[InstCombine] Add pattern 'if (C1 & C2) == C2 then (X & C1) - (X & C2) -> X & (C1 ^ C2)'

https://alive2.llvm.org/ce/z/BveKM5

Author: chenglin.bi

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2280,6 +2280,15 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
   if (match(Op0, m_OneUse(m_Add(m_Value(X), m_AllOnes()))))
     return BinaryOperator::CreateAdd(Builder.CreateNot(Op1), X);
 
+  const APInt *C1, *C2;
+  // if (C1 & C2) == C2 then (X & C1) - (X & C2) -> X & (C1 ^ C2)
+  if (match(Op0, m_And(m_Value(X), m_APInt(C1))) &&
+      match(Op1, m_And(m_Specific(X), m_APInt(C2)))) {
+    if (C2->eq(*C1 & *C2))
+      return BinaryOperator::CreateAnd(
+          X, ConstantInt::get(I.getType(), *C1 ^ *C2));
+  }
+
   // Reassociate sub/add sequences to create more add instructions and
   // reduce dependency chains:
   // ((X - Y) + Z) - Op1 --> (X + Z) - (Y + Op1)

llvm/test/Transforms/InstCombine/X86/and-sub-combine.ll

@@ -3,9 +3,7 @@
 
 define i8 @and_sub(i8 %a) {
 ; CHECK-LABEL: @and_sub(
-; CHECK-NEXT:    [[AND1:%.*]] = and i8 [[A:%.*]], 15
-; CHECK-NEXT:    [[AND2:%.*]] = and i8 [[A]], 3
-; CHECK-NEXT:    [[RET:%.*]] = sub nsw i8 [[AND1]], [[AND2]]
+; CHECK-NEXT:    [[RET:%.*]] = and i8 [[A:%.*]], 12
 ; CHECK-NEXT:    ret i8 [[RET]]
 ;
   %and1 = and i8 %a, 15