinkageTypes NewLinkage = GlobalValue::InternalLinkage;
-    bool LinkFromSrc = false;
       GlobalValue::VisibilityTypes NV;
       if (getLinkageResult(DGV, SF, NewLinkage, NV, LinkFromSrc))
         return true;
       NewVisibility = NV;
       HasUnnamedAddr = HasUnnamedAddr && DGV->hasUnnamedAddr();
 
       if (!LinkFromSrc) {
         // Set calculated linkage
         DGV->setLinkage(NewLinkage);
         DGV->setVisibility(*NewVisibility);
         DGV->setUnnamedAddr(HasUnnamedAddr);
+      }
+    }
 
+    if (!LinkFromSrc) {
       // Make sure to remember this mapping.
       ValueMap[SF] = ConstantExpr::getBitCast(DGV, TypeMap.get(SF->getType()));
 
       // Track the function from the source module so we don't attempt to remap
       // it.
       DoNotLinkFromSource.insert(SF);
 
       return false;
     }
   }
 
   // If the function is to be lazily linked, don't create it just yet.
   // The ValueMaterializerTy will deal with creating it if it's used.
   if (!DGV && (SF->hasLocalLinkage() || SF->hasLinkOnceLinkage() ||
                SF->hasAvailableExternallyLinkage())) {
     DoNotLinkFromSource.insert(SF);
     return false;
   }
 
+  // If the Comdat this function was inside of wasn't selected, skip it.
+  if (DC && !DGV && !LinkFromSrc) {
+    DoNotLinkFromSource.insert(SF);
+    return false;
+  }
+
   // If there is no linkage to be performed or we are linking from the source,
   // bring SF over.
   Function *NewDF = Function::Create(TypeMap.get(SF->getFunctionType()),
                                      SF->getLinkage(), SF->getName(), DstM);
   copyGVAttributes(NewDF, SF);
   if (NewVisibility)
     NewDF->setVisibility(*NewVisibility);
   NewDF->setUnnamedAddr(HasUnnamedAddr);
 
+  if (DC)
+    NewDF->setComdat(DC);
+
   if (DGV) {
     // Any uses of DF need to change to NewDF, with cast.
     DGV->replaceAllUsesWith(ConstantExpr::getBitCast(NewDF, DGV->getType()));
     DGV->eraseFromParent();
   }
 
   ValueMap[SF] = NewDF;
   return false;
 }
 
 /// LinkAliasProto - Set up prototypes for any aliases that come over from the
 /// source module.
 bool ModuleLinker::linkAliasProto(GlobalAlias *SGA) {
   GlobalValue *DGV = getLinkedToGlobal(SGA);
   llvm::Optional<GlobalValue::VisibilityTypes> NewVisibility;
   bool HasUnnamedAddr = SGA->hasUnnamedAddr();
 
+  bool LinkFromSrc = false;
+  Comdat *DC = nullptr;
+  if (const Comdat *SC = SGA->getComdat()) {
+    Comdat::SelectionKind SK;
+    std::tie(SK, LinkFromSrc) = ComdatsChosen[SC];
+    DC = DstM->getOrInsertComdat(SC->getName());
+    DC->setSelectionKind(SK);
+  }
+
   if (DGV) {
+    if (!DC) {
       GlobalValue::LinkageTypes NewLinkage = GlobalValue::InternalLinkage;
       GlobalValue::VisibilityTypes NV;
-    bool LinkFromSrc = false;
       if (getLinkageResult(DGV, SGA, NewLinkage, NV, LinkFromSrc))
         return true;
       NewVisibility = NV;
       HasUnnamedAddr = HasUnnamedAddr && DGV->hasUnnamedAddr();
 
       if (!LinkFromSrc) {
         // Set calculated linkage.
         DGV->setLinkage(NewLinkage);
         DGV->setVisibility(*NewVisibility);
         DGV->setUnnamedAddr(HasUnnamedAddr);
+      }
+    }
 
+    if (!LinkFromSrc) {
       // Make sure to remember this mapping.
       ValueMap[SGA] = ConstantExpr::getBitCast(DGV,TypeMap.get(SGA->getType()));
 
