aboutsummaryrefslogtreecommitdiff
path: root/contrib/llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp
diff options
context:
space:
mode:
Diffstat (limited to 'contrib/llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp')
-rw-r--r--contrib/llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp419
1 files changed, 419 insertions, 0 deletions
diff --git a/contrib/llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp b/contrib/llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp
new file mode 100644
index 000000000000..45cdcb2f37dd
--- /dev/null
+++ b/contrib/llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp
@@ -0,0 +1,419 @@
+//===-------- LoopDataPrefetch.cpp - Loop Data Prefetching Pass -----------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a Loop Data Prefetching Pass.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/LoopDataPrefetch.h"
+#include "llvm/InitializePasses.h"
+
+#define DEBUG_TYPE "loop-data-prefetch"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/CodeMetrics.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/OptimizationRemarkEmitter.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/ScalarEvolutionExpander.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 {
+
+/// Loop prefetch implementation class.
+class LoopDataPrefetch {
+public:
+ LoopDataPrefetch(AssumptionCache *AC, DominatorTree *DT, LoopInfo *LI,
+ ScalarEvolution *SE, const TargetTransformInfo *TTI,
+ OptimizationRemarkEmitter *ORE)
+ : AC(AC), DT(DT), LI(LI), SE(SE), TTI(TTI), ORE(ORE) {}
+
+ bool run();
+
+private:
+ bool runOnLoop(Loop *L);
+
+ /// Check if the stride of the accesses is large enough to
+ /// warrant a prefetch.
+ bool isStrideLargeEnough(const SCEVAddRecExpr *AR, unsigned TargetMinStride);
+
+ unsigned getMinPrefetchStride(unsigned NumMemAccesses,
+ unsigned NumStridedMemAccesses,
+ unsigned NumPrefetches,
+ bool HasCall) {
+ if (MinPrefetchStride.getNumOccurrences() > 0)
+ return MinPrefetchStride;
+ return TTI->getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
+ NumPrefetches, HasCall);
+ }
+
+ unsigned getPrefetchDistance() {
+ if (PrefetchDistance.getNumOccurrences() > 0)
+ return PrefetchDistance;
+ return TTI->getPrefetchDistance();
+ }
+
+ unsigned getMaxPrefetchIterationsAhead() {
+ if (MaxPrefetchIterationsAhead.getNumOccurrences() > 0)
+ return MaxPrefetchIterationsAhead;
+ return TTI->getMaxPrefetchIterationsAhead();
+ }
+
+ bool doPrefetchWrites() {
+ if (PrefetchWrites.getNumOccurrences() > 0)
+ return PrefetchWrites;
+ return TTI->enableWritePrefetching();
+ }
+
+ AssumptionCache *AC;
+ DominatorTree *DT;
+ LoopInfo *LI;
+ ScalarEvolution *SE;
+ const TargetTransformInfo *TTI;
+ OptimizationRemarkEmitter *ORE;
+};
+
+/// Legacy class for inserting loop data prefetches.
+class LoopDataPrefetchLegacyPass : public FunctionPass {
+public:
+ static char ID; // Pass ID, replacement for typeid
+ LoopDataPrefetchLegacyPass() : FunctionPass(ID) {
+ initializeLoopDataPrefetchLegacyPassPass(*PassRegistry::getPassRegistry());
+ }
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addRequired<AssumptionCacheTracker>();
+ AU.addRequired<DominatorTreeWrapperPass>();
+ AU.addPreserved<DominatorTreeWrapperPass>();
+ AU.addRequired<LoopInfoWrapperPass>();
+ AU.addPreserved<LoopInfoWrapperPass>();
+ AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
+ AU.addRequired<ScalarEvolutionWrapperPass>();
+ AU.addPreserved<ScalarEvolutionWrapperPass>();
+ AU.addRequired<TargetTransformInfoWrapperPass>();
+ }
+
+ bool runOnFunction(Function &F) override;
+ };
+}
+
+char LoopDataPrefetchLegacyPass::ID = 0;
+INITIALIZE_PASS_BEGIN(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
+ "Loop Data Prefetch", false, false)
+INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
+INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
+INITIALIZE_PASS_END(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
+ "Loop Data Prefetch", false, false)
+
+FunctionPass *llvm::createLoopDataPrefetchPass() {
+ return new LoopDataPrefetchLegacyPass();
+}
+
+bool LoopDataPrefetch::isStrideLargeEnough(const SCEVAddRecExpr *AR,
+ unsigned TargetMinStride) {
+ // 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;
+}
+
+PreservedAnalyses LoopDataPrefetchPass::run(Function &F,
+ FunctionAnalysisManager &AM) {
+ DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
+ LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
+ ScalarEvolution *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
+ AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
+ OptimizationRemarkEmitter *ORE =
+ &AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
+ const TargetTransformInfo *TTI = &AM.getResult<TargetIRAnalysis>(F);
+
+ LoopDataPrefetch LDP(AC, DT, LI, SE, TTI, ORE);
+ bool Changed = LDP.run();
+
+ if (Changed) {
+ PreservedAnalyses PA;
+ PA.preserve<DominatorTreeAnalysis>();
+ PA.preserve<LoopAnalysis>();
+ return PA;
+ }
+
+ return PreservedAnalyses::all();
+}
+
+bool LoopDataPrefetchLegacyPass::runOnFunction(Function &F) {
+ if (skipFunction(F))
+ return false;
+
+ DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+ AssumptionCache *AC =
+ &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
+ OptimizationRemarkEmitter *ORE =
+ &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
+ const TargetTransformInfo *TTI =
+ &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
+
+ LoopDataPrefetch LDP(AC, DT, LI, SE, TTI, ORE);
+ return LDP.run();
+}
+
+bool LoopDataPrefetch::run() {
+ // 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;
+}
+
+/// A record for a potential prefetch made during the initial scan of the
+/// loop. This is used to let a single prefetch target multiple memory accesses.
