[IR] [DAE] Copy comdats during DAE, and don't copy comdats in GlobalO…

…bject::copyAttributesFrom.

Summary: This reverts the changes to Globals.cpp and IRMover.cpp in
"[IR] Copy comdats in GlobalObject::copyAttributesFrom" (D20631,
rL270743).

The DeadArgElim test is left unchanged, and we change DAE to explicitly
copy comdats.

The reverted change breaks copyAttributesFrom when the destination lives
in a different module from the source.  The decision in D21255 was to
revert this patch and handle comdat copying separately from
copyAttributesFrom.

Reviewers: majnemer, rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21403

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272855 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Globals.cpp

@@ -96,7 +96,6 @@ 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

@@ -637,11 +637,6 @@ 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.

lib/Transforms/IPO/DeadArgumentElimination.cpp

@@ -147,6 +147,7 @@ bool DeadArgumentEliminationPass::DeleteDeadVarargs(Function &Fn) {
   // Create the new function body and insert it into the module...
   Function *NF = Function::Create(NFTy, Fn.getLinkage());
   NF->copyAttributesFrom(&Fn);
+  NF->setComdat(Fn.getComdat());
   Fn.getParent()->getFunctionList().insert(Fn.getIterator(), NF);
   NF->takeName(&Fn);
 
@@ -813,6 +814,7 @@ bool DeadArgumentEliminationPass::RemoveDeadStuffFromFunction(Function *F) {
   // Create the new function body and insert it into the module...
   Function *NF = Function::Create(NFTy, F->getLinkage());
   NF->copyAttributesFrom(F);
+  NF->setComdat(F->getComdat());
   NF->setAttributes(NewPAL);
   // Insert the new function before the old function, so we won't be processing
   // it again.