[-cxx-abi microsoft] Stick zero initialized symbols into the .bss section for COFF

Summary:
We need to do two things:

- Initialize BSSSection in MCObjectFileInfo::InitCOFFMCObjectFileInfo
- Teach TargetLoweringObjectFileCOFF::SelectSectionForGlobal what to do
  with it

This fixes PR16861.

Reviewers: rnk

Reviewed By: rnk

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1361

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188244 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -771,15 +771,18 @@ SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
   }
 
   if (Kind.isText())
-    return getTextSection();
+    return TextSection;
 
   if (Kind.isThreadLocal())
-    return getTLSDataSection();
+    return TLSDataSection;
 
-  if (Kind.isReadOnly() && ReadOnlySection != 0)
+  if (Kind.isReadOnly())
     return ReadOnlySection;
 
-  return getDataSection();
+  if (Kind.isBSS())
+    return BSSSection;
+
+  return DataSection;
 }
 
 void TargetLoweringObjectFileCOFF::

lib/MC/MCObjectFileInfo.cpp

@@ -496,6 +496,12 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
 
 void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
   // COFF
+  BSSSection =
+    Ctx->getCOFFSection(".bss",
+                        COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA |
+                        COFF::IMAGE_SCN_MEM_READ |
+                        COFF::IMAGE_SCN_MEM_WRITE,
+                        SectionKind::getBSS());
   TextSection =
     Ctx->getCOFFSection(".text",
                         COFF::IMAGE_SCN_CNT_CODE |

test/MC/COFF/bss_section.ll

@@ -0,0 +1,6 @@
+; RUN: llc -mtriple i386-pc-win32 < %s | FileCheck %s
+
+%struct.foo = type { i32, i32 }
+
+@"\01?thingy@@3Ufoo@@B" = global %struct.foo zeroinitializer, align 4
+; CHECK: .bss