+struct Prefetch {
+ /// The address formula for this prefetch as returned by ScalarEvolution.
+ const SCEVAddRecExpr *LSCEVAddRec;
+ /// The point of insertion for the prefetch instruction.
+ Instruction *InsertPt;
+ /// True if targeting a write memory access.
+ bool Writes;
+ /// The (first seen) prefetched instruction.
+ Instruction *MemI;
+
+ /// Constructor to create a new Prefetch for \p I.
+ Prefetch(const SCEVAddRecExpr *L, Instruction *I)
+ : LSCEVAddRec(L), InsertPt(nullptr), Writes(false), MemI(nullptr) {
+ addInstruction(I);
+ };
+
+ /// Add the instruction \param I to this prefetch. If it's not the first
+ /// one, 'InsertPt' and 'Writes' will be updated as required.
+ /// \param PtrDiff the known constant address difference to the first added
+ /// instruction.
+ void addInstruction(Instruction *I, DominatorTree *DT = nullptr,
+ int64_t PtrDiff = 0) {
+ if (!InsertPt) {
+ MemI = I;
+ InsertPt = I;
+ Writes = isa<StoreInst>(I);
+ } else {
+ BasicBlock *PrefBB = InsertPt->getParent();
+ BasicBlock *InsBB = I->getParent();
+ if (PrefBB != InsBB) {
+ BasicBlock *DomBB = DT->findNearestCommonDominator(PrefBB, InsBB);
+ if (DomBB != PrefBB)
+ InsertPt = DomBB->getTerminator();
+ }
+
+ if (isa<StoreInst>(I) && PtrDiff == 0)
+ Writes = true;
+ }
+ }
+};
+
+bool LoopDataPrefetch::runOnLoop(Loop *L) {
+ bool MadeChange = false;
+
+ // Only prefetch in the inner-most loop
+ if (!L->isInnermost())
+ return MadeChange;
+
+ SmallPtrSet<const Value *, 32> EphValues;
+ CodeMetrics::collectEphemeralValues(L, AC, EphValues);
+
+ // Calculate the number of iterations ahead to prefetch
+ CodeMetrics Metrics;
+ bool HasCall = false;
+ for (const auto BB : L->blocks()) {
+ // If the loop already has prefetches, then assume that the user knows
+ // what they are doing and don't add any more.
+ for (auto &I : *BB) {
+ if (isa<CallInst>(&I) || isa<InvokeInst>(&I)) {
+ if (const Function *F = cast<CallBase>(I).getCalledFunction()) {
+ if (F->getIntrinsicID() == Intrinsic::prefetch)
+ return MadeChange;
+ if (TTI->isLoweredToCall(F))
+ HasCall = true;
+ } else { // indirect call.
