1 //===-------- LoopDataPrefetch.cpp - Loop Data Prefetching Pass -----------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements a Loop Data Prefetching Pass.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/Transforms/Scalar/LoopDataPrefetch.h"
14 #include "llvm/InitializePasses.h"
15
16 #include "llvm/ADT/DepthFirstIterator.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/Analysis/AssumptionCache.h"
19 #include "llvm/Analysis/CodeMetrics.h"
20 #include "llvm/Analysis/LoopInfo.h"
21 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
22 #include "llvm/Analysis/ScalarEvolution.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/Analysis/TargetTransformInfo.h"
25 #include "llvm/IR/Dominators.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/IR/Module.h"
28 #include "llvm/Support/CommandLine.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Transforms/Utils.h"
32 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
33
34 #define DEBUG_TYPE "loop-data-prefetch"
35
36 using namespace llvm;
37
38 // By default, we limit this to creating 16 PHIs (which is a little over half
39 // of the allocatable register set).
40 static cl::opt<bool>
41 PrefetchWrites("loop-prefetch-writes", cl::Hidden, cl::init(false),
42 cl::desc("Prefetch write addresses"));
43
44 static cl::opt<unsigned>
45 PrefetchDistance("prefetch-distance",
46 cl::desc("Number of instructions to prefetch ahead"),
47 cl::Hidden);
48
49 static cl::opt<unsigned>
50 MinPrefetchStride("min-prefetch-stride",
51 cl::desc("Min stride to add prefetches"), cl::Hidden);
52
53 static cl::opt<unsigned> MaxPrefetchIterationsAhead(
54 "max-prefetch-iters-ahead",
55 cl::desc("Max number of iterations to prefetch ahead"), cl::Hidden);
56
57 STATISTIC(NumPrefetches, "Number of prefetches inserted");
58
59 namespace {
60
61 /// Loop prefetch implementation class.
62 class LoopDataPrefetch {
63 public:
LoopDataPrefetch(AssumptionCache * AC,DominatorTree * DT,LoopInfo * LI,ScalarEvolution * SE,const TargetTransformInfo * TTI,OptimizationRemarkEmitter * ORE)64 LoopDataPrefetch(AssumptionCache *AC, DominatorTree *DT, LoopInfo *LI,
65 ScalarEvolution *SE, const TargetTransformInfo *TTI,
66 OptimizationRemarkEmitter *ORE)
67 : AC(AC), DT(DT), LI(LI), SE(SE), TTI(TTI), ORE(ORE) {}
68
69 bool run();
70
71 private:
72 bool runOnLoop(Loop *L);
73
74 /// Check if the stride of the accesses is large enough to
75 /// warrant a prefetch.
76 bool isStrideLargeEnough(const SCEVAddRecExpr *AR, unsigned TargetMinStride);
77
getMinPrefetchStride(unsigned NumMemAccesses,unsigned NumStridedMemAccesses,unsigned NumPrefetches,bool HasCall)78 unsigned getMinPrefetchStride(unsigned NumMemAccesses,
79 unsigned NumStridedMemAccesses,
80 unsigned NumPrefetches,
81 bool HasCall) {
82 if (MinPrefetchStride.getNumOccurrences() > 0)
83 return MinPrefetchStride;
84 return TTI->getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
85 NumPrefetches, HasCall);
86 }
87
getPrefetchDistance()88 unsigned getPrefetchDistance() {
89 if (PrefetchDistance.getNumOccurrences() > 0)
90 return PrefetchDistance;
91 return TTI->getPrefetchDistance();
92 }
93
getMaxPrefetchIterationsAhead()94 unsigned getMaxPrefetchIterationsAhead() {
95 if (MaxPrefetchIterationsAhead.getNumOccurrences() > 0)
96 return MaxPrefetchIterationsAhead;
97 return TTI->getMaxPrefetchIterationsAhead();
98 }
99
doPrefetchWrites()100 bool doPrefetchWrites() {
101 if (PrefetchWrites.getNumOccurrences() > 0)
102 return PrefetchWrites;
103 return TTI->enableWritePrefetching();
104 }
105
106 AssumptionCache *AC;
107 DominatorTree *DT;
108 LoopInfo *LI;
109 ScalarEvolution *SE;
110 const TargetTransformInfo *TTI;
111 OptimizationRemarkEmitter *ORE;
112 };
113
114 /// Legacy class for inserting loop data prefetches.
