[InstCombine] Fold ((X - Y) - Z) to X - (Y + Z) (PR49858)

https://alive2.llvm.org/ce/z/67w-wQ

We prefer `add`s over `sub`, and this particular xform
allows further folds to happen:

Fixes https://bugs.llvm.org/show_bug.cgi?id=49858

Author: LebedevRI

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1801,6 +1801,12 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
     return BinaryOperator::CreateSub(XZ, YW);
   }
 
+  // ((X - Y) - Op1)  -->  X - (Y + Op1)
+  if (match(Op0, m_OneUse(m_Sub(m_Value(X), m_Value(Y))))) {
+    Value *Add = Builder.CreateAdd(Y, Op1);
+    return BinaryOperator::CreateSub(X, Add);
+  }
+
   auto m_AddRdx = [](Value *&Vec) {
     return m_OneUse(m_Intrinsic<Intrinsic::vector_reduce_add>(m_Value(Vec)));
   };

llvm/test/Transforms/InstCombine/abs-1.ll

@@ -305,9 +305,9 @@ define i32 @nabs_canonical_8(i32 %a) {
 define i32 @nabs_canonical_9(i32 %a, i32 %b) {
 ; CHECK-LABEL: @nabs_canonical_9(
 ; CHECK-NEXT:    [[T1:%.*]] = sub i32 [[A:%.*]], [[B:%.*]]
-; CHECK-NEXT:    [[T2:%.*]] = sub i32 [[B]], [[A]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.abs.i32(i32 [[T1]], i1 false)
-; CHECK-NEXT:    [[ADD:%.*]] = sub i32 [[T2]], [[TMP1]]
+; CHECK-NEXT:    [[TMP2:%.*]] = add i32 [[TMP1]], [[A]]
+; CHECK-NEXT:    [[ADD:%.*]] = sub i32 [[B]], [[TMP2]]
 ; CHECK-NEXT:    ret i32 [[ADD]]
 ;
   %t1 = sub i32 %a, %b

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

@@ -8,8 +8,8 @@ declare void @use8(i8)
 ; Basic test
 define i8 @t0(i8 %x, i8 %y, i8 %z) {
 ; CHECK-LABEL: @t0(
-; CHECK-NEXT:    [[I0:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = sub i8 [[I0]], [[Z:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[Y:%.*]], [[Z:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = sub i8 [[X:%.*]], [[TMP1]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %i0 = sub i8 %x, %y
@@ -20,8 +20,8 @@ define i8 @t0(i8 %x, i8 %y, i8 %z) {
 ; No flags are propagated
 define i8 @t1_flags(i8 %x, i8 %y, i8 %z) {
 ; CHECK-LABEL: @t1_flags(
-; CHECK-NEXT:    [[O0:%.*]] = sub nuw nsw i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = sub nuw nsw i8 [[O0]], [[Z:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[Y:%.*]], [[Z:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = sub i8 [[X:%.*]], [[TMP1]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %o0 = sub nuw nsw i8 %x, %y
@@ -47,8 +47,8 @@ define i8 @n2(i8 %x, i8 %y, i8 %z) {
 
 define i8 @t3_c0(i8 %y, i8 %z) {
 ; CHECK-LABEL: @t3_c0(
-; CHECK-NEXT:    [[I0:%.*]] = sub i8 42, [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = sub i8 [[I0]], [[Z:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[Y:%.*]], [[Z:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = sub i8 42, [[TMP1]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %i0 = sub i8 42, %y