       // Track the alias from the source module so we don't attempt to remap it.
       DoNotLinkFromSource.insert(SGA);
 
       return false;
     }
   }
 
+  // If the Comdat this alias was inside of wasn't selected, skip it.
+  if (DC && !DGV && !LinkFromSrc) {
+    DoNotLinkFromSource.insert(SGA);
+    return false;
+  }
+
   // If there is no linkage to be performed or we're linking from the source,
   // bring over SGA.
   auto *PTy = cast<PointerType>(TypeMap.get(SGA->getType()));
   auto *NewDA =
       GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
                           SGA->getLinkage(), SGA->getName(), DstM);
   copyGVAttributes(NewDA, SGA);
   if (NewVisibility)
if (!SrcM->getModuleInlineAsm().empty()) {
     else
       DstM->setModuleInlineAsm(DstM->getModuleInlineAsm()+"\n"+
                                SrcM->getModuleInlineAsm());
   }
 
   // Loop over all of the linked values to compute type mappings.
   computeTypeMapping();
 
+  ComdatsChosen.clear();
+  for (const StringMapEntry<llvm::Comdat> &SMEC : SrcM->getComdatSymbolTable()) {
+    const Comdat &C = SMEC.getValue();
+    if (ComdatsChosen.count(&C))
+      continue;
+    Comdat::SelectionKind SK;
+    bool LinkFromSrc;
+    if (getComdatResult(&C, SK, LinkFromSrc))
+      return true;
+    ComdatsChosen[&C] = std::make_pair(SK, LinkFromSrc);
+  }
+
   // Insert all of the globals in src into the DstM module... without linking
   // initializers (which could refer to functions not yet mapped over).
   for (Module::global_iterator I = SrcM->global_begin(),
        E = SrcM->global_end(); I != E; ++I)
     if (linkGlobalProto(I))
       return true;
 
   // Link the functions together between the two modules, without doing function

lib/MC/MCContext.cpp

 
 const MCSectionCOFF *MCContext::getCOFFSection(StringRef Section,
                                                unsigned Characteristics,
                                                SectionKind Kind,
                                                StringRef COMDATSymName,
                                                int Selection) {
   // Do the lookup, if we have a hit, return it.
 
-  SectionGroupPair P(Section, COMDATSymName);
-  auto IterBool = COFFUniquingMap.insert(std::make_pair(P, nullptr));
+  SectionGroupTriple T(Section, COMDATSymName, Selection);
+  auto IterBool = COFFUniquingMap.insert(std::make_pair(T, nullptr));
   auto Iter = IterBool.first;
   if (!IterBool.second)
     return Iter->second;
 
   const MCSymbol *COMDATSymbol = nullptr;
   if (!COMDATSymName.empty())
     COMDATSymbol = GetOrCreateSymbol(COMDATSymName);
 
-  StringRef CachedName = Iter->first.first;
+  StringRef CachedName = std::get<0>(Iter->first);
   MCSectionCOFF *Result = new (*this)
       MCSectionCOFF(CachedName, Characteristics, COMDATSymbol, Selection, Kind);
 
   Iter->second = Result;
   return Result;
 }
 
 const MCSectionCOFF *
 MCContext::getCOFFSection(StringRef Section, unsigned Characteristics,
                           SectionKind Kind) {
   return getCOFFSection(Section, Characteristics, Kind, "", 0);
 }
 
 const MCSectionCOFF *MCContext::getCOFFSection(StringRef Section) {
-  SectionGroupPair P(Section, "");
-  auto Iter = COFFUniquingMap.find(P);
+  SectionGroupTriple T(Section, "", 0);
+  auto Iter = COFFUniquingMap.find(T);
   if (Iter == COFFUniquingMap.end())
     return nullptr;
   return Iter->second;
 }
 
 //===----------------------------------------------------------------------===//
 // Dwarf Management
 //===----------------------------------------------------------------------===//

lib/MC/MCParser/COFFAsmParser.cpp

bool COFFAsmParser::ParseDirectiveSymbolAttribute(StringRef Directive, SMLoc) {
 
   Lex();
   return false;
 }
 
 bool COFFAsmParser::ParseSectionSwitch(StringRef Section,
                                        unsigned Characteristics,
                                        SectionKind Kind) {
-  return ParseSectionSwitch(Section, Characteristics, Kind, "",
-                            COFF::IMAGE_COMDAT_SELECT_ANY);
+  return ParseSectionSwitch(Section, Characteristics, Kind, "", (COFF::COMDATType)0);
 }
 
 bool COFFAsmParser::ParseSectionSwitch(StringRef Section,
                                        unsigned Characteristics,
                                        SectionKind Kind,
                                        StringRef COMDATSymName,
                                        COFF::COMDATType Type) {
   if (getLexer().isNot(AsmToken::EndOfStatement))
if (getLexer().is(AsmToken::Comma)) {
 
