[GlobalValue] Make dso_local function work with comdat nodeduplicate

[MaskRay]

Summary

This fixes -fno-semantic-interposition -fsanitize-coverage incompatibility.

-fPIC -fno-semantic-interposition may add dso_local to an external linkage
function. -fsanitize-coverage instrumentation does not clear dso_local when
adding comdat nodeduplicate. This causes a compatibility issue: the function
symbol may be referenced by a PC-relative relocation without using the local
alias. In -shared mode, ld will report a relocation error.

The fix is to either clear dso_local when adding comdat nodeduplicate, or
supporting comdat nodeduplicate. The latter is more appropriate, because a
comdat nodeduplicate is like not using comdat.

Note: The comdat condition was originally added by D77429 to not use local alias
for a hidden external linkage function in a deduplicate comdat. The condition
has been unused since the code was refactored to only use local alias for
default visibility symbols.
Note: canBenefitFromLocalAlias is used by clang/lib/CodeGen/CodeGenModule.cpp
and we don't want to add dso_local to default visibility external linkage comdat any
(clang/test/CodeGenCUDA/usual-deallocators.cu).

[ljmf00]

I had to look into the documentation to understand how comdat works. Compared to the diffs I generated early this seems fine. I also did a full bootstrap multi-stage build of clang with this patch and the patches and flags used by my host clang. I can't reproduce the issue I described on D102453, so it seems fixed. Thanks!

[MaskRay]

Chatted with @leonardchan: let me just submit it.

[MaskRay]

Fix CodeGenCUDA/usual-deallocators.cu (strange default visibility external linkage comdat)

llvm/lib/IR/Globals.cpp

 bool GlobalValue::isInterposable() const {
   if (isInterposableLinkage(getLinkage()))
     return true;
   return getParent() && getParent()->getSemanticInterposition() &&
          !isDSOLocal();
 }
 
 bool GlobalValue::canBenefitFromLocalAlias() const {
-  // See AsmPrinter::getSymbolPreferLocal().
+  // See AsmPrinter::getSymbolPreferLocal(). For a deduplicate comdat kind,
+  // references to this symbol table entry from outside the group are not
+  // allowed, so exclude it.
+  auto isDeduplicateComdat = [](const Comdat *C) {
+    return C && C->getSelectionKind() != Comdat::NoDeduplicate;
+  };
   return hasDefaultVisibility() &&
          GlobalObject::isExternalLinkage(getLinkage()) && !isDeclaration() &&
-         !isa<GlobalIFunc>(this) && !hasComdat();
+         !isa<GlobalIFunc>(this) && !isDeduplicateComdat(getComdat());
 }
 
 unsigned GlobalValue::getAddressSpace() const {
   PointerType *PtrTy = getType();
   return PtrTy->getAddressSpace();
 }
 
 void GlobalObject::setAlignment(MaybeAlign Align) {

llvm/test/CodeGen/X86/linux-preemption.ll

 declare dso_preemptable void @external_preemptable_function()
 define void()* @get_external_preemptable_function() {
   ret void()* @external_preemptable_function
 }
 ; CHECK: movq external_preemptable_function@GOTPCREL(%rip), %rax
 ; STATIC: movq external_preemptable_function@GOTPCREL(%rip), %rax
 ; CHECK32: movl external_preemptable_function@GOT(%eax), %eax
 
+$comdat_nodeduplicate_local = comdat nodeduplicate
+$comdat_nodeduplicate_preemptable = comdat nodeduplicate
+$comdat_any_local = comdat any
+
+;; -fpic -fno-semantic-interposition may add dso_local. Some instrumentation
+;; may add comdat nodeduplicate. We should use local aliases to make the symbol
+;; non-preemptible in the linker.
+define dso_local i8* @comdat_nodeduplicate_local() comdat {
+  ret i8* bitcast (i8* ()* @comdat_nodeduplicate_local to i8*)
+}
+; CHECK: leaq .Lcomdat_nodeduplicate_local$local(%rip), %rax
+; STATIC: movl $comdat_nodeduplicate_local, %eax
+
+define dso_preemptable i8* @comdat_nodeduplicate_preemptable() comdat {
+  ret i8* bitcast (i8* ()* @comdat_nodeduplicate_preemptable to i8*)
+}
+; CHECK: movq comdat_nodeduplicate_preemptable@GOTPCREL(%rip), %rax
+; STATIC: movq comdat_nodeduplicate_preemptable@GOTPCREL(%rip), %rax
+
+;; The desired behavior is unclear.
+define dso_local i8* @comdat_any_local() comdat {
+  ret i8* bitcast (i8* ()* @comdat_any_local to i8*)
+}
+; CHECK: leaq comdat_any_local(%rip), %rax
+; STATIC: movl $comdat_any_local, %eax
+
 !llvm.module.flags = !{!0}
 !0 = !{i32 7, !"PIC Level", i32 2}
 
 ; COMMON:     {{^}}strong_local_global:
 ; CHECK-NEXT: .Lstrong_local_global$local:
 
 ; COMMON:      .globl strong_default_alias
 ; COMMON-NEXT: .set strong_default_alias, aliasee