[IR] Copy comdats in GlobalObject::copyAttributesFrom

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.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D20631

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270743 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Globals.cpp

@@ -96,6 +96,7 @@ void GlobalObject::copyAttributesFrom(const GlobalValue *Src) {
   if (const auto *GV = dyn_cast<GlobalObject>(Src)) {
     setAlignment(GV->getAlignment());
     setSection(GV->getSection());
+    setComdat(const_cast<GlobalObject *>(GV)->getComdat());
   }
 }
 

lib/Linker/IRMover.cpp

@@ -622,6 +622,11 @@ GlobalValue *IRLinker::copyGlobalValueProto(const GlobalValue *SGV,
 
   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.

test/Transforms/DeadArgElim/comdat.ll

@@ -0,0 +1,14 @@
+; 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) {