     StringRef FlagsStr = getTok().getStringContents();
     Lex();
 
     if (ParseSectionFlags(FlagsStr, &Flags))
       return true;
   }
 
-  COFF::COMDATType Type = COFF::IMAGE_COMDAT_SELECT_ANY;
+  COFF::COMDATType Type = (COFF::COMDATType)0;
   StringRef COMDATSymName;
   if (getLexer().is(AsmToken::Comma)) {
+    Type = COFF::IMAGE_COMDAT_SELECT_ANY;;
     Lex();
 
     Flags |= COFF::IMAGE_SCN_LNK_COMDAT;
 
     if (parseCOMDATType(Type))
       return true;
 
     if (getLexer().isNot(AsmToken::Comma))

lib/Transforms/IPO/GlobalDCE.cpp

 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
 
 bool GlobalDCE::runOnModule(Module &M) {
   bool Changed = false;
 
   // Remove empty functions from the global ctors list.
   Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
 
+  typedef std::multimap<const Comdat *, GlobalValue *> ComdatGVPairsTy;
+  ComdatGVPairsTy ComdatGVPairs;
+
   // Loop over the module, adding globals which are obviously necessary.
   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
     Changed |= RemoveUnusedGlobalValue(*I);
     // Functions with external linkage are needed if they have a body
-    if (!I->isDiscardableIfUnused() &&
-        !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
+    if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
+      if (!I->isDiscardableIfUnused())
         GlobalIsNeeded(I);
+      else if (const Comdat *C = I->getComdat())
+        ComdatGVPairs.insert(std::make_pair(C, I));
+    }
   }
 
   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
        I != E; ++I) {
     Changed |= RemoveUnusedGlobalValue(*I);
     // Externally visible & appending globals are needed, if they have an
     // initializer.
-    if (!I->isDiscardableIfUnused() &&
-        !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
+    if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
+      if (!I->isDiscardableIfUnused())
         GlobalIsNeeded(I);
+      else if (const Comdat *C = I->getComdat())
+        ComdatGVPairs.insert(std::make_pair(C, I));
+    }
   }
 
   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
        I != E; ++I) {
     Changed |= RemoveUnusedGlobalValue(*I);
     // Externally visible aliases are needed.
-    if (!I->isDiscardableIfUnused())
+    if (!I->isDiscardableIfUnused()) {
       GlobalIsNeeded(I);
+    } else if (const Comdat *C = I->getComdat()) {
+      ComdatGVPairs.insert(std::make_pair(C, I));
+    }
+  }
+
+  for (ComdatGVPairsTy::iterator I = ComdatGVPairs.begin(),
+                                 E = ComdatGVPairs.end();
+       I != E;) {
+    ComdatGVPairsTy::iterator UB = ComdatGVPairs.upper_bound(I->first);
+    bool CanDiscard = std::all_of(I, UB, [](ComdatGVPairsTy::value_type Pair) {
+      return Pair.second->isDiscardableIfUnused();
+    });
+    if (!CanDiscard) {
+      std::for_each(I, UB, [this](ComdatGVPairsTy::value_type Pair) {
+        GlobalIsNeeded(Pair.second);
+      });
+    }
+    I = UB;
   }
 
   // Now that all globals which are needed are in the AliveGlobals set, we loop
   // through the program, deleting those which are not alive.
   //
 
   // The first pass is to drop initializers of global variables which are dead.
   std::vector<GlobalVariable*> DeadGlobalVars;   // Keep track of dead globals

lib/Transforms/IPO/GlobalOpt.cpp

 // variables only stored to, etc.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/IPO.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/IR/CallSite.h"
 #include "llvm/IR/CallingConv.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
static bool TryToShrinkGlobalToBoolean(GlobalVariable *GV, Constant *OtherVal) {
   return true;
 }
 
 
 /// ProcessGlobal - Analyze the specified global variable and optimize it if
 /// possible.  If we make a change, return true.
 bool GlobalOpt::ProcessGlobal(GlobalVariable *GV,
                               Module::global_iterator &GVI) {
-  if (!GV->isDiscardableIfUnused())
-    return false;
-
   // Do more involved optimizations if the global is internal.
   GV->removeDeadConstantUsers();
 
   if (GV->use_empty()) {
     DEBUG(dbgs() << "GLOBAL DEAD: " << *GV);
     GV->eraseFromParent();
     ++NumDeleted;
     return true;
if (F->isDefTriviallyDead()) {
       }
     }
   }
   return Changed;
 }
 
