Index: llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -755,7 +755,7 @@
 static const char *getCOFFSectionNameForUniqueGlobal(SectionKind Kind) {
   if (Kind.isText())
     return ".text";
-  if (Kind.isBSS ())
+  if (Kind.isBSS())
     return ".bss";
   if (Kind.isThreadLocal())
     return ".tls$";
@@ -781,7 +781,7 @@
   // 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()) {
+      !GV->hasPrivateLinkage() && !Kind.isCommon()) {
     const char *Name = getCOFFSectionNameForUniqueGlobal(Kind);
     unsigned Characteristics = getCOFFSectionFlags(Kind);
 
@@ -802,7 +802,10 @@
   if (Kind.isReadOnly())
     return ReadOnlySection;
 
-  if (Kind.isBSS())
+  // Note: we claim that common symbols are put in BSSSection, but they are
+  // really emitted with the magic .comm directive, which creates a symbol table
+  // entry but not a section.
+  if (Kind.isBSS() || Kind.isCommon())
     return BSSSection;
 
   return DataSection;
Index: llvm/trunk/lib/MC/MCObjectFileInfo.cpp
===================================================================
--- llvm/trunk/lib/MC/MCObjectFileInfo.cpp
+++ llvm/trunk/lib/MC/MCObjectFileInfo.cpp
@@ -548,6 +548,10 @@
 
 
 void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
+  // The object file format cannot represent common symbols with explicit
+  // alignments.
+  CommDirectiveSupportsAlignment = false;
+
   // COFF
   BSSSection =
     Ctx->getCOFFSection(".bss",
Index: llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp
===================================================================
--- llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp
+++ llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp
@@ -394,7 +394,7 @@
   SectionMap[&SectionData.getSection()] = coff_section;
 }
 
-/// This function takes a section data object from the assembler
+/// This function takes a symbol data object from the assembler
 /// and creates the associated COFF symbol staging object.
 void WinCOFFObjectWriter::DefineSymbol(MCSymbolData const &SymbolData,
                                        MCAssembler &Assembler,
@@ -443,6 +443,8 @@
       int64_t Addr;
       if (Symbol.getVariableValue()->EvaluateAsAbsolute(Addr, Layout))
         coff_symbol->Data.Value = Addr;
+    } else if (SymbolData.isExternal() && SymbolData.isCommon()) {
+      coff_symbol->Data.Value = SymbolData.getCommonSize();
     }
 
     coff_symbol->Data.Type         = (ResSymData.getFlags() & 0x0000FFFF) >>  0;
Index: llvm/trunk/lib/MC/WinCOFFStreamer.cpp
===================================================================
--- llvm/trunk/lib/MC/WinCOFFStreamer.cpp
+++ llvm/trunk/lib/MC/WinCOFFStreamer.cpp
@@ -45,9 +45,6 @@
                   MCCodeEmitter &CE,
                   raw_ostream &OS);
 
-  void AddCommonSymbol(MCSymbol *Symbol, uint64_t Size,
-                       unsigned ByteAlignment, bool External);
-
   // MCStreamer interface
 
   void InitSections() override;
@@ -101,26 +98,6 @@
                                  MCCodeEmitter &CE, raw_ostream &OS)
     : MCObjectStreamer(Context, MAB, OS, &CE), CurSymbol(NULL) {}
 
