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[InferAlignment] Implement InferAlignmentPass
This pass aims to infer alignment for instructions as a separate pass, to reduce redundant work done by InstCombine running multiple times. It runs late in the pipeline, just before the back-end passes where this information is most useful. Differential Revision: https://reviews.llvm.org/D158529
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| //===- InferAlignment.h -----------------------------------------*- C++ -*-===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
| // | ||
| // Infer alignment for load, stores and other memory operations based on | ||
| // trailing zero known bits information. | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #ifndef LLVM_TRANSFORMS_SCALAR_INFERALIGNMENT_H | ||
| #define LLVM_TRANSFORMS_SCALAR_INFERALIGNMENT_H | ||
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| #include "llvm/IR/PassManager.h" | ||
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| namespace llvm { | ||
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| struct InferAlignmentPass : public PassInfoMixin<InferAlignmentPass> { | ||
| PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); | ||
| }; | ||
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| } // namespace llvm | ||
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| #endif // LLVM_TRANSFORMS_SCALAR_INFERALIGNMENT_H |
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,91 @@ | ||
| //===- InferAlignment.cpp -------------------------------------------------===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
| // | ||
| // Infer alignment for load, stores and other memory operations based on | ||
| // trailing zero known bits information. | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #include "llvm/Transforms/Scalar/InferAlignment.h" | ||
| #include "llvm/Analysis/AssumptionCache.h" | ||
| #include "llvm/Analysis/ValueTracking.h" | ||
| #include "llvm/IR/Instructions.h" | ||
| #include "llvm/InitializePasses.h" | ||
| #include "llvm/Support/KnownBits.h" | ||
| #include "llvm/Transforms/Scalar.h" | ||
| #include "llvm/Transforms/Utils/Local.h" | ||
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| using namespace llvm; | ||
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| static bool tryToImproveAlign( | ||
| const DataLayout &DL, Instruction *I, | ||
| function_ref<Align(Value *PtrOp, Align OldAlign, Align PrefAlign)> Fn) { | ||
| if (auto *LI = dyn_cast<LoadInst>(I)) { | ||
| Value *PtrOp = LI->getPointerOperand(); | ||
| Align OldAlign = LI->getAlign(); | ||
| Align NewAlign = Fn(PtrOp, OldAlign, DL.getPrefTypeAlign(LI->getType())); | ||
| if (NewAlign > OldAlign) { | ||
| LI->setAlignment(NewAlign); | ||
| return true; | ||
| } | ||
| } else if (auto *SI = dyn_cast<StoreInst>(I)) { | ||
| Value *PtrOp = SI->getPointerOperand(); | ||
| Value *ValOp = SI->getValueOperand(); | ||
| Align OldAlign = SI->getAlign(); | ||
| Align NewAlign = Fn(PtrOp, OldAlign, DL.getPrefTypeAlign(ValOp->getType())); | ||
| if (NewAlign > OldAlign) { | ||
| SI->setAlignment(NewAlign); | ||
| return true; | ||
| } | ||
| } | ||
| // TODO: Also handle memory intrinsics. | ||
| return false; | ||
| } | ||
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| bool inferAlignment(Function &F, AssumptionCache &AC, DominatorTree &DT) { | ||
| const DataLayout &DL = F.getParent()->getDataLayout(); | ||
| bool Changed = false; | ||
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| // Enforce preferred type alignment if possible. We do this as a separate | ||
| // pass first, because it may improve the alignments we infer below. | ||
| for (BasicBlock &BB : F) { | ||
| for (Instruction &I : BB) { | ||
| Changed |= tryToImproveAlign( | ||
| DL, &I, [&](Value *PtrOp, Align OldAlign, Align PrefAlign) { | ||
| if (PrefAlign > OldAlign) | ||
| return std::max(OldAlign, | ||
| tryEnforceAlignment(PtrOp, PrefAlign, DL)); | ||
| return OldAlign; | ||
| }); | ||
| } | ||
| } | ||
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| // Compute alignment from known bits. | ||
| for (BasicBlock &BB : F) { | ||
| for (Instruction &I : BB) { | ||
| Changed |= tryToImproveAlign( | ||
| DL, &I, [&](Value *PtrOp, Align OldAlign, Align PrefAlign) { | ||
| KnownBits Known = computeKnownBits(PtrOp, DL, 0, &AC, &I, &DT); | ||
| unsigned TrailZ = std::min(Known.countMinTrailingZeros(), | ||
| +Value::MaxAlignmentExponent); | ||
| return Align(1ull << std::min(Known.getBitWidth() - 1, TrailZ)); | ||
| }); | ||
| } | ||
| } | ||
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| return Changed; | ||
| } | ||
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| PreservedAnalyses InferAlignmentPass::run(Function &F, | ||
| FunctionAnalysisManager &AM) { | ||
| AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(F); | ||
| DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F); | ||
| inferAlignment(F, AC, DT); | ||
| // Changes to alignment shouldn't invalidated analyses. | ||
| return PreservedAnalyses::all(); | ||
| } |
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