[llvm-commits] [llvm] r48791 - in /llvm/trunk: lib/Transforms/Scalar/InstructionCombining.cpp test/Transforms/InstCombine/xor-undef.ll

[Evan Cheng]

Author: evancheng
Date: Tue Mar 25 15:07:13 2008
New Revision: 48791

URL: http://llvm.org/viewvc/llvm-project?rev=48791&view=rev
Log:
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.

Added:
    llvm/trunk/test/Transforms/InstCombine/xor-undef.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp?rev=48791&r1=48790&r2=48791&view=diff

==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp Tue Mar 25 15:07:13 2008
@@ -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))) {

Added: llvm/trunk/test/Transforms/InstCombine/xor-undef.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/xor-undef.ll?rev=48791&view=auto

==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/xor-undef.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/xor-undef.ll Tue Mar 25 15:07:13 2008
@@ -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
+}