 bool GlobalOpt::OptimizeGlobalVars(Module &M) {
   bool Changed = false;
+
+  SmallSet<const Comdat *, 8> NotDiscardableComdats;
+  for (const GlobalVariable &GV : M.globals())
+    if (const Comdat *C = GV.getComdat())
+      if (!GV.isDiscardableIfUnused())
+        NotDiscardableComdats.insert(C);
+
   for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
        GVI != E; ) {
     GlobalVariable *GV = GVI++;
     // Global variables without names cannot be referenced outside this module.
     if (!GV->hasName() && !GV->isDeclaration())
       GV->setLinkage(GlobalValue::InternalLinkage);
     // Simplify the initializer.
     if (GV->hasInitializer())
       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(GV->getInitializer())) {
         Constant *New = ConstantFoldConstantExpression(CE, DL, TLI);
         if (New && New != CE)
           GV->setInitializer(New);
       }
 
+    if (GV->isDiscardableIfUnused()) {
+      if (const Comdat *C = GV->getComdat())
+        if (NotDiscardableComdats.count(C))
+          continue;
       Changed |= ProcessGlobal(GV, GVI);
     }
+  }
   return Changed;
 }
 
 static inline bool
 isSimpleEnoughValueToCommit(Constant *C,
                             SmallPtrSet<Constant*, 8> &SimpleConstants,
                             const DataLayout *DL);
 

test/Assembler/invalid-comdat.ll

+; RUN: not llvm-as < %s 2>&1 | FileCheck %s
+
+@v = global i32 0, comdat $v
+; CHECK: use of undefined comdat '$v'

test/Assembler/invalid-comdat2.ll

+; RUN: not llvm-as < %s 2>&1 | FileCheck %s
+
+$v = comdat any
+$v = comdat any
+; CHECK: redefinition of comdat '$v'

test/CodeGen/X86/coff-comdat.ll

+; RUN: llc -mtriple i386-pc-win32 < %s | FileCheck %s
+
+$f1 = comdat any
+@v1 = global i32 0, comdat $f1
+define void @f1() comdat $f1 {
+  ret void
+}
+
+$f2 = comdat exactmatch
+@v2 = global i32 0, comdat $f2
+define void @f2() comdat $f2 {
+  ret void
+}
+
+$f3 = comdat largest
+@v3 = global i32 0, comdat $f3
+define void @f3() comdat $f3 {
+  ret void
+}
+
+$f4 = comdat noduplicates
+@v4 = global i32 0, comdat $f4
+define void @f4() comdat $f4 {
+  ret void
+}
+
+$f5 = comdat samesize
+@v5 = global i32 0, comdat $f5
+define void @f5() comdat $f5 {
+  ret void
+}
+
+$f6 = comdat samesize
+@v6 = global i32 0, comdat $f6
+@f6 = global i32 0, comdat $f6
+
+$"\01@f7@0" = comdat any
+define x86_fastcallcc void @"\01@v7@0"() comdat $"\01@f7@0" {
+  ret void
+}
+define x86_fastcallcc void @"\01@f7@0"() comdat $"\01@f7@0" {
+  ret void
+}
+
+$f8 = comdat any
+define x86_fastcallcc void @v8() comdat $f8 {
+  ret void
+}
+define x86_fastcallcc void @f8() comdat $f8 {
+  ret void
+}
+
+; CHECK: .section        .text,"xr",discard,_f1
+; CHECK: .globl  _f1
+; CHECK: .section        .text,"xr",same_contents,_f2
+; CHECK: .globl  _f2
+; CHECK: .section        .text,"xr",largest,_f3
+; CHECK: .globl  _f3
+; CHECK: .section        .text,"xr",one_only,_f4
+; CHECK: .globl  _f4
+; CHECK: .section        .text,"xr",same_size,_f5
+; CHECK: .globl  _f5
+; CHECK: .section        .text,"xr",associative,@f7@0
+; CHECK: .globl  @v7@0
+; CHECK: .section        .text,"xr",discard,@f7@0
+; CHECK: .globl  @f7@0
+; CHECK: .section        .text,"xr",associative,@f8@0
+; CHECK: .globl  @v8@0
+; CHECK: .section        .text,"xr",discard,@f8@0
+; CHECK: .globl  @f8@0
+; CHECK: .section        .bss,"bw",associative,_f1
+; CHECK: .globl  _v1
+; CHECK: .section        .bss,"bw",associative,_f2
+; CHECK: .globl  _v2
+; CHECK: .section        .bss,"bw",associative,_f3
+; CHECK: .globl  _v3
+; CHECK: .section        .bss,"bw",associative,_f4
+; CHECK: .globl  _v4
+; CHECK: .section        .bss,"bw",associative,_f5
+; CHECK: .globl  _v5
+; CHECK: .section        .bss,"bw",associative,_f6
+; CHECK: .globl  _v6
+; CHECK: .section        .bss,"bw",same_size,_f6
+; CHECK: .globl  _f6

