PHINode::hasConstantValue(): return undef if the PHI is fully recursive.

Thanks Duncan for the idea

llvm-svn: 159687

2 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index d245783ec65..cea34e16e7b 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -694,7 +694,7 @@ void llvm::ComputeMaskedBits(Value *V, APInt &KnownZero, APInt &KnownOne,
     // taking conservative care to avoid excessive recursion.
     if (Depth < MaxDepth - 1 && !KnownZero && !KnownOne) {
       // Skip if every incoming value references to ourself.
-      if (P->hasConstantValue() == P)
+      if (dyn_cast_or_null<UndefValue>(P->hasConstantValue()))
         break;
 
       KnownZero = APInt::getAllOnesValue(BitWidth);
diff --git a/llvm/lib/VMCore/Instructions.cpp b/llvm/lib/VMCore/Instructions.cpp
index 5aef4592501..9af98e8a9b3 100644
--- a/llvm/lib/VMCore/Instructions.cpp
+++ b/llvm/lib/VMCore/Instructions.cpp
@@ -167,6 +167,8 @@ Value *PHINode::hasConstantValue() const {
        // The case where the first value is this PHI.
       ConstantValue = getIncomingValue(i);
     }
+  if (ConstantValue == this)
+    return UndefValue::get(getType());
   return ConstantValue;
 }