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

Thanks Duncan for the idea

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159687 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 3 insertions, 1 deletions

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index d245783ec6..cea34e16e7 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/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/lib/VMCore/Instructions.cpp b/lib/VMCore/Instructions.cpp
index 5aef459250..9af98e8a9b 100644
--- a/lib/VMCore/Instructions.cpp
+++ b/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;
 }