COFF: Do not assign linker-weak symbols to selectany comdat sections.

It is mandatory to specify a comdat in order to receive comdat semantics
for a symbol. We were previously getting this wrong in -function-sections
mode; linker-weak symbols were being emitted in a selectany comdat. This
change causes such symbols to use a noduplicates comdat instead, fixing
the inconsistency.

Also correct an inaccuracy in the docs.

Differential Revision: http://reviews.llvm.org/D10828

llvm-svn: 241103

llvm/docs/LangRef.rst

@@ -823,9 +823,11 @@ with the same name.  This is necessary because both globals belong to different
 COMDAT groups and COMDATs, at the object file level, are represented by
 sections.
 
-Note that certain IR constructs like global variables and functions may create
-COMDATs in the object file in addition to any which are specified using COMDAT
-IR.  This arises, for example, when a global variable has linkonce_odr linkage.
+Note that certain IR constructs like global variables and functions may
+create COMDATs in the object file in addition to any which are specified using
+COMDAT IR.  This arises when the code generator is configured to emit globals
+in individual sections (e.g. when `-data-sections` or `-function-sections`
+is supplied to `llc`).
 
 .. _namedmetadatastructure:
 

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -840,8 +840,6 @@ static int getSelectionForCOFF(const GlobalValue *GV) {
     } else {
       return COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE;
     }
-  } else if (GV->isWeakForLinker()) {
-    return COFF::IMAGE_COMDAT_SELECT_ANY;
   }
   return 0;
 }

llvm/test/CodeGen/X86/coff-weak.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
+}