[RS4GC] Lazily set changed flag when folding single entry phis

The function FoldSingleEntryPHINodes() is changed to return if
it has changed IR or not. This return value is used by RS4GC to
set the MadeChange flag respectively.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D93810

llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h

@@ -74,7 +74,7 @@ bool EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU = nullptr,
 /// in it, fold them away. This handles the case when all entries to the PHI
 /// nodes in a block are guaranteed equal, such as when the block has exactly
 /// one predecessor.
-void FoldSingleEntryPHINodes(BasicBlock *BB,
+bool FoldSingleEntryPHINodes(BasicBlock *BB,
                              MemoryDependenceResults *MemDep = nullptr);
 
 /// Examine each PHI in the given block and delete it if it is dead. Also

llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

@@ -2735,10 +2735,8 @@ bool RewriteStatepointsForGC::runOnFunction(Function &F, DominatorTree &DT,
   // of liveness sets for no good reason.  It may be harder to do this post
   // insertion since relocations and base phis can confuse things.
   for (BasicBlock &BB : F)
-    if (BB.getUniquePredecessor()) {
-      MadeChange = true;
-      FoldSingleEntryPHINodes(&BB);
-    }
+    if (BB.getUniquePredecessor())
+      MadeChange |= FoldSingleEntryPHINodes(&BB);
 
   // Before we start introducing relocations, we want to tweak the IR a bit to
   // avoid unfortunate code generation effects.  The main example is that we

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -136,9 +136,10 @@ bool llvm::EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU,
   return !DeadBlocks.empty();
 }
 
-void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
+bool llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
                                    MemoryDependenceResults *MemDep) {
-  if (!isa<PHINode>(BB->begin())) return;
+  if (!isa<PHINode>(BB->begin()))
+    return false;
 
   while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
     if (PN->getIncomingValue(0) != PN)
@@ -151,6 +152,7 @@ void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
 
     PN->eraseFromParent();
   }
+  return true;
 }
 
 bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI,