[AsmPrinter] Emit .weak directive for weak linkage on COFF for symbols without a comdat

MC already knows how to emulate the .weak directive (with its ELF
semantics; i.e., an undefined weak symbol resolves to 0, and a defined
weak symbol has lower link precedence than a strong symbol of the same
name) using COFF weak externals. Plumb this through the ASM printer too,
so that definitions marked with __attribute__((weak)) at the language
level (which gets translated to weak linkage at the IR level) have the
corresponding .weak directive emitted. Note that declarations marked
with __attribute__((weak)) at the language level (which translates to
extern_weak at the IR level) already have .weak directives emitted.

Weak*/linkonce* symbols without an associated comdat (in particular, ones
generated with __attribute__((weak)) in C/C++) were earlier emitted as
normal unique globals, as the comdat is required to provide the linkonce
semantics. This change makes sure they are emitted as .weak instead,
allowing other symbols to override them.

Rename the existing coff-weak.ll test to coff-linkonce.ll. I'm not
quite sure what that test covers, since the behavior being tested in it
(the emission of a one_only section) is just a result of passing
-function-sections to llc; the linkonce_odr makes no difference.

Add a new coff-weak.ll which tests the new directive emission.

Based on an previous patch by Shoaib Meenai.

Differential Revision: https://reviews.llvm.org/D44543

llvm/include/llvm/MC/MCAsmInfo.h

@@ -322,9 +322,10 @@ protected:
   /// symbol that can be hidden (unexported).  Defaults to false.
   bool HasWeakDefCanBeHiddenDirective = false;
 
-  /// True if we have a .linkonce directive.  This is used on cygwin/mingw.
+  /// True if we should mark symbols as global instead of weak, for
+  /// weak*/linkonce*, if the symbol has a comdat.
   /// Defaults to false.
-  bool HasLinkOnceDirective = false;
+  bool AvoidWeakIfComdat = false;
 
   /// True if we have a .lglobl directive, which is used to emit the information
   /// of a static symbol into the symbol table. Defaults to false.
@@ -590,7 +591,7 @@ public:
     return HasWeakDefCanBeHiddenDirective;
   }
 
-  bool hasLinkOnceDirective() const { return HasLinkOnceDirective; }
+  bool avoidWeakIfComdat() const { return AvoidWeakIfComdat; }
 
   bool hasDotLGloblDirective() const { return HasDotLGloblDirective; }
 

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -414,7 +414,7 @@ void AsmPrinter::emitLinkage(const GlobalValue *GV, MCSymbol *GVSym) const {
         OutStreamer->emitSymbolAttribute(GVSym, MCSA_WeakDefinition);
       else
         OutStreamer->emitSymbolAttribute(GVSym, MCSA_WeakDefAutoPrivate);
-    } else if (MAI->hasLinkOnceDirective()) {
+    } else if (MAI->avoidWeakIfComdat() && GV->hasComdat()) {
       // .globl _foo
       OutStreamer->emitSymbolAttribute(GVSym, MCSA_Global);
       //NOTE: linkonce is handled by the section the symbol was assigned to.

llvm/lib/MC/MCAsmInfoCOFF.cpp

@@ -26,7 +26,7 @@ MCAsmInfoCOFF::MCAsmInfoCOFF() {
   HasDotTypeDotSizeDirective = false;
   HasSingleParameterDotFile = true;
   WeakRefDirective = "\t.weak\t";
-  HasLinkOnceDirective = true;
+  AvoidWeakIfComdat = true;
 
   // Doesn't support visibility:
   HiddenVisibilityAttr = HiddenDeclarationVisibilityAttr = MCSA_Invalid;

