It is possible for SimplifyCFG to cause PHI nodes to become redundant too late in...

pipeline to be caught by instcombine, and it's not feasible to catch them in SimplifyCFG because the
use-lists are in an inconsistent state at the point where it could know that it need to simplify them.
Instead, have CodeGenPrepare look for trivially redundant PHIs as part of its general cleanup effort.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122516 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/CodeGenPrepare.cpp b/lib/Transforms/Scalar/CodeGenPrepare.cpp
index 60c7f7565ebe2e52418f1710c151d16fa2e499ae..1f0930df06ca5d02d1dac493a45e8ab8189c59eb 100644 (file)
--- a/lib/Transforms/Scalar/CodeGenPrepare.cpp
+++ b/lib/Transforms/Scalar/CodeGenPrepare.cpp
@@ -22,6 +22,7 @@
 #include "llvm/Instructions.h"
 #include "llvm/IntrinsicInst.h"
 #include "llvm/Pass.h"
+#include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/ProfileInfo.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Target/TargetLowering.h"
@@ -959,7 +960,15 @@ bool CodeGenPrepare::OptimizeBlock(BasicBlock &BB) {
   for (BasicBlock::iterator BBI = BB.begin(), E = BB.end(); BBI != E; ) {
     Instruction *I = BBI++;
 
-    if (CastInst *CI = dyn_cast<CastInst>(I)) {
+    if (PHINode *P = dyn_cast<PHINode>(I)) {
+      // It is possible for very late stage optimizations (such as SimplifyCFG)
+      // to introduce PHI nodes too late to be cleaned up.  If we detect such a
+      // trivial PHI, go ahead and zap it here.
+      if (Value *V = SimplifyInstruction(P)) {
+        P->replaceAllUsesWith(V);
+        P->eraseFromParent();
+      }
+    } else if (CastInst *CI = dyn_cast<CastInst>(I)) {
       // If the source of the cast is a constant, then this should have
       // already been constant folded.  The only reason NOT to constant fold
       // it is if something (e.g. LSR) was careful to place the constant