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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.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48791 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng 2008-03-25 20:07:13 +00:00
parent 1cf962b874
commit d34af7875b
2 changed files with 12 additions and 1 deletions
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7
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -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))) {

6
test/Transforms/InstCombine/xor-undef.ll Normal file
View File

@ -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
}