[AsmPrinter] Do not define local aliases for global objects in a comdat

[leonardchan]

Summary

A global symbol that is defined in a comdat should not generate an alias since call sites that would've referred to that symbol will refer to their own independent local aliases rather than the surviving global comdat one. This could result in something that looks like:

ld.lld: error: relocation refers to a discarded section: .text._ZN3fbl8internal18NullFunctionTargetIvJjjPjEED1Ev.stub
>>> defined in user-x64-clang/obj/system/ulib/minfs/libminfs.a(minfs._sources.file.cc.o)
>>> section group signature: _ZN3fbl8internal18NullFunctionTargetIvJjjPjEED1Ev.stub
>>> prevailing definition is in user-x64-clang/obj/system/ulib/minfs/libminfs.a(minfs._sources.vnode.cc.o)
>>> referenced by function.h:169 (../../zircon/system/ulib/fbl/include/fbl/function.h:169)
>>>               minfs._sources.file.cc.o:(minfs::File::AllocateAndCommitData(std::__2::unique_ptr<minfs::Transaction, std::__2::default_delete<minfs::Transaction> >)) in archive user-x64-clang/obj/system/ulib/minfs/libminfs.a

We ran into this when experimenting with a new C++ ABI for fuchsia (refer to D72959) which takes relative offsets between comdat'd functions which is why the normal C++ user wouldn't run into this.

[MaskRay]

Thanks. This is correct.

llvm/test/CodeGen/X86/reject-local-symbol-for-comdat.ll

1 ↗

(On Diff #254897)

This may be called semantic-interposition-comdat.ll or similar, just to make the filename clear it is related to semantic interposition.

[MaskRay]

Thanks!

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

MCSymbol *AsmPrinter::getSymbolPreferLocal(const GlobalValue &GV) const {
   // exact definion (intersection of GlobalValue::hasExactDefinition() and
   // !isInterposable()). These linkages include: external, appending, internal,
   // private. It may be profitable to use a local alias for external. The
   // assembler would otherwise be conservative and assume a global default
   // visibility symbol can be interposable, even if the code generator already
   // assumed it.
   if (TM.getTargetTriple().isOSBinFormatELF() &&
       GlobalObject::isExternalLinkage(GV.getLinkage()) && GV.isDSOLocal() &&
-      !GV.isDeclaration() && !isa<GlobalIFunc>(GV))
+      !GV.isDeclaration() && !isa<GlobalIFunc>(GV) && !GV.hasComdat())
     return getSymbolWithGlobalValueBase(&GV, "$local");
   return TM.getSymbol(&GV);
 }
 
 /// EmitGlobalVariable - Emit the specified global variable to the .s file.
 void AsmPrinter::emitGlobalVariable(const GlobalVariable *GV) {
   bool IsEmuTLSVar = TM.useEmulatedTLS() && GV->isThreadLocal();
   assert(!(IsEmuTLSVar && GV->hasCommonLinkage()) &&

llvm/test/CodeGen/X86/semantic-interposition-comdat.ll

+; RUN: llc -mtriple x86_64-unknown-linux-gnu %s -o - | FileCheck %s
+
+$comdat_func = comdat any
+
+; CHECK-LABEL: func2:
+; CHECK-NEXT: .Lfunc2$local
+
+declare void @func()
+
+define hidden void @func2() {
+entry:
+  call void @func()
+  ret void
+}
+
+; CHECK: comdat_func:
+; CHECK-NOT: .Lcomdat_func$local
+
+define hidden void @comdat_func() comdat {
+entry:
+  call void @func()
+  ret void
+}