Index: llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -404,7 +404,8 @@
   case GlobalValue::LinkOnceAnyLinkage:
   case GlobalValue::LinkOnceODRLinkage:
   case GlobalValue::WeakAnyLinkage:
-  case GlobalValue::WeakODRLinkage:
+  case GlobalValue::WeakODRLinkage: {
+    const Triple &TT = TM.getTargetTriple();
     if (MAI->hasWeakDefDirective()) {
       // .globl _foo
       OutStreamer->emitSymbolAttribute(GVSym, MCSA_Global);
@@ -414,6 +415,13 @@
         OutStreamer->emitSymbolAttribute(GVSym, MCSA_WeakDefinition);
       else
         OutStreamer->emitSymbolAttribute(GVSym, MCSA_WeakDefAutoPrivate);
+    } else if (TT.isOSWindows() && TT.isOSBinFormatCOFF() && !GV->hasComdat()) {
+      // hasLinkOnceDirective is true for COFF, but the linkonce directive
+      // can only be expressed if the symbol is associated with a comdat group.
+      // Thus emit weak if we can't express the intent with a linkonce
+      // directive.
+      // .weak _foo
+      OutStreamer->emitSymbolAttribute(GVSym, MCSA_Weak);
     } else if (MAI->hasLinkOnceDirective()) {
       // .globl _foo
       OutStreamer->emitSymbolAttribute(GVSym, MCSA_Global);
@@ -423,6 +431,7 @@
       OutStreamer->emitSymbolAttribute(GVSym, MCSA_Weak);
     }
     return;
+  }
   case GlobalValue::ExternalLinkage:
     // If external, declare as a global symbol: .globl _foo
     OutStreamer->emitSymbolAttribute(GVSym, MCSA_Global);
Index: llvm/test/CodeGen/X86/coff-linkonce.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/coff-linkonce.ll
@@ -0,0 +1,9 @@
+; RUN: llc -function-sections -o - %s | FileCheck %s
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+; CHECK: .section{{.*}}one_only
+define linkonce_odr void @foo() {
+  ret void
+}
Index: llvm/test/CodeGen/X86/coff-weak.ll
===================================================================
--- llvm/test/CodeGen/X86/coff-weak.ll
+++ llvm/test/CodeGen/X86/coff-weak.ll
@@ -1,9 +1,50 @@
-; RUN: llc -function-sections -o - %s | FileCheck %s
+; RUN: llc -mtriple=i686-windows-msvc -o - %s | FileCheck -check-prefix=X86 %s
+; RUN: llc -mtriple=x86_64-windows-msvc -o - %s | FileCheck -check-prefix=X64 %s
 
-target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
-target triple = "x86_64-pc-windows-msvc"
+; X86: .weak _foo
+; X64: .weak foo
+define weak void @foo() {
+  ret void
+}
+
+; X86: .weak _bar
+; X64: .weak bar
+define weak_odr void @bar() {
+  ret void
+}
+
+; X86-NOT: .weak _bar_comdat
+; X64-NOT: .weak bar_comdat
+$bar_comdat = comdat any
+
+define weak_odr void @bar_comdat() comdat {
+  ret void
+}
+
+; X86: .weak _baz
+; X64: .weak baz
+define linkonce void @baz() {
+  ret void
+}
+
+; X86-NOT: .weak _baz_comdat
+; X64-NOT: .weak baz_comdat
+$baz_comdat = comdat any
+
+define linkonce void @baz_comdat() comdat {
+  ret void
+}
+
+; X86: .weak _quux
+; X64: .weak quux
+define linkonce_odr void @quux() {
+  ret void
+}
+
+; X86-NOT: .weak _quux_comdat
+; X64-NOT: .weak quux_comdat
+$quux_comdat = comdat any
 
-; CHECK: .section{{.*}}one_only
-define linkonce_odr void @foo() {
+define linkonce_odr void @quux_comdat() comdat {
   ret void
 }
Index: llvm/test/CodeGen/X86/dllexport-x86_64.ll
===================================================================
--- llvm/test/CodeGen/X86/dllexport-x86_64.ll
+++ llvm/test/CodeGen/X86/dllexport-x86_64.ll
@@ -20,16 +20,20 @@
 }
 
 ; CHECK: .globl lnk1
-define linkonce_odr dllexport void @lnk1() {
+$lnk1 = comdat any
+
+define linkonce_odr dllexport void @lnk1() comdat {
 	ret void
 }
 
 ; CHECK: .globl lnk2
-define linkonce_odr dllexport void @lnk2() alwaysinline {
+$lnk2 = comdat any
+
+define linkonce_odr dllexport void @lnk2() alwaysinline comdat {
 	ret void
 }
 
-; CHECK: .globl weak1
+; CHECK: .weak weak1
 define weak_odr dllexport void @weak1() {
 	ret void
 }
@@ -46,14 +50,14 @@
 ; CHECK: .comm Var3
 @Var3 = common dllexport global i32 0, align 4
 
-; CHECK: .globl WeakVar1
+; CHECK: .weak WeakVar1
 @WeakVar1 = weak_odr dllexport global i32 1, align 4
 
-; CHECK: .globl WeakVar2
+; CHECK: .weak WeakVar2
 @WeakVar2 = weak_odr dllexport unnamed_addr constant i32 1
 
 ; CHECK: .bss
-; CHECK: .globl WeakVar3
+; CHECK: .weak WeakVar3
 @WeakVar3 = weak_odr dllexport global i32 0, align 4
 
 
Index: llvm/test/CodeGen/X86/dllexport.ll
===================================================================
--- llvm/test/CodeGen/X86/dllexport.ll
+++ llvm/test/CodeGen/X86/dllexport.ll
@@ -45,16 +45,20 @@
 }
 
 ; CHECK: .globl _lnk1
-define linkonce_odr dllexport void @lnk1() {
+$lnk1 = comdat any
+
+define linkonce_odr dllexport void @lnk1() comdat {
 	ret void
 }
 
 ; CHECK: .globl _lnk2
-define linkonce_odr dllexport void @lnk2() alwaysinline {
+$lnk2 = comdat any
+
+define linkonce_odr dllexport void @lnk2() alwaysinline comdat {
 	ret void
 }
 
-; CHECK: .globl _weak1
+; CHECK: .weak _weak1
 define weak_odr dllexport void @weak1() {
 	ret void
 }
@@ -71,10 +75,10 @@
 ; CHECK: .comm _Var3
 @Var3 = common dllexport global i32 0, align 4
 
-; CHECK: .globl _WeakVar1
+; CHECK: .weak _WeakVar1
 @WeakVar1 = weak_odr dllexport global i32 1, align 4
 
-; CHECK: .globl _WeakVar2
+; CHECK: .weak _WeakVar2
 @WeakVar2 = weak_odr dllexport unnamed_addr constant i32 1