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

2 files changed, 12 insertions, 1 deletions

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 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
   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))) {
diff --git a/test/Transforms/InstCombine/xor-undef.ll b/test/Transforms/InstCombine/xor-undef.ll
new file mode 100644
index 0000000..c090223
--- /dev/null
+++ b/test/Transforms/InstCombine/xor-undef.ll
@@ -0,0 +1,6 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep zeroinitializer
+
+define <2 x i64> @f() {
+	%tmp = xor <2 x i64> undef, undef
+        ret <2 x i64> %tmp
+}