forked from OSchip/llvm-project
[InstCombine] Combine neg of shl of sub (PR44529)
Fixes https://bugs.llvm.org/show_bug.cgi?id=44529. We already have a combine to sink a negation through a left-shift, but it currently only works if the shift operand is negatable without creating any instructions. This patch introduces freelyNegateValue() as a more powerful extension of dyn_castNegVal(), which allows negating a value as long as this doesn't end up increasing instruction count. Specifically, this patch adds support for negating A-B to B-A. This mechanism could in the future be extended to handle general negation chains that a) start at a proper 0-X negation and b) only require one operand to be freely negatable. This would end up as a weaker form of D68408 aimed at the most obviously profitable subset that eliminates a negation entirely. Differential Revision: https://reviews.llvm.org/D72978
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llvm
lib/Transforms/InstCombine
test/Transforms/InstCombine
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@ -1985,7 +1985,7 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
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// 0 - (X << Y) -> (-X << Y) when X is freely negatable.
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if (match(Op1, m_Shl(m_Value(X), m_Value(Y))) && match(Op0, m_Zero()))
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if (Value *XNeg = dyn_castNegVal(X))
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if (Value *XNeg = freelyNegateValue(X))
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return BinaryOperator::CreateShl(XNeg, Y);
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// Subtracting -1/0 is the same as adding 1/0:
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@ -474,6 +474,7 @@ private:
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bool shouldChangeType(unsigned FromBitWidth, unsigned ToBitWidth) const;
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bool shouldChangeType(Type *From, Type *To) const;
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Value *dyn_castNegVal(Value *V) const;
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Value *freelyNegateValue(Value *V);
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Type *FindElementAtOffset(PointerType *PtrTy, int64_t Offset,
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SmallVectorImpl<Value *> &NewIndices);
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@ -856,6 +856,23 @@ Value *InstCombiner::dyn_castNegVal(Value *V) const {
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return nullptr;
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}
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/// Get negated V (that is 0-V) without increasing instruction count,
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/// assuming that the original V will become unused.
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Value *InstCombiner::freelyNegateValue(Value *V) {
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if (Value *NegV = dyn_castNegVal(V))
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return NegV;
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if (!V->hasOneUse())
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return nullptr;
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Value *A, *B;
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// 0-(A-B) => B-A
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if (match(V, m_Sub(m_Value(A), m_Value(B))))
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return Builder.CreateSub(B, A);
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return nullptr;
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}
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static Value *foldOperationIntoSelectOperand(Instruction &I, Value *SO,
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InstCombiner::BuilderTy &Builder) {
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if (auto *Cast = dyn_cast<CastInst>(&I))
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@ -543,9 +543,8 @@ define i32 @test26(i32 %x) {
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define i64 @test_neg_shl_sub(i64 %a, i64 %b) {
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; CHECK-LABEL: @test_neg_shl_sub(
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; CHECK-NEXT: [[SUB:%.*]] = sub i64 [[A:%.*]], [[B:%.*]]
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; CHECK-NEXT: [[MUL:%.*]] = shl i64 [[SUB]], 2
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; CHECK-NEXT: [[NEG:%.*]] = sub i64 0, [[MUL]]
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; CHECK-NEXT: [[TMP1:%.*]] = sub i64 [[B:%.*]], [[A:%.*]]
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; CHECK-NEXT: [[NEG:%.*]] = shl i64 [[TMP1]], 2
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; CHECK-NEXT: ret i64 [[NEG]]
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;
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%sub = sub i64 %a, %b
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