Index: llvm/trunk/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
===================================================================
--- llvm/trunk/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
+++ llvm/trunk/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
@@ -134,6 +134,11 @@
 class TargetLoweringObjectFileCOFF : public TargetLoweringObjectFile {
   mutable unsigned NextUniqueID = 0;
 
+  /// Append "$symbol" to the section name when targetting mingw. The ld.bfd
+  /// COFF linker will not properly handle comdats otherwise.
+  void appendComdatSymbolForMinGW(SmallVectorImpl<char> &SecName,
+                                  StringRef Symbol, const DataLayout &DL) const;
+
 public:
   ~TargetLoweringObjectFileCOFF() override = default;
 
Index: llvm/trunk/include/llvm/MC/MCAsmInfo.h
===================================================================
--- llvm/trunk/include/llvm/MC/MCAsmInfo.h
+++ llvm/trunk/include/llvm/MC/MCAsmInfo.h
@@ -84,6 +84,11 @@
   /// directive for emitting thread local BSS Symbols.  Default is false.
   bool HasMachoTBSSDirective = false;
 
+  /// True if this is a non-GNU COFF target. The COFF port of the GNU linker
+  /// doesn't handle associative comdats in the way that we would like to use
+  /// them.
+  bool HasCOFFAssociativeComdats = false;
+
   /// This is the maximum possible length of an instruction, which is needed to
   /// compute the size of an inline asm.  Defaults to 4.
   unsigned MaxInstLength = 4;
@@ -463,6 +468,7 @@
 
   bool hasMachoZeroFillDirective() const { return HasMachoZeroFillDirective; }
   bool hasMachoTBSSDirective() const { return HasMachoTBSSDirective; }
+  bool hasCOFFAssociativeComdats() const { return HasCOFFAssociativeComdats; }
   unsigned getMaxInstLength() const { return MaxInstLength; }
   unsigned getMinInstAlignment() const { return MinInstAlignment; }
   bool getDollarIsPC() const { return DollarIsPC; }
Index: llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -1060,7 +1060,7 @@
                                      Selection);
 }
 
-static const char *getCOFFSectionNameForUniqueGlobal(SectionKind Kind) {
+static StringRef getCOFFSectionNameForUniqueGlobal(SectionKind Kind) {
   if (Kind.isText())
     return ".text";
   if (Kind.isBSS())
@@ -1072,6 +1072,15 @@
   return ".data";
 }
 
+void TargetLoweringObjectFileCOFF::appendComdatSymbolForMinGW(
+    SmallVectorImpl<char> &SecName, StringRef Symbol,
+    const DataLayout &DL) const {
+  if (getTargetTriple().isWindowsGNUEnvironment()) {
+    SecName.push_back('$');
+    getMangler().getNameWithPrefix(SecName, Symbol, DL);
+  }
+}
+
 MCSection *TargetLoweringObjectFileCOFF::SelectSectionForGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   // If we have -ffunction-sections then we should emit the global value to a
@@ -1083,7 +1092,8 @@
     EmitUniquedSection = TM.getDataSections();
 
   if ((EmitUniquedSection && !Kind.isCommon()) || GO->hasComdat()) {
-    const char *Name = getCOFFSectionNameForUniqueGlobal(Kind);
+    SmallString<256> Name = getCOFFSectionNameForUniqueGlobal(Kind);
+
     unsigned Characteristics = getCOFFSectionFlags(Kind, TM);
 
     Characteristics |= COFF::IMAGE_SCN_LNK_COMDAT;
@@ -1103,6 +1113,8 @@
     if (!ComdatGV->hasPrivateLinkage()) {
       MCSymbol *Sym = TM.getSymbol(ComdatGV);
       StringRef COMDATSymName = Sym->getName();
+      appendComdatSymbolForMinGW(Name, COMDATSymName,
+                                 GO->getParent()->getDataLayout());
       return getContext().getCOFFSection(Name, Characteristics, Kind,
                                          COMDATSymName, Selection, UniqueID);
     } else {
@@ -1160,13 +1172,14 @@
   StringRef COMDATSymName = Sym->getName();
 
   SectionKind Kind = SectionKind::getReadOnly();
-  const char *Name = getCOFFSectionNameForUniqueGlobal(Kind);
+  StringRef SecName = getCOFFSectionNameForUniqueGlobal(Kind);
   unsigned Characteristics = getCOFFSectionFlags(Kind, TM);
   Characteristics |= COFF::IMAGE_SCN_LNK_COMDAT;
   unsigned UniqueID = NextUniqueID++;
 
-  return getContext().getCOFFSection(Name, Characteristics, Kind, COMDATSymName,
-                                     COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE, UniqueID);
+  return getContext().getCOFFSection(
+      SecName, Characteristics, Kind, COMDATSymName,
+      COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE, UniqueID);
 }
 
 void TargetLoweringObjectFileCOFF::emitModuleMetadata(MCStreamer &Streamer,
Index: llvm/trunk/lib/MC/MCAsmInfoCOFF.cpp
===================================================================
--- llvm/trunk/lib/MC/MCAsmInfoCOFF.cpp
+++ llvm/trunk/lib/MC/MCAsmInfoCOFF.cpp
@@ -41,6 +41,10 @@
 
