[InstCombine] Don't use dominating conditions to transform sub into xor.

Other passes are unable to reverse this transform if we use dominating
conditions.

Fixes #88239.

Author: topperc

llvm/include/llvm/Analysis/SimplifyQuery.h

@@ -113,6 +113,12 @@ struct SimplifyQuery {
     using namespace PatternMatch;
     return match(V, m_Undef());
   }
+
+  SimplifyQuery getWithoutDomCondCache() const {
+    SimplifyQuery Copy(*this);
+    Copy.DC = nullptr;
+    return Copy;
+  }
 };
 
 } // end namespace llvm

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2281,8 +2281,10 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
   if (match(Op0, m_APInt(Op0C))) {
     if (Op0C->isMask()) {
       // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
-      // zero.
-      KnownBits RHSKnown = computeKnownBits(Op1, 0, &I);
+      // zero. We don't use information from dominating conditions so this
+      // transform is easier to reverse if necessary.
+      KnownBits RHSKnown = llvm::computeKnownBits(
+          Op1, 0, SQ.getWithInstruction(&I).getWithoutDomCondCache());
       if ((*Op0C | RHSKnown.Zero).isAllOnes())
         return BinaryOperator::CreateXor(Op1, Op0);
     }

llvm/test/Transforms/InstCombine/sub-xor.ll

@@ -158,13 +158,15 @@ define <2 x i8> @xor_add_splat_undef(<2 x i8> %x) {
   ret <2 x i8> %add
 }
 
+; Make sure we don't convert sub to xor using dominating condition. That makes
+; it hard for other passe to reverse.
 define i32 @xor_dominating_cond(i32 %x) {
 ; CHECK-LABEL: @xor_dominating_cond(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[COND:%.*]] = icmp ult i32 [[X:%.*]], 256
 ; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_END:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[A:%.*]] = xor i32 [[X]], 255
+; CHECK-NEXT:    [[A:%.*]] = sub nuw nsw i32 255, [[X]]
 ; CHECK-NEXT:    ret i32 [[A]]
 ; CHECK:       if.end:
 ; CHECK-NEXT:    ret i32 [[X]]