test/CodeGen/X86/coff-comdat2.ll

+; RUN: not llc %s -o /dev/null 2>&1 | FileCheck %s
+
+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat largest
+@foo = global i32 0
+@bar = global i32 0, comdat $foo
+; CHECK: Associative COMDAT symbol 'foo' is not a key for it's COMDAT.

test/CodeGen/X86/coff-comdat3.ll

+; RUN: not llc %s -o /dev/null 2>&1 | FileCheck %s
+
+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat largest
+@bar = global i32 0, comdat $foo
+; CHECK: Associative COMDAT symbol 'foo' does not exist.

test/CodeGen/X86/elf-comdat.ll

+; RUN: llc -mtriple x86_64-pc-linux-gnu < %s | FileCheck %s
+
+$f = comdat any
+@v = global i32 0, comdat $f
+define void @f() comdat $f {
+  ret void
+}
+; CHECK: .section        .text.f,"axG",@progbits,f,comdat
+; CHECK: .globl  f
+; CHECK: .section        .bss.v,"aGw",@nobits,f,comdat
+; CHECK: .globl  v

test/CodeGen/X86/elf-comdat2.ll

+; RUN: llc -mtriple x86_64-pc-linux-gnu < %s | FileCheck %s
+
+$foo = comdat any
+@bar = global i32 42, comdat $foo
+@foo = global i32 42
+
+; CHECK:      .type   bar,@object
+; CHECK-NEXT: .section        .data.bar,"aGw",@progbits,foo,comdat
+; CHECK-NEXT: .globl  bar
+; CHECK:      .type   foo,@object
+; CHECK-NEXT: .data
+; CHECK-NEXT: .globl  foo

test/CodeGen/X86/macho-comdat.ll

+; RUN: not llc -mtriple x86_64-apple-darwin < %s 2> %t
+; RUN: FileCheck < %t %s
+
+$f = comdat any
+@v = global i32 0, comdat $f
+; CHECK: LLVM ERROR: MachO doesn't support COMDATs, 'f' cannot be lowered.

test/Feature/comdat.ll

+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+$f = comdat any
+; CHECK: $f = comdat any
+
+$f2 = comdat any
+; CHECK-NOT: f2
+
+@v = global i32 0, comdat $f
+; CHECK: @v = global i32 0, comdat $f
+
+define void @f() comdat $f {
+  ret void
+}
+; CHECK: define void @f() comdat $f

test/Instrumentation/AddressSanitizer/do-not-instrument-llvm-metadata.ll

 ; This test checks that we are not instrumenting globals in llvm.metadata.
 ; RUN: opt < %s -asan -asan-module -S | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @.str_noinst = private unnamed_addr constant [4 x i8] c"aaa\00", section "llvm.metadata"
-@.str_inst = private unnamed_addr constant [4 x i8] c"aaa\00",
+@.str_inst = private unnamed_addr constant [4 x i8] c"aaa\00"
 
 ; CHECK-NOT: {{asan_gen.*str_noinst}}
 ; CHECK: {{asan_gen.*str_inst}}
 ; CHECK: @asan.module_ctor

test/Linker/Inputs/comdat.ll

+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat largest
+@foo = global i64 43, comdat $foo
+
+define i32 @bar() comdat $foo {
+  ret i32 43
+}
+
+$qux = comdat largest
+@qux = global i32 13, comdat $qux
+@in_unselected_group = global i32 13, comdat $qux
+
+define i32 @baz() comdat $qux {
+  ret i32 13
+}
+
+$any = comdat any
+@any = global i64 7, comdat $any

test/Linker/Inputs/comdat2.ll

+$foo = comdat largest
+@foo = global i64 43, comdat $foo

test/Linker/Inputs/comdat3.ll

+$foo = comdat noduplicates
+@foo = global i64 43, comdat $foo

test/Linker/Inputs/comdat4.ll

+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat samesize
+@foo = global i64 42, comdat $foo

