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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205811 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index e41fbfc6d3..523d219daa 100644
--- a/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -755,7 +755,7 @@ const MCSection *TargetLoweringObjectFileCOFF::getExplicitSectionGlobal(
 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 @@ SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
   // 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 @@ SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
   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;