[IR] Copy comdats in GlobalObject::copyAttributesFrom

[rnk]

Summary

This is probably correct for all uses except cross-module IR linking,
where we need to move the comdat from the source module to the
destination module.

Fixes PR27870.

[majnemer]

LGTM

lib/IR/Globals.cpp

void GlobalObject::setGlobalObjectSubClassData(unsigned Val) {
   assert(getGlobalObjectSubClassData() == Val && "representation error");
 }
 
 void GlobalObject::copyAttributesFrom(const GlobalValue *Src) {
   GlobalValue::copyAttributesFrom(Src);
   if (const auto *GV = dyn_cast<GlobalObject>(Src)) {
     setAlignment(GV->getAlignment());
     setSection(GV->getSection());
+    setComdat(const_cast<GlobalObject *>(GV)->getComdat());
   }
 }
 
 std::string GlobalValue::getGlobalIdentifier(StringRef Name,
                                              GlobalValue::LinkageTypes Linkage,
                                              StringRef FileName) {
 
   // Value names may be prefixed with a binary '1' to indicate

lib/Linker/IRMover.cpp

GlobalValue *IRLinker::copyGlobalValueProto(const GlobalValue *SGV,
 
   if (ForDefinition)
     NewGV->setLinkage(SGV->getLinkage());
   else if (SGV->hasExternalWeakLinkage())
     NewGV->setLinkage(GlobalValue::ExternalWeakLinkage);
 
   NewGV->copyAttributesFrom(SGV);
 
+  // Don't copy the comdat, it's from the original module. We'll handle it
+  // later.
+  if (auto *NewGO = dyn_cast<GlobalObject>(NewGV))
+    NewGO->setComdat(nullptr);
+
   // Remove these copied constants in case this stays a declaration, since
   // they point to the source module. If the def is linked the values will
   // be mapped in during linkFunctionBody.
   if (auto *NewF = dyn_cast<Function>(NewGV)) {
     NewF->setPersonalityFn(nullptr);
     NewF->setPrefixData(nullptr);
     NewF->setPrologueData(nullptr);
   }

test/Transforms/DeadArgElim/comdat.ll

+; RUN: opt -S < %s -deadargelim | FileCheck %s
+
+$f = comdat any
+
+define void @f() comdat {
+  call void @g(i32 0)
+  ret void
+}
+
+define internal void @g(i32 %dead) comdat($f) {
+  ret void
+}
+
+; CHECK: define internal void @g() comdat($f) {