[PGO] Create weak alias for the renamed Comdat function

[xur]

Summary

Add a weak alias to the renamed Comdat function in IR level instrumentation, using it's original name. This ensures the same behavior w/ and w/o IR instrumentation, even for non standard conforming code.

[davidxl]

lgtm

llvm/trunk/lib/Transforms/Instrumentation/PGOInstrumentation.cpp

static bool canRenameComdat(
   return true;
 }
 
 // Append the CFGHash to the Comdat function name.
 template <class Edge, class BBInfo>
 void FuncPGOInstrumentation<Edge, BBInfo>::renameComdatFunction() {
   if (!canRenameComdat(F, ComdatMembers))
     return;
+  std::string OrigName = F.getName().str();
   std::string NewFuncName =
       Twine(F.getName() + "." + Twine(FunctionHash)).str();
   F.setName(Twine(NewFuncName));
+  GlobalAlias::create(GlobalValue::WeakAnyLinkage, OrigName, &F);
   FuncName = Twine(FuncName + "." + Twine(FunctionHash)).str();
   Comdat *NewComdat;
   Module *M = F.getParent();
   // For AvailableExternallyLinkage functions, change the linkage to
   // LinkOnceODR and put them into comdat. This is because after renaming, there
   // is no backup external copy available for the function.
   if (!F.hasComdat()) {
     assert(F.getLinkage() == GlobalValue::AvailableExternallyLinkage);
std::string NewComdatName =
       Twine(OrigComdat->getName() + "." + Twine(FunctionHash)).str();
   NewComdat = M->getOrInsertComdat(StringRef(NewComdatName));
   NewComdat->setSelectionKind(OrigComdat->getSelectionKind());
 
   for (auto &&CM : make_range(ComdatMembers.equal_range(OrigComdat))) {
     if (GlobalAlias *GA = dyn_cast<GlobalAlias>(CM.second)) {
       // For aliases, change the name directly.
       assert(dyn_cast<Function>(GA->getAliasee()->stripPointerCasts()) == &F);
+      std::string OrigGAName = GA->getName().str();
       GA->setName(Twine(GA->getName() + "." + Twine(FunctionHash)));
+      GlobalAlias::create(GlobalValue::WeakAnyLinkage, OrigGAName, GA);
       continue;
     }
     // Must be a function.
     Function *CF = dyn_cast<Function>(CM.second);
     assert(CF);
     CF->setComdat(NewComdat);
   }
 }

llvm/trunk/test/Transforms/PGOProfile/comdat_rename.ll

 ; COFFONLY: @af.[[SINGLEBB_HASH]] = alias void (...), bitcast (void ()* @f_with_alias.[[SINGLEBB_HASH]] to
 ; ELFONLY-DAG: @af.[[SINGLEBB_HASH]] = alias void (...), bitcast (void ()* @f_with_alias.[[SINGLEBB_HASH]] to
 define linkonce_odr void @f_with_alias() comdat($f_with_alias) {
   ret void
 }
 
 ; Rename AvailableExternallyLinkage functions
 ; ELFONLY-DAG: $aef.[[SINGLEBB_HASH]] = comdat any
+
+; ELFONLY: @f = weak alias void (), void ()* @f.[[SINGLEBB_HASH]]
+; ELFONLY: @f_with_alias = weak alias void (), void ()* @f_with_alias.[[SINGLEBB_HASH]]
+; ELFONLY: @af = weak alias void (...), void (...)* @af.[[SINGLEBB_HASH]]
+; ELFONLY: @aef = weak alias void (), void ()* @aef.[[SINGLEBB_HASH]]
+
 define available_externally void @aef() {
 ; ELFONLY: define linkonce_odr void @aef.[[SINGLEBB_HASH]]() comdat {
 ; COFFONLY: define available_externally void @aef() {
   ret void
 }