-void WinCOFFStreamer::AddCommonSymbol(MCSymbol *Symbol, uint64_t Size,
-                                      unsigned ByteAlignment, bool External) {
-  assert(!Symbol->isInSection() && "Symbol must not already have a section!");
-
-  const MCSection *Section = getContext().getObjectFileInfo()->getBSSSection();
-  MCSectionData &SectionData = getAssembler().getOrCreateSectionData(*Section);
-  if (SectionData.getAlignment() < ByteAlignment)
-    SectionData.setAlignment(ByteAlignment);
-
-  MCSymbolData &SymbolData = getAssembler().getOrCreateSymbolData(*Symbol);
-  SymbolData.setExternal(External);
-
-  AssignSection(Symbol, Section);
-
-  if (ByteAlignment != 1)
-      new MCAlignFragment(ByteAlignment, 0, 0, ByteAlignment, &SectionData);
-
-  SymbolData.setFragment(new MCFillFragment(0, 0, Size, &SectionData));
-}
-
 // MCStreamer interface
 
 void WinCOFFStreamer::InitSections() {
@@ -244,15 +221,38 @@
   assert((Symbol->isInSection()
          ? Symbol->getSection().getVariant() == MCSection::SV_COFF
          : true) && "Got non-COFF section in the COFF backend!");
-  AddCommonSymbol(Symbol, Size, ByteAlignment, true);
+
+  if (ByteAlignment > 32)
+    report_fatal_error(
+        "The linker won't align common symbols beyond 32 bytes.");
+
+  AssignSection(Symbol, NULL);
+
+  MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
+  SD.setExternal(true);
+  SD.setCommon(Size, ByteAlignment);
 }
 
 void WinCOFFStreamer::EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                                             unsigned ByteAlignment) {
-  assert((Symbol->isInSection()
-         ? Symbol->getSection().getVariant() == MCSection::SV_COFF
-         : true) && "Got non-COFF section in the COFF backend!");
-  AddCommonSymbol(Symbol, Size, ByteAlignment, false);
+  assert(!Symbol->isInSection() && "Symbol must not already have a section!");
+
+  const MCSection *Section = getContext().getObjectFileInfo()->getBSSSection();
+  MCSectionData &SectionData = getAssembler().getOrCreateSectionData(*Section);
+  if (SectionData.getAlignment() < ByteAlignment)
+    SectionData.setAlignment(ByteAlignment);
+
+  MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
+  SD.setExternal(false);
+
+  AssignSection(Symbol, Section);
+
+  if (ByteAlignment != 1)
+    new MCAlignFragment(ByteAlignment, /*_Value=*/0, /*_ValueSize=*/0,
+                        ByteAlignment, &SectionData);
+
+  SD.setFragment(
+      new MCFillFragment(/*_Value=*/0, /*_ValueSize=*/0, Size, &SectionData));
 }
 
 void WinCOFFStreamer::EmitZerofill(const MCSection *Section, MCSymbol *Symbol,
Index: llvm/trunk/test/MC/COFF/comm.ll
===================================================================
--- llvm/trunk/test/MC/COFF/comm.ll
+++ llvm/trunk/test/MC/COFF/comm.ll
@@ -9,5 +9,5 @@
 ; CHECK: .lcomm	_a,1
 ; CHECK: .lcomm	_b,8,8
 ; .comm uses log2 alignment
-; CHECK: .comm	_c,1,0
-; CHECK: .comm	_d,8,3
+; CHECK: .comm	_c,1
+; CHECK: .comm	_d,8
Index: llvm/trunk/test/MC/COFF/comm.s
===================================================================
--- llvm/trunk/test/MC/COFF/comm.s
+++ llvm/trunk/test/MC/COFF/comm.s
@@ -1,7 +1,7 @@
 // RUN: llvm-mc -filetype=obj -triple i686-pc-win32 %s | llvm-readobj -t | FileCheck %s
 
 .lcomm _a,4,4
-.comm	_b, 4, 2
+.comm	_b, 4
 
 
 // CHECK:       Symbol {
@@ -17,7 +17,7 @@
 // CHECK:       Symbol {
 // CHECK:         Name: _b
 // CHECK-NEXT:    Value: 4
-// CHECK-NEXT:    Section: .bss
+// CHECK-NEXT:    Section:  (0)
 // CHECK-NEXT:    BaseType: Null
 // CHECK-NEXT:    ComplexType: Null
 // CHECK-NEXT:    StorageClass: External