902ddda120
This adds a new pass for dropping assumes that are unlikely to be useful for further optimization. It works by discarding any assumes whose affected values are one-use (which implies that they are only used by the assume, i.e. ephemeral). This pass currently runs at the start of the module optimization pipeline, that is post-inline and post-link. Before that point, it is more likely for previously "useless" assumes to become useful again, e.g. because an additional user of the value is introduced after inlining + CSE.
63 lines
2.1 KiB
C++
63 lines
2.1 KiB
C++
//===------------------------------------------------------------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/DropUnnecessaryAssumes.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/Transforms/Utils/Local.h"
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using namespace llvm;
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using namespace llvm::PatternMatch;
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PreservedAnalyses
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DropUnnecessaryAssumesPass::run(Function &F, FunctionAnalysisManager &FAM) {
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AssumptionCache &AC = FAM.getResult<AssumptionAnalysis>(F);
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bool Changed = false;
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for (AssumptionCache::ResultElem &Elem : AC.assumptions()) {
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auto *Assume = cast_or_null<AssumeInst>(Elem.Assume);
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if (!Assume)
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continue;
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// TODO: Handle assumes with operand bundles.
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if (Assume->hasOperandBundles())
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continue;
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Value *Cond = Assume->getArgOperand(0);
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// Don't drop type tests, which have special semantics.
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if (match(Cond, m_Intrinsic<Intrinsic::type_test>()))
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continue;
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SmallPtrSet<Value *, 8> Affected;
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findValuesAffectedByCondition(Cond, /*IsAssume=*/true,
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[&](Value *A) { Affected.insert(A); });
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// If all the affected uses have only one use (part of the assume), then
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// the assume does not provide useful information. Note that additional
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// users may appear as a result of inlining and CSE, so we should only
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// make this assumption late in the optimization pipeline.
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// TODO: Handle dead cyclic usages.
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// TODO: Handle multiple dead assumes on the same value.
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if (!all_of(Affected, match_fn(m_OneUse(m_Value()))))
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continue;
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Assume->eraseFromParent();
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RecursivelyDeleteTriviallyDeadInstructions(Cond);
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Changed = true;
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}
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if (Changed) {
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PreservedAnalyses PA;
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PA.preserveSet<CFGAnalyses>();
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return PA;
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}
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return PreservedAnalyses::all();
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}
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