blob: 7f7c6de76450de2ae48d1250991dab59e23a8c78 [file] [log] [blame]
//===-------- 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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LoopDataPrefetch.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/ScalarEvolutionExpander.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/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, LoopInfo *LI, ScalarEvolution *SE,
const TargetTransformInfo *TTI,
OptimizationRemarkEmitter *ORE)
: AC(AC), LI(LI), SE(SE), TTI(TTI), ORE(ORE) {}
bool run();
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;
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.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 = 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;
}
PreservedAnalyses LoopDataPrefetchPass::run(Function &F,
FunctionAnalysisManager &AM) {
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, 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;
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, 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;
}
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 (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 (CallInst *CI = dyn_cast<CallInst>(&I))
if (Function *F = CI->getCalledFunction())
if (F->getIntrinsicID() == Intrinsic::prefetch)
return MadeChange;
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;
DEBUG(dbgs() << "Prefetching " << ItersAhead
<< " iterations ahead (loop size: " << LoopSize << ") in "
<< L->getHeader()->getParent()->getName() << ": " << *L);
SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
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 (!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(BB->getContext(), PtrAddrSpace);
SCEVExpander SCEVE(*SE, I.getModule()->getDataLayout(), "prefaddr");
Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
IRBuilder<> Builder(MemI);
Module *M = BB->getParent()->getParent();
Type *I32 = Type::getInt32Ty(BB->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");
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "Prefetched", MemI)
<< "prefetched memory access";
});
MadeChange = true;
}
}
return MadeChange;
}