forked from OSchip/llvm-project
422 lines
15 KiB
C++
422 lines
15 KiB
C++
//===-------- LoopDataPrefetch.cpp - Loop Data Prefetching Pass -----------===//
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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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//
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// This file implements a Loop Data Prefetching Pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/LoopDataPrefetch.h"
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#include "llvm/InitializePasses.h"
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#define DEBUG_TYPE "loop-data-prefetch"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/CodeMetrics.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/OptimizationRemarkEmitter.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionExpander.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/CallSite.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/ValueMapper.h"
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using namespace llvm;
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// By default, we limit this to creating 16 PHIs (which is a little over half
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// of the allocatable register set).
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static cl::opt<bool>
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PrefetchWrites("loop-prefetch-writes", cl::Hidden, cl::init(false),
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cl::desc("Prefetch write addresses"));
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static cl::opt<unsigned>
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PrefetchDistance("prefetch-distance",
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cl::desc("Number of instructions to prefetch ahead"),
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cl::Hidden);
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static cl::opt<unsigned>
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MinPrefetchStride("min-prefetch-stride",
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cl::desc("Min stride to add prefetches"), cl::Hidden);
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static cl::opt<unsigned> MaxPrefetchIterationsAhead(
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"max-prefetch-iters-ahead",
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cl::desc("Max number of iterations to prefetch ahead"), cl::Hidden);
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STATISTIC(NumPrefetches, "Number of prefetches inserted");
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namespace {
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/// Loop prefetch implementation class.
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class LoopDataPrefetch {
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public:
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LoopDataPrefetch(AssumptionCache *AC, DominatorTree *DT, LoopInfo *LI,
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ScalarEvolution *SE, const TargetTransformInfo *TTI,
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OptimizationRemarkEmitter *ORE)
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: AC(AC), DT(DT), LI(LI), SE(SE), TTI(TTI), ORE(ORE) {}
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bool run();
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private:
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bool runOnLoop(Loop *L);
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/// Check if the stride of the accesses is large enough to
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/// warrant a prefetch.
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bool isStrideLargeEnough(const SCEVAddRecExpr *AR, unsigned TargetMinStride);
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unsigned getMinPrefetchStride(unsigned NumMemAccesses,
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unsigned NumStridedMemAccesses,
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unsigned NumPrefetches,
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bool HasCall) {
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if (MinPrefetchStride.getNumOccurrences() > 0)
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return MinPrefetchStride;
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return TTI->getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
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NumPrefetches, HasCall);
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}
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unsigned getPrefetchDistance() {
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if (PrefetchDistance.getNumOccurrences() > 0)
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return PrefetchDistance;
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return TTI->getPrefetchDistance();
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}
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unsigned getMaxPrefetchIterationsAhead() {
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if (MaxPrefetchIterationsAhead.getNumOccurrences() > 0)
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return MaxPrefetchIterationsAhead;
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return TTI->getMaxPrefetchIterationsAhead();
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}
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bool doPrefetchWrites() {
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if (PrefetchWrites.getNumOccurrences() > 0)
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return PrefetchWrites;
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return TTI->enableWritePrefetching();
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}
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AssumptionCache *AC;
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DominatorTree *DT;
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LoopInfo *LI;
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ScalarEvolution *SE;
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const TargetTransformInfo *TTI;
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OptimizationRemarkEmitter *ORE;
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};
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/// Legacy class for inserting loop data prefetches.
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class LoopDataPrefetchLegacyPass : public FunctionPass {
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public:
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static char ID; // Pass ID, replacement for typeid
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LoopDataPrefetchLegacyPass() : FunctionPass(ID) {
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initializeLoopDataPrefetchLegacyPassPass(*PassRegistry::getPassRegistry());
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<AssumptionCacheTracker>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
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AU.addRequired<ScalarEvolutionWrapperPass>();
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AU.addPreserved<ScalarEvolutionWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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}
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bool runOnFunction(Function &F) override;
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};
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}
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char LoopDataPrefetchLegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
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"Loop Data Prefetch", false, false)
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INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
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INITIALIZE_PASS_END(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
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"Loop Data Prefetch", false, false)
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FunctionPass *llvm::createLoopDataPrefetchPass() {
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return new LoopDataPrefetchLegacyPass();
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}
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bool LoopDataPrefetch::isStrideLargeEnough(const SCEVAddRecExpr *AR,
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unsigned TargetMinStride) {
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// No need to check if any stride goes.
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if (TargetMinStride <= 1)
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return true;
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const auto *ConstStride = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*SE));
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// If MinStride is set, don't prefetch unless we can ensure that stride is
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// larger.
