[llvm] 9c375a4 - [InstCombine] Transform `(add (shl (neg X), Cnt))` -> `(sub (shl X, Cnt))`

[Noah Goldstein via llvm-commits]

Author: Noah Goldstein
Date: 2023-08-16T22:43:04-05:00
New Revision: 9c375a457e1c3b4b559348b3cc63ec0f0f5ae968

URL: https://github.com/llvm/llvm-project/commit/9c375a457e1c3b4b559348b3cc63ec0f0f5ae968
DIFF: https://github.com/llvm/llvm-project/commit/9c375a457e1c3b4b559348b3cc63ec0f0f5ae968.diff

LOG: [InstCombine] Transform `(add (shl (neg X), Cnt))` -> `(sub (shl X, Cnt))`

Just a simple instruction save.

Proof: https://alive2.llvm.org/ce/z/Vb484j

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D154807

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/test/Transforms/InstCombine/add-shift.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index c0afcc90d8337f..3f14bb90e669dc 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1180,6 +1180,21 @@ static Instruction *foldToUnsignedSaturatedAdd(BinaryOperator &I) {
   return nullptr;
 }
 
+// Transform:
+//  (add A, (shl (neg B), Y))
+//      -> (sub A, (shl B, Y))
+static Instruction *combineAddSubWithShlAddSub(InstCombiner::BuilderTy &Builder,
+                                               const BinaryOperator &I) {
+  Value *A, *B, *Cnt;
+  if (match(&I,
+            m_c_Add(m_OneUse(m_Shl(m_OneUse(m_Neg(m_Value(B))), m_Value(Cnt))),
+                    m_Value(A)))) {
+    Value *NewShl = Builder.CreateShl(B, Cnt);
+    return BinaryOperator::CreateSub(A, NewShl);
+  }
+  return nullptr;
+}
+
 /// Try to reduce signed division by power-of-2 to an arithmetic shift right.
 static Instruction *foldAddToAshr(BinaryOperator &Add) {
   // Division must be by power-of-2, but not the minimum signed value.
@@ -1420,6 +1435,9 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
   if (Instruction *R = foldBinOpShiftWithShift(I))
     return R;
 
+  if (Instruction *R = combineAddSubWithShlAddSub(Builder, I))
+    return R;
+
   Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
   Type *Ty = I.getType();
   if (Ty->isIntOrIntVectorTy(1))

diff  --git a/llvm/test/Transforms/InstCombine/add-shift.ll b/llvm/test/Transforms/InstCombine/add-shift.ll
index a0a252b30e6867..9b7e5846a93260 100644
--- a/llvm/test/Transforms/InstCombine/add-shift.ll
+++ b/llvm/test/Transforms/InstCombine/add-shift.ll
@@ -3,9 +3,8 @@
 define i8 @flip_add_of_shift_neg(i8 %v, i8 %sh, i8 %x) {
 ; CHECK-LABEL: define i8 @flip_add_of_shift_neg
 ; CHECK-SAME: (i8 [[V:%.*]], i8 [[SH:%.*]], i8 [[X:%.*]]) {
-; CHECK-NEXT:    [[NV:%.*]] = sub i8 0, [[V]]
-; CHECK-NEXT:    [[SV:%.*]] = shl nuw nsw i8 [[NV]], [[SH]]
-; CHECK-NEXT:    [[R:%.*]] = add i8 [[SV]], [[X]]
+; CHECK-NEXT:    [[TMP1:%.*]] = shl i8 [[V]], [[SH]]
+; CHECK-NEXT:    [[R:%.*]] = sub i8 [[X]], [[TMP1]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %nv = sub i8 0, %v
@@ -18,9 +17,8 @@ define <2 x i8> @flip_add_of_shift_neg_vec(<2 x i8> %v, <2 x i8> %sh, <2 x i8> %
 ; CHECK-LABEL: define <2 x i8> @flip_add_of_shift_neg_vec
 ; CHECK-SAME: (<2 x i8> [[V:%.*]], <2 x i8> [[SH:%.*]], <2 x i8> [[XX:%.*]]) {
 ; CHECK-NEXT:    [[X:%.*]] = mul <2 x i8> [[XX]], [[XX]]
-; CHECK-NEXT:    [[NV:%.*]] = sub <2 x i8> zeroinitializer, [[V]]
-; CHECK-NEXT:    [[SV:%.*]] = shl <2 x i8> [[NV]], [[SH]]
-; CHECK-NEXT:    [[R:%.*]] = add <2 x i8> [[X]], [[SV]]
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i8> [[V]], [[SH]]
+; CHECK-NEXT:    [[R:%.*]] = sub <2 x i8> [[X]], [[TMP1]]
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %x = mul <2 x i8> %xx, %xx