[InstComb] Fold ashr (xor x, y), x -> ashr y, x

Proof: https://alive2.llvm.org/ce/z/yWCmMd

Co-authored-by: John Regehr <regehr@cs.utah.edu>

Author: artagnon

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -1836,6 +1836,11 @@ Instruction *InstCombinerImpl::visitAShr(BinaryOperator &I) {
     return Lshr;
   }
 
+  // ashr (xor %x, %y), %x --> ashr %y, %x
+  Value *Y;
+  if (match(Op0, m_Xor(m_Value(X), m_Value(Y))) && Op1 == X)
+    return BinaryOperator::CreateAShr(Y, X);
+
   // ashr (xor %x, -1), %y  -->  xor (ashr %x, %y), -1
   if (match(Op0, m_OneUse(m_Not(m_Value(X))))) {
     // Note that we must drop 'exact'-ness of the shift!

llvm/test/Transforms/InstCombine/shift-logic.ll

@@ -228,8 +228,7 @@ define <2 x i32> @ashr_poison_poison_xor(<2 x i32> %x, <2 x i32> %y) {
 
 define i32 @ashr_xor_operand_match(i32 %x, i32 %y) {
 ; CHECK-LABEL: @ashr_xor_operand_match(
-; CHECK-NEXT:    [[SH1:%.*]] = xor i32 [[TMP1:%.*]], [[TMP2:%.*]]
-; CHECK-NEXT:    [[RET:%.*]] = ashr i32 [[SH1]], [[TMP1]]
+; CHECK-NEXT:    [[RET:%.*]] = ashr i32 [[SH1:%.*]], [[TMP1:%.*]]
 ; CHECK-NEXT:    ret i32 [[RET]]
 ;
   %r = xor i32 %x, %y