test/Linker/Inputs/comdat5.ll

+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+%MSRTTICompleteObjectLocator = type { i32, i32, i32, i8*, %MSRTTIClassHierarchyDescriptor* }
+%MSRTTIClassHierarchyDescriptor = type { i32, i32, i32, %MSRTTIBaseClassDescriptor** }
+%MSRTTIBaseClassDescriptor = type { i8*, i32, i32, i32, i32, i32, %MSRTTIClassHierarchyDescriptor* }
+%struct.S = type { i32 (...)** }
+
+$"\01??_7S@@6B@" = comdat largest
+
+@"\01??_R4S@@6B@" = external constant %MSRTTICompleteObjectLocator
+@some_name = linkonce_odr unnamed_addr constant [2 x i8*] [i8* bitcast (%MSRTTICompleteObjectLocator* @"\01??_R4S@@6B@" to i8*), i8* bitcast (void (%struct.S*, i32)* @"\01??_GS@@UAEPAXI@Z" to i8*)]
+@"\01??_7S@@6B@" = alias getelementptr([2 x i8*]* @some_name, i32 0, i32 1)
+
+declare x86_thiscallcc void @"\01??_GS@@UAEPAXI@Z"(%struct.S*, i32) unnamed_addr

test/Linker/comdat.ll

+; RUN: llvm-link %s %p/Inputs/comdat.ll -S -o - | FileCheck %s
+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat largest
+@foo = global i32 42, comdat $foo
+
+define i32 @bar() comdat $foo {
+  ret i32 42
+}
+
+$qux = comdat largest
+@qux = global i64 12, comdat $qux
+
+define i32 @baz() comdat $qux {
+  ret i32 12
+}
+
+$any = comdat any
+@any = global i64 6, comdat $any
+
+; CHECK: $qux = comdat largest
+; CHECK: $foo = comdat largest
+; CHECK: $any = comdat any
+
+; CHECK: @qux = global i64 12, comdat $qux
+; CHECK: @any = global i64 6, comdat $any
+; CHECK: @foo = global i64 43, comdat $foo
+; CHECK-NOT: @in_unselected_group = global i32 13, comdat $qux
+
+; CHECK: define i32 @baz() comdat $qux
+; CHECK: define i32 @bar() comdat $foo

test/Linker/comdat2.ll

+; RUN: not llvm-link %s %p/Inputs/comdat.ll -S -o - 2>&1 | FileCheck %s
+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat samesize
+@foo = global i32 42, comdat $foo
+; CHECK: Linking COMDATs named 'foo': invalid selection kinds!

test/Linker/comdat3.ll

+; RUN: not llvm-link %s %p/Inputs/comdat2.ll -S -o - 2>&1 | FileCheck %s
+
+$foo = comdat largest
+@foo = global i32 43, comdat $foo
+; CHECK: Linking COMDATs named 'foo': can't do size dependent selection without DataLayout!

test/Linker/comdat4.ll

+; RUN: not llvm-link %s %p/Inputs/comdat3.ll -S -o - 2>&1 | FileCheck %s
+
+$foo = comdat noduplicates
+@foo = global i64 43, comdat $foo
+; CHECK: Linking COMDATs named 'foo': noduplicates has been violated!

test/Linker/comdat5.ll

+; RUN: not llvm-link %s %p/Inputs/comdat4.ll -S -o - 2>&1 | FileCheck %s
+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+$foo = comdat samesize
+@foo = global i32 42, comdat $foo
+; CHECK: Linking COMDATs named 'foo': SameSize violated!

test/Linker/comdat6.ll

+; RUN: llvm-link %s %p/Inputs/comdat5.ll -S -o - 2>&1 | FileCheck %s
+; RUN: llvm-link %p/Inputs/comdat5.ll %s -S -o - 2>&1 | FileCheck %s
+target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+%struct.S = type { i32 (...)** }
+
+$"\01??_7S@@6B@" = comdat largest
+@"\01??_7S@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast (void (%struct.S*, i32)* @"\01??_GS@@UAEPAXI@Z" to i8*)]
+
+; CHECK: @"\01??_7S@@6B@" = alias getelementptr inbounds ([2 x i8*]* @some_name, i32 0, i32 1)
+
+declare x86_thiscallcc void @"\01??_GS@@UAEPAXI@Z"(%struct.S*, i32) unnamed_addr

test/Verifier/comdat.ll

+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+$v = comdat any
+@v = common global i32 0, comdat $v
+; CHECK: 'common' global may not be in a Comdat!

test/Verifier/comdat2.ll

+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+$v = comdat any
+@v = private global i32 0, comdat $v
+; CHECK: comdat global value has local linkage