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+//===-------- LoopDataPrefetch.cpp - Loop Data Prefetching Pass -----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a Loop Data Prefetching Pass.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "loop-data-prefetch"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/CodeMetrics.h"
+#include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
+#include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/DiagnosticInfo.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+using namespace llvm;
+
+// By default, we limit this to creating 16 PHIs (which is a little over half
+// of the allocatable register set).
+static cl::opt<bool>
+PrefetchWrites("loop-prefetch-writes", cl::Hidden, cl::init(false),
+ cl::desc("Prefetch write addresses"));
+
+static cl::opt<unsigned>
+ PrefetchDistance("prefetch-distance",
+ cl::desc("Number of instructions to prefetch ahead"),
+ cl::Hidden);
+
+static cl::opt<unsigned>
+ MinPrefetchStride("min-prefetch-stride",
+ cl::desc("Min stride to add prefetches"), cl::Hidden);
+
+static cl::opt<unsigned> MaxPrefetchIterationsAhead(
+ "max-prefetch-iters-ahead",
+ cl::desc("Max number of iterations to prefetch ahead"), cl::Hidden);
+
+STATISTIC(NumPrefetches, "Number of prefetches inserted");
+
+namespace llvm {
+ void initializeLoopDataPrefetchPass(PassRegistry&);
+}
+
+namespace {
+
+ class LoopDataPrefetch : public FunctionPass {
+ public:
+ static char ID; // Pass ID, replacement for typeid
+ LoopDataPrefetch() : FunctionPass(ID) {
+ initializeLoopDataPrefetchPass(*PassRegistry::getPassRegistry());
+ }
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addRequired<AssumptionCacheTracker>();
+ AU.addPreserved<DominatorTreeWrapperPass>();
+ AU.addRequired<LoopInfoWrapperPass>();
+ AU.addPreserved<LoopInfoWrapperPass>();
+ AU.addRequired<ScalarEvolutionWrapperPass>();
+ // FIXME: For some reason, preserving SE here breaks LSR (even if
+ // this pass changes nothing).
+ // AU.addPreserved<ScalarEvolutionWrapperPass>();
+ AU.addRequired<TargetTransformInfoWrapperPass>();
+ }
+
+ bool runOnFunction(Function &F) override;
+
+ private:
+ bool runOnLoop(Loop *L);
+
+ /// \brief Check if the the stride of the accesses is large enough to
+ /// warrant a prefetch.
+ bool isStrideLargeEnough(const SCEVAddRecExpr *AR);
+
+ unsigned getMinPrefetchStride() {
+ if (MinPrefetchStride.getNumOccurrences() > 0)
+ return MinPrefetchStride;
+ return TTI->getMinPrefetchStride();
+ }
+
+ unsigned getPrefetchDistance() {
+ if (PrefetchDistance.getNumOccurrences() > 0)
+ return PrefetchDistance;
+ return TTI->getPrefetchDistance();
+ }
+
+ unsigned getMaxPrefetchIterationsAhead() {
+ if (MaxPrefetchIterationsAhead.getNumOccurrences() > 0)
+ return MaxPrefetchIterationsAhead;
+ return TTI->getMaxPrefetchIterationsAhead();
+ }
+
+ AssumptionCache *AC;
+ LoopInfo *LI;
+ ScalarEvolution *SE;
+ const TargetTransformInfo *TTI;
+ const DataLayout *DL;
+ };
+}
+
+char LoopDataPrefetch::ID = 0;
+INITIALIZE_PASS_BEGIN(LoopDataPrefetch, "loop-data-prefetch",
+ "Loop Data Prefetch", false, false)
+INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
+INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
+INITIALIZE_PASS_END(LoopDataPrefetch, "loop-data-prefetch",
+ "Loop Data Prefetch", false, false)
+
+FunctionPass *llvm::createLoopDataPrefetchPass() { return new LoopDataPrefetch(); }
+
+bool LoopDataPrefetch::isStrideLargeEnough(const SCEVAddRecExpr *AR) {
+ unsigned TargetMinStride = getMinPrefetchStride();
+ // No need to check if any stride goes.
+ if (TargetMinStride <= 1)
+ return true;
+
+ const auto *ConstStride = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*SE));
+ // If MinStride is set, don't prefetch unless we can ensure that stride is
+ // larger.
+ if (!ConstStride)
+ return false;
+
+ unsigned AbsStride = std::abs(ConstStride->getAPInt().getSExtValue());
+ return TargetMinStride <= AbsStride;
+}
+
+bool LoopDataPrefetch::runOnFunction(Function &F) {
+ if (skipFunction(F))
+ return false;
+
+ LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+ DL = &F.getParent()->getDataLayout();
+ AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
+ TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
+
+ // If PrefetchDistance is not set, don't run the pass. This gives an
+ // opportunity for targets to run this pass for selected subtargets only
+ // (whose TTI sets PrefetchDistance).
