Merge from mainline because Owen said so.

llvm-svn: 42080

1 files changed, 16 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 75edd8e..7f809e7 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -673,6 +673,7 @@ namespace {
     void dump(DenseMap<BasicBlock*, Value*>& d);
     bool iterateOnFunction(Function &F);
     Value* CollapsePhi(PHINode* p);
+    bool isSafeReplacement(PHINode* p, Instruction* inst);
   };
   
   char GVN::ID = 0;
@@ -731,7 +732,8 @@ Value* GVN::CollapsePhi(PHINode* p) {
   if (constVal) {
     if (Instruction* inst = dyn_cast<Instruction>(constVal)) {
       if (DT.dominates(inst, p))
-        return inst;
+        if (isSafeReplacement(p, inst))
+          return inst;
     } else {
       return constVal;
     }
@@ -740,6 +742,19 @@ Value* GVN::CollapsePhi(PHINode* p) {
   return 0;
 }
 
+bool GVN::isSafeReplacement(PHINode* p, Instruction* inst) {
+  if (!isa<PHINode>(inst))
+    return true;
+  
+  for (Instruction::use_iterator UI = p->use_begin(), E = p->use_end();
+       UI != E; ++UI)
+    if (PHINode* use_phi = dyn_cast<PHINode>(UI))
+      if (use_phi->getParent() == inst->getParent())
+        return false;
+  
+  return true;
+}
+
 /// GetValueForBlock - Get the value to use within the specified basic block.
 /// available values are in Phis.
 Value *GVN::GetValueForBlock(BasicBlock *BB, LoadInst* orig,