[InstCombine] Fold X - Y + Y * C -> X + Y * (C - 1)

Alive2 Proof:
https://alive2.llvm.org/ce/z/gS9Wb7

This is based on and closes #108451

Note that no nsw or nuw saving works because they all result in poison or undef mismatches.

Author: AZero13

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1612,6 +1612,18 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
       Value *Sub = Builder.CreateSub(B, A, "reass.sub");
       return BinaryOperator::CreateAdd(Sub, C1);
     }
+
+    // A - B + B * C -> A + B * (C - 1)
+    const APInt *C;
+    if (((match(LHS, m_Sub(m_Value(A), m_Value(B))) &&
+          match(RHS, m_Mul(m_Specific(B), m_APInt(C)))) ||
+         (match(RHS, m_Sub(m_Value(A), m_Value(B))) &&
+          match(LHS, m_Mul(m_Specific(B), m_APInt(C))))) &&
+        LHS->hasOneUse() && RHS->hasOneUse()) {
+      Value *Mul =
+          Builder.CreateMul(B, ConstantInt::get(RHS->getType(), *C - 1));
+      return BinaryOperator::CreateAdd(A, Mul);
+    }
   }
 
   // X % C0 + (( X / C0 ) % C1) * C0 => X % (C0 * C1)

llvm/test/Transforms/InstCombine/addsub-constant-folding.ll

@@ -753,9 +753,8 @@ define i8 @sub_from_constant_extra_use(i8 %x, i8 %y) {
 
 define i32 @sub_plus_mul(i32 %x, i32 %y) {
 ; CHECK-LABEL: @sub_plus_mul(
-; CHECK-NEXT:    [[A:%.*]] = sub i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[B:%.*]] = mul i32 [[Y]], 10
-; CHECK-NEXT:    [[C:%.*]] = add i32 [[A]], [[B]]
+; CHECK-NEXT:    [[B:%.*]] = mul i32 [[Y:%.*]], 9
+; CHECK-NEXT:    [[C:%.*]] = add i32 [[A:%.*]], [[B]]
 ; CHECK-NEXT:    ret i32 [[C]]
 ;
   %a = sub i32 %x, %y
@@ -766,9 +765,8 @@ define i32 @sub_plus_mul(i32 %x, i32 %y) {
 
 define i32 @sub_plus_mul_2(i32 %x, i32 %y) {
 ; CHECK-LABEL: @sub_plus_mul_2(
-; CHECK-NEXT:    [[A:%.*]] = sub i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[B:%.*]] = mul i32 [[Y]], -10
-; CHECK-NEXT:    [[C:%.*]] = add i32 [[A]], [[B]]
+; CHECK-NEXT:    [[B:%.*]] = mul i32 [[Y:%.*]], -11
+; CHECK-NEXT:    [[C:%.*]] = add i32 [[A:%.*]], [[B]]
 ; CHECK-NEXT:    ret i32 [[C]]
 ;
   %a = sub i32 %x, %y
@@ -779,9 +777,8 @@ define i32 @sub_plus_mul_2(i32 %x, i32 %y) {
 
 define i32 @sub_plus_mul3(i32 %x, i32 %y) {
 ; CHECK-LABEL: @sub_plus_mul3(
-; CHECK-NEXT:    [[A:%.*]] = mul i32 [[Y:%.*]], 10
-; CHECK-NEXT:    [[B:%.*]] = sub i32 [[X:%.*]], [[Y]]
-; CHECK-NEXT:    [[C:%.*]] = add i32 [[B]], [[A]]
+; CHECK-NEXT:    [[A:%.*]] = mul i32 [[Y:%.*]], 9
+; CHECK-NEXT:    [[C:%.*]] = add i32 [[B:%.*]], [[A]]
 ; CHECK-NEXT:    ret i32 [[C]]
 ;
   %a = mul i32 %y, 10
@@ -792,9 +789,8 @@ define i32 @sub_plus_mul3(i32 %x, i32 %y) {
 
 define i32 @sub_plus_mul4(i32 %x, i32 %y) {
 ; CHECK-LABEL: @sub_plus_mul4(
-; CHECK-NEXT:    [[A:%.*]] = mul i32 [[Y:%.*]], -10
-; CHECK-NEXT:    [[B:%.*]] = sub i32 [[X:%.*]], [[Y]]
-; CHECK-NEXT:    [[C:%.*]] = add i32 [[B]], [[A]]
+; CHECK-NEXT:    [[A:%.*]] = mul i32 [[Y:%.*]], -11
+; CHECK-NEXT:    [[C:%.*]] = add i32 [[B:%.*]], [[A]]
 ; CHECK-NEXT:    ret i32 [[C]]
 ;
   %a = mul i32 %y, -10
@@ -805,9 +801,8 @@ define i32 @sub_plus_mul4(i32 %x, i32 %y) {
 
 define <2 x i8> @sub_plus_mul_splat(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @sub_plus_mul_splat(
-; CHECK-NEXT:    [[A:%.*]] = sub <2 x i8> [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[B:%.*]] = mul <2 x i8> [[Y]], splat (i8 7)
-; CHECK-NEXT:    [[C:%.*]] = add <2 x i8> [[A]], [[B]]
+; CHECK-NEXT:    [[B:%.*]] = mul <2 x i8> [[Y:%.*]], splat (i8 6)
+; CHECK-NEXT:    [[C:%.*]] = add <2 x i8> [[A:%.*]], [[B]]
 ; CHECK-NEXT:    ret <2 x i8> [[C]]
 ;
   %a = sub <2 x i8> %x, %y
@@ -832,8 +827,8 @@ define i32 @neg_sub_plus_mul_pow2(i32 %x, i32 %y) {
   ret i32 %c
 }
 
-define <2 x i8> @sub_plus_mul_splat_shl(<2 x i8> %x, <2 x i8> %y) {
-; CHECK-LABEL: @sub_plus_mul_splat_shl(
+define <2 x i8> @neg_sub_plus_mul_splat_shl(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @neg_sub_plus_mul_splat_shl(
 ; CHECK-NEXT:    [[A:%.*]] = sub <2 x i8> [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    [[B:%.*]] = shl <2 x i8> [[Y]], splat (i8 3)
 ; CHECK-NEXT:    [[C:%.*]] = add <2 x i8> [[A]], [[B]]