[InstCombine] Fold dependent IVs #81151
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@@ -1378,6 +1378,58 @@ static Value *simplifyUsingControlFlow(InstCombiner &Self, PHINode &PN, | |
| return nullptr; | ||
| } | ||
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| // Fold iv = phi(start, iv.next = iv2.next op start) | ||
| // where iv2 = phi(iv2.start, iv2.next = iv2 + iv2.step) | ||
| // and iv2.start op start = start | ||
| // to iv = iv2 op start | ||
| static Value *foldDependentIVs(PHINode &PN, IRBuilderBase &Builder) { | ||
| BasicBlock *BB = PN.getParent(); | ||
| if (PN.getNumIncomingValues() != 2) | ||
| return nullptr; | ||
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| Value *Start; | ||
| Instruction *IvNext; | ||
| BinaryOperator *Iv2Next; | ||
| auto MatchOuterIV = [&](Value *V1, Value *V2) { | ||
| if (match(V2, m_c_BinOp(m_Specific(V1), m_BinOp(Iv2Next))) || | ||
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llvm-project/llvm/include/llvm/IR/PatternMatch.h Lines 911 to 927 in 8f7ae64 llvm-project/llvm/include/llvm/IR/PatternMatch.h Lines 2410 to 2414 in 8f7ae64 It doesn't cause miscompilation because
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There was a problem hiding this comment. I'm aware -- this is why there is an isCommutative() assertion below, in case the behavior of getBinOpIdentity() ever changes. |
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| match(V2, m_GEP(m_Specific(V1), m_BinOp(Iv2Next)))) { | ||
| Start = V1; | ||
| IvNext = cast<Instruction>(V2); | ||
| return true; | ||
| } | ||
| return false; | ||
| }; | ||
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| if (!MatchOuterIV(PN.getIncomingValue(0), PN.getIncomingValue(1)) && | ||
| !MatchOuterIV(PN.getIncomingValue(1), PN.getIncomingValue(0))) | ||
| return nullptr; | ||
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| PHINode *Iv2; | ||
| Value *Iv2Start, *Iv2Step; | ||
| if (!matchSimpleRecurrence(Iv2Next, Iv2, Iv2Start, Iv2Step) || | ||
| Iv2->getParent() != BB) | ||
| return nullptr; | ||
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| auto *BO = dyn_cast<BinaryOperator>(IvNext); | ||
| Constant *Identity = | ||
| BO ? ConstantExpr::getBinOpIdentity(BO->getOpcode(), Iv2Start->getType()) | ||
| : Constant::getNullValue(Iv2Start->getType()); | ||
| if (Iv2Start != Identity) | ||
| return nullptr; | ||
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| Builder.SetInsertPoint(&*BB, BB->getFirstInsertionPt()); | ||
| if (!BO) { | ||
| auto *GEP = cast<GEPOperator>(IvNext); | ||
| return Builder.CreateGEP(GEP->getSourceElementType(), Start, Iv2, "", | ||
| cast<GEPOperator>(IvNext)->isInBounds()); | ||
| } | ||
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| assert(BO->isCommutative() && "Must be commutative"); | ||
| Value *Res = Builder.CreateBinOp(BO->getOpcode(), Iv2, Start); | ||
| cast<Instruction>(Res)->copyIRFlags(BO); | ||
| return Res; | ||
| } | ||
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| // PHINode simplification | ||
| // | ||
| Instruction *InstCombinerImpl::visitPHINode(PHINode &PN) { | ||
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@@ -1595,5 +1647,8 @@ Instruction *InstCombinerImpl::visitPHINode(PHINode &PN) { | |
| if (auto *V = simplifyUsingControlFlow(*this, PN, DT)) | ||
| return replaceInstUsesWith(PN, V); | ||
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| if (Value *Res = foldDependentIVs(PN, Builder)) | ||
| return replaceInstUsesWith(PN, Res); | ||
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| return nullptr; | ||
| } | ||
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nit: Maybe add a todo for detecting if
Iv2Nextis rhs/lhs to cover more binops (like sub).