+ HasCall = true;
+ }
+ }
+ }
+ Metrics.analyzeBasicBlock(BB, *TTI, EphValues);
+ }
+ unsigned LoopSize = Metrics.NumInsts;
+ if (!LoopSize)
+ LoopSize = 1;
+
+ unsigned ItersAhead = getPrefetchDistance() / LoopSize;
+ if (!ItersAhead)
+ ItersAhead = 1;
+
+ if (ItersAhead > getMaxPrefetchIterationsAhead())
+ return MadeChange;
+
+ unsigned ConstantMaxTripCount = SE->getSmallConstantMaxTripCount(L);
+ if (ConstantMaxTripCount && ConstantMaxTripCount < ItersAhead + 1)
+ return MadeChange;
+
+ unsigned NumMemAccesses = 0;
+ unsigned NumStridedMemAccesses = 0;
+ SmallVector<Prefetch, 16> Prefetches;
+ for (const auto BB : L->blocks())
+ for (auto &I : *BB) {
+ Value *PtrValue;
+ Instruction *MemI;
+
+ if (LoadInst *LMemI = dyn_cast<LoadInst>(&I)) {
+ MemI = LMemI;
+ PtrValue = LMemI->getPointerOperand();
+ } else if (StoreInst *SMemI = dyn_cast<StoreInst>(&I)) {
+ if (!doPrefetchWrites()) continue;
+ MemI = SMemI;
+ PtrValue = SMemI->getPointerOperand();
+ } else continue;
+
+ unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
+ if (PtrAddrSpace)
+ continue;
+ NumMemAccesses++;
+ if (L->isLoopInvariant(PtrValue))
+ continue;
+
+ const SCEV *LSCEV = SE->getSCEV(PtrValue);
+ const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
+ if (!LSCEVAddRec)
+ continue;
+ NumStridedMemAccesses++;
+
+ // We don't want to double prefetch individual cache lines. If this
+ // access is known to be within one cache line of some other one that
+ // has already been prefetched, then don't prefetch this one as well.
+ bool DupPref = false;
+ for (auto &Pref : Prefetches) {
+ const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, Pref.LSCEVAddRec);
+ if (const SCEVConstant *ConstPtrDiff =
+ dyn_cast<SCEVConstant>(PtrDiff)) {
+ int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
+ if (PD < (int64_t) TTI->getCacheLineSize()) {
+ Pref.addInstruction(MemI, DT, PD);
+ DupPref = true;
+ break;
+ }
+ }
+ }
+ if (!DupPref)
+ Prefetches.push_back(Prefetch(LSCEVAddRec, MemI));
+ }
+
+ unsigned TargetMinStride =
+ getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
+ Prefetches.size(), HasCall);
+
+ LLVM_DEBUG(dbgs() << "Prefetching " << ItersAhead
+ << " iterations ahead (loop size: " << LoopSize << ") in "
+ << L->getHeader()->getParent()->getName() << ": " << *L);
+ LLVM_DEBUG(dbgs() << "Loop has: "
+ << NumMemAccesses << " memory accesses, "
+ << NumStridedMemAccesses << " strided memory accesses, "
+ << Prefetches.size() << " potential prefetch(es), "
+ << "a minimum stride of " << TargetMinStride << ", "
+ << (HasCall ? "calls" : "no calls") << ".\n");
+
+ for (auto &P : Prefetches) {
+ // Check if the stride of the accesses is large enough to warrant a
+ // prefetch.
+ if (!isStrideLargeEnough(P.LSCEVAddRec, TargetMinStride))
+ continue;
+
+ const SCEV *NextLSCEV = SE->getAddExpr(P.LSCEVAddRec, SE->getMulExpr(
+ SE->getConstant(P.LSCEVAddRec->getType(), ItersAhead),
+ P.LSCEVAddRec->getStepRecurrence(*SE)));
+ if (!isSafeToExpand(NextLSCEV, *SE))
+ continue;
+
+ BasicBlock *BB = P.InsertPt->getParent();
+ Type *I8Ptr = Type::getInt8PtrTy(BB->getContext(), 0/*PtrAddrSpace*/);
+ SCEVExpander SCEVE(*SE, BB->getModule()->getDataLayout(), "prefaddr");
+ Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, P.InsertPt);
+
+ IRBuilder<> Builder(P.InsertPt);
+ Module *M = BB->getParent()->getParent();
+ Type *I32 = Type::getInt32Ty(BB->getContext());
+ Function *PrefetchFunc = Intrinsic::getDeclaration(
+ M, Intrinsic::prefetch, PrefPtrValue->getType());
+ Builder.CreateCall(
+ PrefetchFunc,
+ {PrefPtrValue,
+ ConstantInt::get(I32, P.Writes),
+ ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
+ ++NumPrefetches;
+ LLVM_DEBUG(dbgs() << " Access: "
+ << *P.MemI->getOperand(isa<LoadInst>(P.MemI) ? 0 : 1)
+ << ", SCEV: " << *P.LSCEVAddRec << "\n");
+ ORE->emit([&]() {
+ return OptimizationRemark(DEBUG_TYPE, "Prefetched", P.MemI)
+ << "prefetched memory access";
+ });
+
+ MadeChange = true;
+ }
+
+ return MadeChange;
+}
generated by cgit v1.2.3 (git 2.25.1) at 2025-10-10 16:55:51 +0000