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llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp

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//===-------- 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;
}
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