[InstCombine] Perform C-(X+C2) --> (C-C2)-X transform before using Negator

In particular, it makes it fire for C=0, because negator doesn't want
to perform that fold since in general it's not beneficial.

Author: LebedevRI

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1723,6 +1723,15 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
     return Res;
   }
 
+  if (Constant *C = dyn_cast<Constant>(Op0)) {
+    Value *X;
+    Constant *C2;
+
+    // C-(X+C2) --> (C-C2)-X
+    if (match(Op1, m_Add(m_Value(X), m_Constant(C2))))
+      return BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
+  }
+
   auto TryToNarrowDeduceFlags = [this, &I, &Op0, &Op1]() -> Instruction * {
     if (Instruction *Ext = narrowMathIfNoOverflow(I))
       return Ext;
@@ -1834,10 +1843,6 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
     // C-(C2-X) --> X+(C-C2)
     if (match(Op1, m_Sub(m_Constant(C2), m_Value(X))) && !isa<ConstantExpr>(C2))
       return BinaryOperator::CreateAdd(X, ConstantExpr::getSub(C, C2));
-
-    // C-(X+C2) --> (C-C2)-X
-    if (match(Op1, m_Add(m_Value(X), m_Constant(C2))))
-      return BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
   }
 
   const APInt *Op0C;

llvm/test/Transforms/InstCombine/sub-of-negatible.ll

@@ -311,8 +311,8 @@ define i8 @n14(i8 %x, i8 %y, i8 %z) {
 
 define i8 @neg_of_add_with_constant(i8 %x) {
 ; CHECK-LABEL: @neg_of_add_with_constant(
-; CHECK-NEXT:    [[S_NEG:%.*]] = sub i8 -42, [[X:%.*]]
-; CHECK-NEXT:    ret i8 [[S_NEG]]
+; CHECK-NEXT:    [[R:%.*]] = sub i8 -42, [[X:%.*]]
+; CHECK-NEXT:    ret i8 [[R]]
 ;
   %s = add i8 %x, 42
   %r = sub i8 0, %s
@@ -323,7 +323,7 @@ define i8 @neg_of_add_with_constant_multi_use(i8 %x) {
 ; CHECK-LABEL: @neg_of_add_with_constant_multi_use(
 ; CHECK-NEXT:    [[S:%.*]] = add i8 [[X:%.*]], 42
 ; CHECK-NEXT:    call void @use8(i8 [[S]])
-; CHECK-NEXT:    [[R:%.*]] = sub i8 0, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = sub i8 -42, [[X]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %s = add i8 %x, 42
@@ -1247,8 +1247,8 @@ define i8 @negate_left_shift_by_constant_extrause(i8 %x, i8 %y, i8 %z, i8 %k) {
 ; `add` with single negatible operand is still negatible
 define i8 @negate_add_with_single_negatible_operand(i8 %x, i8 %y) {
 ; CHECK-LABEL: @negate_add_with_single_negatible_operand(
-; CHECK-NEXT:    [[T0_NEG:%.*]] = sub i8 -42, [[X:%.*]]
-; CHECK-NEXT:    ret i8 [[T0_NEG]]
+; CHECK-NEXT:    [[T1:%.*]] = sub i8 -42, [[X:%.*]]
+; CHECK-NEXT:    ret i8 [[T1]]
 ;
   %t0 = add i8 %x, 42
   %t1 = sub i8 0, %t0
@@ -1271,7 +1271,7 @@ define i8 @negate_add_with_single_negatible_operand_extrause(i8 %x, i8 %y) {
 ; CHECK-LABEL: @negate_add_with_single_negatible_operand_extrause(
 ; CHECK-NEXT:    [[T0:%.*]] = add i8 [[X:%.*]], 42
 ; CHECK-NEXT:    call void @use8(i8 [[T0]])
-; CHECK-NEXT:    [[T1:%.*]] = sub i8 0, [[T0]]
+; CHECK-NEXT:    [[T1:%.*]] = sub i8 -42, [[X]]
 ; CHECK-NEXT:    ret i8 [[T1]]
 ;
   %t0 = add i8 %x, 42