[InstCombine] Handle commuted patterns in foldSelectWithSRem (#121896)

Closes #121771.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -2823,9 +2823,9 @@ static Instruction *foldSelectWithSRem(SelectInst &SI, InstCombinerImpl &IC,
   // %cnd = icmp slt i32 %rem, 0
   // %add = add i32 %rem, %n
   // %sel = select i1 %cnd, i32 %add, i32 %rem
-  if (match(TrueVal, m_Add(m_Specific(RemRes), m_Value(Remainder))) &&
+  if (match(TrueVal, m_c_Add(m_Specific(RemRes), m_Value(Remainder))) &&
       match(RemRes, m_SRem(m_Value(Op), m_Specific(Remainder))) &&
-      IC.isKnownToBeAPowerOfTwo(Remainder, /*OrZero*/ true) &&
+      IC.isKnownToBeAPowerOfTwo(Remainder, /*OrZero=*/true) &&
       FalseVal == RemRes)
     return FoldToBitwiseAnd(Remainder);
 

llvm/test/Transforms/InstCombine/select-divrem.ll

@@ -322,6 +322,21 @@ define i8 @rem_euclid_non_const_pow2(i8 %0, i8 %1) {
   ret i8 %sel
 }
 
+define i8 @rem_euclid_non_const_pow2_commuted(i8 %0, i8 %1) {
+; CHECK-LABEL: @rem_euclid_non_const_pow2_commuted(
+; CHECK-NEXT:    [[NOTMASK:%.*]] = shl nsw i8 -1, [[TMP0:%.*]]
+; CHECK-NEXT:    [[TMP3:%.*]] = xor i8 [[NOTMASK]], -1
+; CHECK-NEXT:    [[SEL:%.*]] = and i8 [[TMP1:%.*]], [[TMP3]]
+; CHECK-NEXT:    ret i8 [[SEL]]
+;
+  %pow2 = shl i8 1, %0
+  %rem = srem i8 %1, %pow2
+  %cond = icmp slt i8 %rem, 0
+  %add = add i8 %pow2, %rem
+  %sel = select i1 %cond, i8 %add, i8 %rem
+  ret i8 %sel
+}
+
 define i32 @rem_euclid_pow2_true_arm_folded(i32 %n) {
 ; CHECK-LABEL: @rem_euclid_pow2_true_arm_folded(
 ; CHECK-NEXT:    [[RES:%.*]] = and i32 [[N:%.*]], 1