[InstCombine] Don't transform sub X, ~Y -> add X, -Y unless Y is actually negatable

This combine was previously adding instruction in some cases (see the
tests).

Closes #72767

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp

@@ -447,9 +447,11 @@ std::array<Value *, 2> Negator::getSortedOperandsOfBinOp(Instruction *I) {
     // `xor` is negatible if one of its operands is invertible.
     // FIXME: InstCombineInverter? But how to connect Inverter and Negator?
     if (auto *C = dyn_cast<Constant>(Ops[1])) {
-      Value *Xor = Builder.CreateXor(Ops[0], ConstantExpr::getNot(C));
-      return Builder.CreateAdd(Xor, ConstantInt::get(Xor->getType(), 1),
-                               I->getName() + ".neg");
+      if (IsTrulyNegation) {
+        Value *Xor = Builder.CreateXor(Ops[0], ConstantExpr::getNot(C));
+        return Builder.CreateAdd(Xor, ConstantInt::get(Xor->getType(), 1),
+                                 I->getName() + ".neg");
+      }
     }
     return nullptr;
   }

llvm/test/Analysis/ValueTracking/knownbits-and-or-xor-lowbit.ll

@@ -106,9 +106,8 @@ define <2 x i1> @sub_XY_and_bit0_is_zero_fail(<2 x i8> %x, <2 x i8> %C) nounwind
 
 define i1 @sub_XY_xor_bit0_is_one_fail(i8 %x, i8 %C) nounwind {
 ; CHECK-LABEL: @sub_XY_xor_bit0_is_one_fail(
-; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[C:%.*]], -2
-; CHECK-NEXT:    [[C1_NEG:%.*]] = add i8 [[TMP1]], 1
-; CHECK-NEXT:    [[Y:%.*]] = add i8 [[C1_NEG]], [[X:%.*]]
+; CHECK-NEXT:    [[C1:%.*]] = xor i8 [[C:%.*]], 1
+; CHECK-NEXT:    [[Y:%.*]] = sub i8 [[X:%.*]], [[C1]]
 ; CHECK-NEXT:    [[W:%.*]] = xor i8 [[Y]], [[X]]
 ; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[W]], 10
 ; CHECK-NEXT:    ret i1 [[R]]

llvm/test/Transforms/InstCombine/fold-sub-of-not-to-inc-of-add.ll

@@ -25,9 +25,8 @@ define i32 @p0_scalar(i32 %x, i32 %y) {
 define i8 @p0_scalar_not_truly_negatable(i8 %x, i8 %y) {
 ; CHECK-LABEL: @p0_scalar_not_truly_negatable(
 ; CHECK-NEXT:    [[XX:%.*]] = xor i8 [[X:%.*]], 123
-; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[Y:%.*]], -46
-; CHECK-NEXT:    [[YY_NEG:%.*]] = add i8 [[TMP1]], 1
-; CHECK-NEXT:    [[R:%.*]] = add i8 [[YY_NEG]], [[XX]]
+; CHECK-NEXT:    [[YY:%.*]] = xor i8 [[Y:%.*]], 45
+; CHECK-NEXT:    [[R:%.*]] = sub i8 [[XX]], [[YY]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %xx = xor i8 %x, 123
@@ -99,8 +98,8 @@ define i32 @n4(i32 %x, i32 %y) {
 
 define i32 @n5_is_not_not(i32 %x, i32 %y) {
 ; CHECK-LABEL: @n5_is_not_not(
-; CHECK-NEXT:    [[T0_NEG:%.*]] = add i32 [[X:%.*]], -2147483647
-; CHECK-NEXT:    [[T1:%.*]] = add i32 [[T0_NEG]], [[Y:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = xor i32 [[X:%.*]], 2147483647
+; CHECK-NEXT:    [[T1:%.*]] = sub i32 [[Y:%.*]], [[T0]]
 ; CHECK-NEXT:    ret i32 [[T1]]
 ;
   %t0 = xor i32 %x, 2147483647  ; not -1
@@ -110,8 +109,8 @@ define i32 @n5_is_not_not(i32 %x, i32 %y) {
 
 define <2 x i32> @n5_is_not_not_vec_splat(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @n5_is_not_not_vec_splat(
-; CHECK-NEXT:    [[T0_NEG:%.*]] = add <2 x i32> [[X:%.*]], <i32 -2147483647, i32 -2147483647>
-; CHECK-NEXT:    [[T1:%.*]] = add <2 x i32> [[T0_NEG]], [[Y:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = xor <2 x i32> [[X:%.*]], <i32 2147483647, i32 2147483647>
+; CHECK-NEXT:    [[T1:%.*]] = sub <2 x i32> [[Y:%.*]], [[T0]]
 ; CHECK-NEXT:    ret <2 x i32> [[T1]]
 ;
   %t0 = xor <2 x i32> %x, <i32 2147483647, i32 2147483647>  ; signmask, but not -1