[InstSimpify] Simplifying (xor (sub C_Mask, X), C_Mask) -> X

Helps address regressions with folding `clz(Pow2)`.

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

Author: goldsteinn

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -871,6 +871,14 @@ static Value *simplifySubInst(Value *Op0, Value *Op1, bool IsNSW, bool IsNUW,
   if (Value *V = simplifyByDomEq(Instruction::Sub, Op0, Op1, Q, MaxRecurse))
     return V;
 
+  // (sub nuw C_Mask, (xor X, C_Mask)) -> X
+  if (IsNUW) {
+    Value *X;
+    if (match(Op1, m_Xor(m_Value(X), m_Specific(Op0))) &&
+        match(Op0, m_CheckedInt([](const APInt &C) { return C.isMask(); })))
+      return X;
+  }
+
   return nullptr;
 }
 
@@ -2540,6 +2548,14 @@ static Value *simplifyXorInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
   if (Value *V = simplifyByDomEq(Instruction::Xor, Op0, Op1, Q, MaxRecurse))
     return V;
 
+  // (xor (sub nuw C_Mask, X), C_Mask) -> X
+  {
+    Value *X;
+    if (match(Op0, m_NUWSub(m_Specific(Op1), m_Value(X))) &&
+        match(Op1, m_CheckedInt([](const APInt &C) { return C.isMask(); })))
+      return X;
+  }
+
   return nullptr;
 }
 

llvm/test/Transforms/InstSimplify/subnuw-with-xor.ll

@@ -52,9 +52,7 @@ define i8 @xor_w_sub_fail_not_mask(i8 range(i8 0, 16) %x) {
 define i8 @xor_w_sub_okay(i8 range(i8 0, 16) %x) {
 ; CHECK-LABEL: define i8 @xor_w_sub_okay(
 ; CHECK-SAME: i8 range(i8 0, 16) [[X:%.*]]) {
-; CHECK-NEXT:    [[XOR:%.*]] = xor i8 [[X]], 31
-; CHECK-NEXT:    [[R:%.*]] = sub nuw nsw i8 31, [[XOR]]
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 [[X]]
 ;
   %xor = xor i8 %x, 31
   %r = sub nsw nuw i8 31, %xor
@@ -112,9 +110,7 @@ define i8 @sub_w_sub_fail_not_mask(i8 range(i8 0, 16) %x) {
 define i8 @sub_w_sub_okay(i8 range(i8 0, 16) %x) {
 ; CHECK-LABEL: define i8 @sub_w_sub_okay(
 ; CHECK-SAME: i8 range(i8 0, 16) [[X:%.*]]) {
-; CHECK-NEXT:    [[SUB:%.*]] = sub nuw nsw i8 31, [[X]]
-; CHECK-NEXT:    [[R:%.*]] = xor i8 [[SUB]], 31
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 [[X]]
 ;
   %sub = sub nsw nuw i8 31, %x
   %r = xor i8 %sub, 31