+ if (getPrefetchDistance() == 0)
+ return false;
+ assert(TTI->getCacheLineSize() && "Cache line size is not set for target");
+
+ bool MadeChange = false;
+
+ for (Loop *I : *LI)
+ for (auto L = df_begin(I), LE = df_end(I); L != LE; ++L)
+ MadeChange |= runOnLoop(*L);
+
+ return MadeChange;
+}
+
+bool LoopDataPrefetch::runOnLoop(Loop *L) {
+ bool MadeChange = false;
+
+ // Only prefetch in the inner-most loop
+ if (!L->empty())
+ return MadeChange;
+
+ SmallPtrSet<const Value *, 32> EphValues;
+ CodeMetrics::collectEphemeralValues(L, AC, EphValues);
+
+ // Calculate the number of iterations ahead to prefetch
+ CodeMetrics Metrics;
+ for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
+ I != IE; ++I) {
+
+ // If the loop already has prefetches, then assume that the user knows
+ // what they are doing and don't add any more.
+ for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
+ J != JE; ++J)
+ if (CallInst *CI = dyn_cast<CallInst>(J))
+ if (Function *F = CI->getCalledFunction())
+ if (F->getIntrinsicID() == Intrinsic::prefetch)
+ return MadeChange;
+
+ Metrics.analyzeBasicBlock(*I, *TTI, EphValues);
+ }
+ unsigned LoopSize = Metrics.NumInsts;
+ if (!LoopSize)
+ LoopSize = 1;
+
+ unsigned ItersAhead = getPrefetchDistance() / LoopSize;
+ if (!ItersAhead)
+ ItersAhead = 1;
+
+ if (ItersAhead > getMaxPrefetchIterationsAhead())
+ return MadeChange;
+
+ Function *F = L->getHeader()->getParent();
+ DEBUG(dbgs() << "Prefetching " << ItersAhead
+ << " iterations ahead (loop size: " << LoopSize << ") in "
+ << F->getName() << ": " << *L);
+
+ SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
+ for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
+ I != IE; ++I) {
+ for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
+ J != JE; ++J) {
+ Value *PtrValue;
+ Instruction *MemI;
+
+ if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
+ MemI = LMemI;
+ PtrValue = LMemI->getPointerOperand();
+ } else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
+ if (!PrefetchWrites) continue;
+ MemI = SMemI;
+ PtrValue = SMemI->getPointerOperand();
+ } else continue;
+
+ unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
+ if (PtrAddrSpace)
+ continue;
+
+ if (L->isLoopInvariant(PtrValue))
+ continue;
+
+ const SCEV *LSCEV = SE->getSCEV(PtrValue);
+ const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
+ if (!LSCEVAddRec)
+ continue;
+
+ // Check if the the stride of the accesses is large enough to warrant a
+ // prefetch.
+ if (!isStrideLargeEnough(LSCEVAddRec))
+ continue;
+
+ // We don't want to double prefetch individual cache lines. If this load
+ // is known to be within one cache line of some other load that has
+ // already been prefetched, then don't prefetch this one as well.
+ bool DupPref = false;
+ for (const auto &PrefLoad : PrefLoads) {
+ const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, PrefLoad.second);
+ if (const SCEVConstant *ConstPtrDiff =
+ dyn_cast<SCEVConstant>(PtrDiff)) {
+ int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
+ if (PD < (int64_t) TTI->getCacheLineSize()) {
+ DupPref = true;
+ break;
+ }
+ }
+ }
+ if (DupPref)
+ continue;
+
+ const SCEV *NextLSCEV = SE->getAddExpr(LSCEVAddRec, SE->getMulExpr(
+ SE->getConstant(LSCEVAddRec->getType(), ItersAhead),
+ LSCEVAddRec->getStepRecurrence(*SE)));
+ if (!isSafeToExpand(NextLSCEV, *SE))
+ continue;
+
+ PrefLoads.push_back(std::make_pair(MemI, LSCEVAddRec));
+
+ Type *I8Ptr = Type::getInt8PtrTy((*I)->getContext(), PtrAddrSpace);
+ SCEVExpander SCEVE(*SE, J->getModule()->getDataLayout(), "prefaddr");
+ Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
+
+ IRBuilder<> Builder(MemI);
+ Module *M = (*I)->getParent()->getParent();
+ Type *I32 = Type::getInt32Ty((*I)->getContext());
+ Value *PrefetchFunc = Intrinsic::getDeclaration(M, Intrinsic::prefetch);
+ Builder.CreateCall(
+ PrefetchFunc,
+ {PrefPtrValue,
+ ConstantInt::get(I32, MemI->mayReadFromMemory() ? 0 : 1),
+ ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
+ ++NumPrefetches;
+ DEBUG(dbgs() << " Access: " << *PtrValue << ", SCEV: " << *LSCEV
+ << "\n");
+ emitOptimizationRemark(F->getContext(), DEBUG_TYPE, *F,
+ MemI->getDebugLoc(), "prefetched memory access");
+
+
+ MadeChange = true;
+ }
+ }
+
+ return MadeChange;
+}
+
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