WinCOFF: Emit common symbols as specified in the COFF spec

Summary:
Local common symbols were properly inserted into the .bss section.
However, putting external common symbols in the .bss section would give
them a strong definition.

Instead, encode them as undefined, external symbols who's symbol value
is equivalent to their size.

Reviewers: Bigcheese, rafael, rnk

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3324

llvm-svn: 205811

3 files changed, 35 insertions, 29 deletions

diff --git a/llvm/lib/MC/MCObjectFileInfo.cpp b/llvm/lib/MC/MCObjectFileInfo.cpp
index 3b011c8bc5a..75147bfb489 100644
--- a/llvm/lib/MC/MCObjectFileInfo.cpp
+++ b/llvm/lib/MC/MCObjectFileInfo.cpp
@@ -548,6 +548,10 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
 
 
 void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
+  // The object file format cannot represent common symbols with explicit
+  // alignments.
+  CommDirectiveSupportsAlignment = false;
+
   // COFF
   BSSSection =
     Ctx->getCOFFSection(".bss",
diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
index 500acd8f14e..f9426c61bde 100644
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -394,7 +394,7 @@ void WinCOFFObjectWriter::DefineSection(MCSectionData const &SectionData) {
   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 @@ void WinCOFFObjectWriter::DefineSymbol(MCSymbolData const &SymbolData,
       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;
diff --git a/llvm/lib/MC/WinCOFFStreamer.cpp b/llvm/lib/MC/WinCOFFStreamer.cpp
index 5bd7b8f0c63..6981bb7982d 100644
--- a/llvm/lib/MC/WinCOFFStreamer.cpp
+++ b/llvm/lib/MC/WinCOFFStreamer.cpp
@@ -45,9 +45,6 @@ public:
                   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 @@ WinCOFFStreamer::WinCOFFStreamer(MCContext &Context, MCAsmBackend &MAB,
                                  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 @@ void WinCOFFStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
   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,