   // At least MSVC inline-asm does AShr.
   UseLogicalShr = false;
+
+  // If this is a COFF target, assume that it supports associative comdats. It's
+  // part of the spec.
+  HasCOFFAssociativeComdats = true;
 }
 
 void MCAsmInfoMicrosoft::anchor() {}
@@ -49,4 +53,9 @@
 
 void MCAsmInfoGNUCOFF::anchor() {}
 
-MCAsmInfoGNUCOFF::MCAsmInfoGNUCOFF() = default;
+MCAsmInfoGNUCOFF::MCAsmInfoGNUCOFF() {
+  // If this is a GNU environment (mingw or cygwin), don't use associative
+  // comdats for jump tables, unwind information, and other data associated with
+  // a function.
+  HasCOFFAssociativeComdats = false;
+}
Index: llvm/trunk/lib/MC/MCStreamer.cpp
===================================================================
--- llvm/trunk/lib/MC/MCStreamer.cpp
+++ llvm/trunk/lib/MC/MCStreamer.cpp
@@ -673,16 +673,31 @@
     return MainCFISec;
 
   const auto *TextSecCOFF = cast<MCSectionCOFF>(TextSec);
+  auto *MainCFISecCOFF = cast<MCSectionCOFF>(MainCFISec);
   unsigned UniqueID = TextSecCOFF->getOrAssignWinCFISectionID(NextWinCFIID);
 
   // If this section is COMDAT, this unwind section should be COMDAT associative
   // with its group.
   const MCSymbol *KeySym = nullptr;
-  if (TextSecCOFF->getCharacteristics() & COFF::IMAGE_SCN_LNK_COMDAT)
+  if (TextSecCOFF->getCharacteristics() & COFF::IMAGE_SCN_LNK_COMDAT) {
     KeySym = TextSecCOFF->getCOMDATSymbol();
 
-  return Context.getAssociativeCOFFSection(cast<MCSectionCOFF>(MainCFISec),
-                                           KeySym, UniqueID);
+    // In a GNU environment, we can't use associative comdats. Instead, do what
+    // GCC does, which is to make plain comdat selectany section named like
+    // ".[px]data$_Z3foov".
+    if (!Context.getAsmInfo()->hasCOFFAssociativeComdats()) {
+      std::string SectionName =
+          (MainCFISecCOFF->getSectionName() + "$" +
+           TextSecCOFF->getSectionName().split('$').second)
+              .str();
+      return Context.getCOFFSection(
+          SectionName,
+          MainCFISecCOFF->getCharacteristics() | COFF::IMAGE_SCN_LNK_COMDAT,
+          MainCFISecCOFF->getKind(), "", COFF::IMAGE_COMDAT_SELECT_ANY);
+    }
+  }
+
+  return Context.getAssociativeCOFFSection(MainCFISecCOFF, KeySym, UniqueID);
 }
 
 MCSection *MCStreamer::getAssociatedPDataSection(const MCSection *TextSec) {
Index: llvm/trunk/test/CodeGen/X86/mingw-comdats.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/mingw-comdats.ll
+++ llvm/trunk/test/CodeGen/X86/mingw-comdats.ll
@@ -0,0 +1,62 @@
+; RUN: llc -mtriple=x86_64-windows-itanium < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-windows-msvc < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-w64-windows-gnu < %s | FileCheck %s --check-prefix=GNU
+; RUN: llc -mtriple=x86_64-w64-windows-gnu < %s -filetype=obj | llvm-objdump - -headers | FileCheck %s --check-prefix=GNUOBJ
+
+; GCC and MSVC handle comdats completely differently. Make sure we do the right
+; thing for each.
+
+; Generated with this C++ source:
+; int bar(int);
+; __declspec(selectany) int gv = 42;
+; inline int foo(int x) { return bar(x) + gv; }
+; int main() { return foo(1); }
+
+$_Z3fooi = comdat any
+
+$gv = comdat any
+
+@gv = weak_odr dso_local global i32 42, comdat, align 4
+
+; Function Attrs: norecurse uwtable
+define dso_local i32 @main() #0 {
+entry:
+  %call = tail call i32 @_Z3fooi(i32 1)
+  ret i32 %call
+}
+
+; CHECK: main:
+; GNU: main:
+
+; Function Attrs: inlinehint uwtable
+define linkonce_odr dso_local i32 @_Z3fooi(i32 %x) #1 comdat {
+entry:
+  %call = tail call i32 @_Z3bari(i32 %x)
+  %0 = load i32, i32* @gv, align 4
+  %add = add nsw i32 %0, %call
+  ret i32 %add
+}
+
+; CHECK: .section        .text,"xr",discard,_Z3fooi
+; CHECK: _Z3fooi:
+; CHECK: .section        .data,"dw",discard,gv
+; CHECK: gv:
+; CHECK: .long 42
+
+; GNU: .section        .text$_Z3fooi,"xr",discard,_Z3fooi
+; GNU: _Z3fooi:
+; GNU: .section        .data$gv,"dw",discard,gv
+; GNU: gv:
+; GNU: .long 42
+
+; Make sure the assembler puts the .xdata and .pdata in sections with the right
+; names.
+; GNUOBJ: .text$_Z3fooi
+; GNUOBJ: .xdata$_Z3fooi
+; GNUOBJ: .data$gv
+; GNUOBJ: .pdata$_Z3fooi
+
+declare dso_local i32 @_Z3bari(i32)
+
+attributes #0 = { norecurse uwtable }
+attributes #1 = { inlinehint uwtable }