[NVPTX] Do not emit .weak symbols for NVPTX

[jingyue]

Summary

".weak" symbols cannot be consumed by ptxas (PR21685). This patch makes the
weak directive in MCAsmPrinter customizable, and disables emitting ".weak"
symbols for NVPTX.

[majnemer]

include/llvm/MC/MCAsmInfo.h

269

Does it default to .weak?

[jingyue]

David, you are right. I believe the comment on GlobalDirective needs updated too.

[jingyue]

+ justin@nvidia

[jholewinski]

This looks fine for now, considering the same approach is taken for the .globl directive. That said, we really need a better way to handle this. Ptxas supports just enough GNU-style syntax to let us get by without a custom AsmStreamer in most cases, but not enough to avoid these annoying issues.

[jingyue]

Thanks Justin. Closed in r223077.

I totally agree with you. What do you think is a good long-term solution? I'm thinking of overriding EmitFunctionHeader to emit the format ptxas can consume.

include/llvm/MC/MCAsmInfo.h

protected:
   /// Defaults to false.
   bool HasIdentDirective;
 
   /// True if this target supports the MachO .no_dead_strip directive.  Defaults
   /// to false.
   bool HasNoDeadStrip;
 
   /// This directive, if non-null, is used to declare a global as being a weak
+  /// symbol.  Defaults to NULL.
+  const char *WeakDirective;
+
+  /// This directive, if non-null, is used to declare a global as being a weak
   /// undefined symbol.  Defaults to NULL.
   const char *WeakRefDirective;
 
   /// True if we have a directive to declare a global as being a weak defined
   /// symbol.  Defaults to false.
   bool HasWeakDefDirective;
 
   /// True if we have a directive to declare a global as being a weak defined
public:
   }
   LCOMM::LCOMMType getLCOMMDirectiveAlignmentType() const {
     return LCOMMDirectiveAlignmentType;
   }
   bool hasDotTypeDotSizeDirective() const { return HasDotTypeDotSizeDirective; }
   bool hasSingleParameterDotFile() const { return HasSingleParameterDotFile; }
   bool hasIdentDirective() const { return HasIdentDirective; }
   bool hasNoDeadStrip() const { return HasNoDeadStrip; }
+  const char *getWeakDirective() const { return WeakDirective; }
   const char *getWeakRefDirective() const { return WeakRefDirective; }
   bool hasWeakDefDirective() const { return HasWeakDefDirective; }
   bool hasWeakDefCanBeHiddenDirective() const {
     return HasWeakDefCanBeHiddenDirective;
   }
   bool hasLinkOnceDirective() const { return HasLinkOnceDirective; }
 
   MCSymbolAttr getHiddenVisibilityAttr() const { return HiddenVisibilityAttr; }

lib/MC/MCAsmInfo.cpp

MCAsmInfo::MCAsmInfo() {
   SetDirectiveSuppressesReloc = false;
   HasAggressiveSymbolFolding = true;
   COMMDirectiveAlignmentIsInBytes = true;
   LCOMMDirectiveAlignmentType = LCOMM::NoAlignment;
   HasDotTypeDotSizeDirective = true;
   HasSingleParameterDotFile = true;
   HasIdentDirective = false;
   HasNoDeadStrip = false;
+  WeakDirective = "\t.weak\t";
   WeakRefDirective = nullptr;
   HasWeakDefDirective = false;
   HasWeakDefCanBeHiddenDirective = false;
   HasLinkOnceDirective = false;
   HiddenVisibilityAttr = MCSA_Hidden;
   HiddenDeclarationVisibilityAttr = MCSA_Hidden;
   ProtectedVisibilityAttr = MCSA_Protected;
   SupportsDebugInformation = false;

lib/MC/MCAsmStreamer.cpp

bool MCAsmStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
   case MCSA_Local:          OS << "\t.local\t";           break;
   case MCSA_NoDeadStrip:    OS << "\t.no_dead_strip\t";   break;
   case MCSA_SymbolResolver: OS << "\t.symbol_resolver\t"; break;
   case MCSA_PrivateExtern:
     OS << "\t.private_extern\t";
     break;
   case MCSA_Protected:      OS << "\t.protected\t";       break;
   case MCSA_Reference:      OS << "\t.reference\t";       break;
-  case MCSA_Weak:           OS << "\t.weak\t";            break;
+  case MCSA_Weak:           OS << MAI->getWeakDirective(); break;
   case MCSA_WeakDefinition:
     OS << "\t.weak_definition\t";
     break;
       // .weak_reference
   case MCSA_WeakReference:  OS << MAI->getWeakRefDirective(); break;
   case MCSA_WeakDefAutoPrivate: OS << "\t.weak_def_can_be_hidden\t"; break;
   }
 

lib/Target/NVPTX/MCTargetDesc/NVPTXMCAsmInfo.cpp

NVPTXMCAsmInfo::NVPTXMCAsmInfo(StringRef TT) {
   Data16bitsDirective = " .b16 ";
   Data32bitsDirective = " .b32 ";
   Data64bitsDirective = " .b64 ";
   ZeroDirective = " .b8";
   AsciiDirective = " .b8";
   AscizDirective = " .b8";
 
   // @TODO: Can we just disable this?
+  WeakDirective = "\t// .weak\t";
   GlobalDirective = "\t// .globl\t";
 }

test/CodeGen/NVPTX/weak-linkage.ll

 ; RUN: llc < %s -march=nvptx -mcpu=sm_20 | FileCheck %s
 
-
+; CHECK: // .weak foo
 ; CHECK: .weak .func foo
 define weak void @foo() {
   ret void
 }
 
+; CHECK: // .weak baz
+; CHECK: .weak .func baz
+define weak_odr void @baz() {
+  ret void
+}
+
 ; CHECK: .visible .func bar
 define void @bar() {
   ret void
 }