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

llvm/test/CodeGen/X86/coff-weak.ll

@@ -1,9 +1,50 @@
-; RUN: llc -function-sections -o - %s | FileCheck %s
+; RUN: llc -mtriple=i686-windows-msvc -o - %s | FileCheck -check-prefix=X86 %s
+; RUN: llc -mtriple=x86_64-windows-msvc -o - %s | FileCheck -check-prefix=X64 %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() {
+; X86: .weak _foo
+; X64: .weak foo
+define weak void @foo() {
+  ret void
+}
+
+; X86: .weak _bar
+; X64: .weak bar
+define weak_odr void @bar() {
+  ret void
+}
+
+; X86-NOT: .weak _bar_comdat
+; X64-NOT: .weak bar_comdat
+$bar_comdat = comdat any
+
+define weak_odr void @bar_comdat() comdat {
+  ret void
+}
+
+; X86: .weak _baz
+; X64: .weak baz
+define linkonce void @baz() {
+  ret void
+}
+
+; X86-NOT: .weak _baz_comdat
+; X64-NOT: .weak baz_comdat
+$baz_comdat = comdat any
+
+define linkonce void @baz_comdat() comdat {
+  ret void
+}
+
+; X86: .weak _quux
+; X64: .weak quux
+define linkonce_odr void @quux() {
+  ret void
+}
+
+; X86-NOT: .weak _quux_comdat
+; X64-NOT: .weak quux_comdat
+$quux_comdat = comdat any
+
+define linkonce_odr void @quux_comdat() comdat {
   ret void
 }

llvm/test/CodeGen/X86/dllexport-x86_64.ll

@@ -20,16 +20,20 @@ define dllexport void @f2() unnamed_addr {
 }
 
 ; CHECK: .globl lnk1
-define linkonce_odr dllexport void @lnk1() {
+$lnk1 = comdat any
+
+define linkonce_odr dllexport void @lnk1() comdat {
 	ret void
 }
 
 ; CHECK: .globl lnk2
-define linkonce_odr dllexport void @lnk2() alwaysinline {
+$lnk2 = comdat any
+
+define linkonce_odr dllexport void @lnk2() alwaysinline comdat {
 	ret void
 }
 
-; CHECK: .globl weak1
+; CHECK: .weak weak1
 define weak_odr dllexport void @weak1() {
 	ret void
 }
@@ -46,14 +50,14 @@ define weak_odr dllexport void @weak1() {
 ; CHECK: .comm Var3
 @Var3 = common dllexport global i32 0, align 4
 
-; CHECK: .globl WeakVar1
+; CHECK: .weak WeakVar1
 @WeakVar1 = weak_odr dllexport global i32 1, align 4
 
-; CHECK: .globl WeakVar2
+; CHECK: .weak WeakVar2
 @WeakVar2 = weak_odr dllexport unnamed_addr constant i32 1
 
 ; CHECK: .bss
-; CHECK: .globl WeakVar3
+; CHECK: .weak WeakVar3
 @WeakVar3 = weak_odr dllexport global i32 0, align 4
 
 

llvm/test/CodeGen/X86/dllexport.ll

@@ -45,16 +45,20 @@ define dllexport x86_thiscallcc void @thisfun() nounwind {
 }
 
 ; CHECK: .globl _lnk1
-define linkonce_odr dllexport void @lnk1() {
+$lnk1 = comdat any
+
+define linkonce_odr dllexport void @lnk1() comdat {
 	ret void
 }
 
 ; CHECK: .globl _lnk2
-define linkonce_odr dllexport void @lnk2() alwaysinline {
+$lnk2 = comdat any
+
+define linkonce_odr dllexport void @lnk2() alwaysinline comdat {
 	ret void
 }
 
-; CHECK: .globl _weak1
+; CHECK: .weak _weak1
 define weak_odr dllexport void @weak1() {
 	ret void
 }
@@ -71,10 +75,10 @@ define weak_odr dllexport void @weak1() {
 ; CHECK: .comm _Var3
 @Var3 = common dllexport global i32 0, align 4
 
-; CHECK: .globl _WeakVar1
+; CHECK: .weak _WeakVar1
 @WeakVar1 = weak_odr dllexport global i32 1, align 4
 
-; CHECK: .globl _WeakVar2
+; CHECK: .weak _WeakVar2
 @WeakVar2 = weak_odr dllexport unnamed_addr constant i32 1