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if (!ConstStride)
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return false;
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unsigned AbsStride = std::abs(ConstStride->getAPInt().getSExtValue());
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return TargetMinStride <= AbsStride;
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}
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PreservedAnalyses LoopDataPrefetchPass::run(Function &F,
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FunctionAnalysisManager &AM) {
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DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
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LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
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ScalarEvolution *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
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AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
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OptimizationRemarkEmitter *ORE =
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&AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
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const TargetTransformInfo *TTI = &AM.getResult<TargetIRAnalysis>(F);
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LoopDataPrefetch LDP(AC, DT, LI, SE, TTI, ORE);
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bool Changed = LDP.run();
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if (Changed) {
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PreservedAnalyses PA;
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PA.preserve<DominatorTreeAnalysis>();
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PA.preserve<LoopAnalysis>();
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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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bool LoopDataPrefetchLegacyPass::runOnFunction(Function &F) {
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if (skipFunction(F))
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return false;
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DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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AssumptionCache *AC =
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&getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
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OptimizationRemarkEmitter *ORE =
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&getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
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const TargetTransformInfo *TTI =
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&getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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LoopDataPrefetch LDP(AC, DT, LI, SE, TTI, ORE);
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return LDP.run();
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}
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bool LoopDataPrefetch::run() {
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// If PrefetchDistance is not set, don't run the pass. This gives an
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// opportunity for targets to run this pass for selected subtargets only
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// (whose TTI sets PrefetchDistance).
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if (getPrefetchDistance() == 0)
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return false;
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assert(TTI->getCacheLineSize() && "Cache line size is not set for target");
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bool MadeChange = false;
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for (Loop *I : *LI)
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for (auto L = df_begin(I), LE = df_end(I); L != LE; ++L)
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MadeChange |= runOnLoop(*L);
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return MadeChange;
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}
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/// A record for a potential prefetch made during the initial scan of the
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/// loop. This is used to let a single prefetch target multiple memory accesses.
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struct Prefetch {
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/// The address formula for this prefetch as returned by ScalarEvolution.
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const SCEVAddRecExpr *LSCEVAddRec;
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/// The point of insertion for the prefetch instruction.
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Instruction *InsertPt;
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/// True if targeting a write memory access.
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bool Writes;
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/// The (first seen) prefetched instruction.
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Instruction *MemI;
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/// Constructor to create a new Prefetch for \param I.
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Prefetch(const SCEVAddRecExpr *L, Instruction *I)
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: LSCEVAddRec(L), InsertPt(nullptr), Writes(false), MemI(nullptr) {
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addInstruction(I);
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};
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/// Add the instruction \param I to this prefetch. If it's not the first
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/// one, 'InsertPt' and 'Writes' will be updated as required.
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/// \param PtrDiff the known constant address difference to the first added
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/// instruction.
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void addInstruction(Instruction *I, DominatorTree *DT = nullptr,
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int64_t PtrDiff = 0) {
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if (!InsertPt) {
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MemI = I;
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InsertPt = I;
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Writes = isa<StoreInst>(I);
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} else {
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BasicBlock *PrefBB = InsertPt->getParent();
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BasicBlock *InsBB = I->getParent();
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if (PrefBB != InsBB) {
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BasicBlock *DomBB = DT->findNearestCommonDominator(PrefBB, InsBB);
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if (DomBB != PrefBB)
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InsertPt = DomBB->getTerminator();
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}
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if (isa<StoreInst>(I) && PtrDiff == 0)
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Writes = true;
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}
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}
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};
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bool LoopDataPrefetch::runOnLoop(Loop *L) {
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bool MadeChange = false;
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// Only prefetch in the inner-most loop
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if (!L->empty())
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return MadeChange;
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SmallPtrSet<const Value *, 32> EphValues;
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CodeMetrics::collectEphemeralValues(L, AC, EphValues);
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// Calculate the number of iterations ahead to prefetch
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CodeMetrics Metrics;
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bool HasCall = false;
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for (const auto BB : L->blocks()) {
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// If the loop already has prefetches, then assume that the user knows
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// what they are doing and don't add any more.
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for (auto &I : *BB) {
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if (isa<CallInst>(&I) || isa<InvokeInst>(&I)) {
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ImmutableCallSite CS(&I);
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if (const Function *F = CS.getCalledFunction()) {
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if (F->getIntrinsicID() == Intrinsic::prefetch)
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return MadeChange;
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if (TTI->isLoweredToCall(F))
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HasCall = true;
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} else { // indirect call.
