[PATCH] D154807: [InstCombine] Transform `(add (shl (neg X), Cnt))` -> `(sub (shl X, Cnt))`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
goldstein.w.n added a reviewer: nikic.
Herald added subscribers: StephenFan, hiraditya.
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Just a simple instruction save.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D154807

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


Index: llvm/test/Transforms/InstCombine/add-shift.ll
===================================================================
--- llvm/test/Transforms/InstCombine/add-shift.ll
+++ 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 @@
 ; 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
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1146,6 +1146,30 @@
   return nullptr;
 }
 
+// Transform:
+//  (add A, (shl (neg X), Y))
+//      -> (sub A, (shl X, Y))
+static Instruction *combineAddSubWithShlAddSub(InstCombiner::BuilderTy &Builder,
+                                               const BinaryOperator &I) {
+  Value *Cnt, *X, *Other;
+  auto MatchShlOfNeg = [&I, &Cnt, &X, &Other](unsigned OpIdx) {
+    // TODO: We could also match multiuse shl and replace:
+    //          (shl (neg X), Y) -> (neg (shl X, Y))
+    //       and continue with the transform.
+    if (!match(I.getOperand(OpIdx),
+               m_OneUse(m_Shl(m_OneUse(m_Neg(m_Value(X))), m_Value(Cnt)))))
+      return false;
+    Other = I.getOperand(1 - OpIdx);
+    return true;
+  };
+
+  if (!MatchShlOfNeg(0) && !MatchShlOfNeg(1))
+    return nullptr;
+
+  Value *NewShl = Builder.CreateShl(X, Cnt);
+  return BinaryOperator::CreateSub(Other, NewShl);
+}
+
 /// 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.
@@ -1386,6 +1410,9 @@
   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))


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