Handle a special case xor undef, undef -> 0. Technically this should be transformed to undef. But this is such a common idiom (misuse) we are going to handle it.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48791 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 8ebd2cc..35715fb 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -4425,8 +4425,13 @@
bool Changed = SimplifyCommutative(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
- if (isa<UndefValue>(Op1))
+ if (isa<UndefValue>(Op1)) {
+ if (isa<UndefValue>(Op0))
+ // Handle undef ^ undef -> 0 special case. This is a common
+ // idiom (misuse).
+ return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
return ReplaceInstUsesWith(I, Op1); // X ^ undef -> undef
+ }
// xor X, X = 0, even if X is nested in a sequence of Xor's.
if (Instruction *Result = AssociativeOpt(I, XorSelf(Op1))) {