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HasCall = true;
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}
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}
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}
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Metrics.analyzeBasicBlock(BB, *TTI, EphValues);
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}
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unsigned LoopSize = Metrics.NumInsts;
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if (!LoopSize)
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LoopSize = 1;
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unsigned ItersAhead = getPrefetchDistance() / LoopSize;
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if (!ItersAhead)
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ItersAhead = 1;
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if (ItersAhead > getMaxPrefetchIterationsAhead())
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return MadeChange;
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unsigned ConstantMaxTripCount = SE->getSmallConstantMaxTripCount(L);
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if (ConstantMaxTripCount && ConstantMaxTripCount < ItersAhead + 1)
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return MadeChange;
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unsigned NumMemAccesses = 0;
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unsigned NumStridedMemAccesses = 0;
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SmallVector<Prefetch, 16> Prefetches;
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for (const auto BB : L->blocks())
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for (auto &I : *BB) {
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Value *PtrValue;
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Instruction *MemI;
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if (LoadInst *LMemI = dyn_cast<LoadInst>(&I)) {
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MemI = LMemI;
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PtrValue = LMemI->getPointerOperand();
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} else if (StoreInst *SMemI = dyn_cast<StoreInst>(&I)) {
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if (!doPrefetchWrites()) continue;
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MemI = SMemI;
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PtrValue = SMemI->getPointerOperand();
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} else continue;
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unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
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if (PtrAddrSpace)
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continue;
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NumMemAccesses++;
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if (L->isLoopInvariant(PtrValue))
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continue;
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const SCEV *LSCEV = SE->getSCEV(PtrValue);
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const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
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if (!LSCEVAddRec)
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continue;
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NumStridedMemAccesses++;
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// We don't want to double prefetch individual cache lines. If this
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// access is known to be within one cache line of some other one that
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// has already been prefetched, then don't prefetch this one as well.
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bool DupPref = false;
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for (auto &Pref : Prefetches) {
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const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, Pref.LSCEVAddRec);
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if (const SCEVConstant *ConstPtrDiff =
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dyn_cast<SCEVConstant>(PtrDiff)) {
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int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
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if (PD < (int64_t) TTI->getCacheLineSize()) {
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Pref.addInstruction(MemI, DT, PD);
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DupPref = true;
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break;
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}
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}
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}
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if (!DupPref)
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Prefetches.push_back(Prefetch(LSCEVAddRec, MemI));
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}
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unsigned TargetMinStride =
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getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
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Prefetches.size(), HasCall);
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LLVM_DEBUG(dbgs() << "Prefetching " << ItersAhead
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<< " iterations ahead (loop size: " << LoopSize << ") in "
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<< L->getHeader()->getParent()->getName() << ": " << *L);
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LLVM_DEBUG(dbgs() << "Loop has: "
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<< NumMemAccesses << " memory accesses, "
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<< NumStridedMemAccesses << " strided memory accesses, "
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<< Prefetches.size() << " potential prefetch(es), "
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<< "a minimum stride of " << TargetMinStride << ", "
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<< (HasCall ? "calls" : "no calls") << ".\n");
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for (auto &P : Prefetches) {
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// Check if the stride of the accesses is large enough to warrant a
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// prefetch.
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if (!isStrideLargeEnough(P.LSCEVAddRec, TargetMinStride))
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continue;
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const SCEV *NextLSCEV = SE->getAddExpr(P.LSCEVAddRec, SE->getMulExpr(
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SE->getConstant(P.LSCEVAddRec->getType(), ItersAhead),
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P.LSCEVAddRec->getStepRecurrence(*SE)));
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if (!isSafeToExpand(NextLSCEV, *SE))
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continue;
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BasicBlock *BB = P.InsertPt->getParent();
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Type *I8Ptr = Type::getInt8PtrTy(BB->getContext(), 0/*PtrAddrSpace*/);
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SCEVExpander SCEVE(*SE, BB->getModule()->getDataLayout(), "prefaddr");
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Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, P.InsertPt);
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IRBuilder<> Builder(P.InsertPt);
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Module *M = BB->getParent()->getParent();
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Type *I32 = Type::getInt32Ty(BB->getContext());
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Function *PrefetchFunc = Intrinsic::getDeclaration(
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M, Intrinsic::prefetch, PrefPtrValue->getType());
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Builder.CreateCall(
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PrefetchFunc,
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{PrefPtrValue,
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ConstantInt::get(I32, P.Writes),
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ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
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++NumPrefetches;
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LLVM_DEBUG(dbgs() << " Access: "
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<< *P.MemI->getOperand(isa<LoadInst>(P.MemI) ? 0 : 1)
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<< ", SCEV: " << *P.LSCEVAddRec << "\n");
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ORE->emit([&]() {
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return OptimizationRemark(DEBUG_TYPE, "Prefetched", P.MemI)
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<< "prefetched memory access";
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});
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MadeChange = true;
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}
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return MadeChange;
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}
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