115 class LoopDataPrefetchLegacyPass : public FunctionPass {
116 public:
117 static char ID; // Pass ID, replacement for typeid
LoopDataPrefetchLegacyPass()118 LoopDataPrefetchLegacyPass() : FunctionPass(ID) {
119 initializeLoopDataPrefetchLegacyPassPass(*PassRegistry::getPassRegistry());
120 }
121
getAnalysisUsage(AnalysisUsage & AU) const122 void getAnalysisUsage(AnalysisUsage &AU) const override {
123 AU.addRequired<AssumptionCacheTracker>();
124 AU.addRequired<DominatorTreeWrapperPass>();
125 AU.addPreserved<DominatorTreeWrapperPass>();
126 AU.addRequired<LoopInfoWrapperPass>();
127 AU.addPreserved<LoopInfoWrapperPass>();
128 AU.addRequiredID(LoopSimplifyID);
129 AU.addPreservedID(LoopSimplifyID);
130 AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
131 AU.addRequired<ScalarEvolutionWrapperPass>();
132 AU.addPreserved<ScalarEvolutionWrapperPass>();
133 AU.addRequired<TargetTransformInfoWrapperPass>();
134 }
135
136 bool runOnFunction(Function &F) override;
137 };
138 }
139
140 char LoopDataPrefetchLegacyPass::ID = 0;
141 INITIALIZE_PASS_BEGIN(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
142 "Loop Data Prefetch", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)143 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
144 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
145 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
146 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
147 INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
148 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
149 INITIALIZE_PASS_END(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
150 "Loop Data Prefetch", false, false)
151
152 FunctionPass *llvm::createLoopDataPrefetchPass() {
153 return new LoopDataPrefetchLegacyPass();
154 }
155
isStrideLargeEnough(const SCEVAddRecExpr * AR,unsigned TargetMinStride)156 bool LoopDataPrefetch::isStrideLargeEnough(const SCEVAddRecExpr *AR,
157 unsigned TargetMinStride) {
158 // No need to check if any stride goes.
159 if (TargetMinStride <= 1)
160 return true;
161
162 const auto *ConstStride = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*SE));
163 // If MinStride is set, don't prefetch unless we can ensure that stride is
164 // larger.
165 if (!ConstStride)
166 return false;
167
168 unsigned AbsStride = std::abs(ConstStride->getAPInt().getSExtValue());
169 return TargetMinStride <= AbsStride;
170 }
171
run(Function & F,FunctionAnalysisManager & AM)172 PreservedAnalyses LoopDataPrefetchPass::run(Function &F,
173 FunctionAnalysisManager &AM) {
174 DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
175 LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
176 ScalarEvolution *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
177 AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
178 OptimizationRemarkEmitter *ORE =
179 &AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
180 const TargetTransformInfo *TTI = &AM.getResult<TargetIRAnalysis>(F);
181
182 LoopDataPrefetch LDP(AC, DT, LI, SE, TTI, ORE);
183 bool Changed = LDP.run();
184
185 if (Changed) {
186 PreservedAnalyses PA;
187 PA.preserve<DominatorTreeAnalysis>();
188 PA.preserve<LoopAnalysis>();
189 return PA;
190 }
191
192 return PreservedAnalyses::all();
193 }
194
runOnFunction(Function & F)195 bool LoopDataPrefetchLegacyPass::runOnFunction(Function &F) {
196 if (skipFunction(F))
197 return false;
198
199 DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
200 LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
201 ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
202 AssumptionCache *AC =
203 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
204 OptimizationRemarkEmitter *ORE =
205 &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
206 const TargetTransformInfo *TTI =
207 &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
208
209 LoopDataPrefetch LDP(AC, DT, LI, SE, TTI, ORE);
210 return LDP.run();
211 }
212
run()213 bool LoopDataPrefetch::run() {
214 // If PrefetchDistance is not set, don't run the pass. This gives an
215 // opportunity for targets to run this pass for selected subtargets only
216 // (whose TTI sets PrefetchDistance and CacheLineSize).
217 if (getPrefetchDistance() == 0 || TTI->getCacheLineSize() == 0) {
218 LLVM_DEBUG(dbgs() << "Please set both PrefetchDistance and CacheLineSize "
219 "for loop data prefetch.\n");
220 return false;
221 }
222
223 bool MadeChange = false;
224
225 for (Loop *I : *LI)
226 for (Loop *L : depth_first(I))
227 MadeChange |= runOnLoop(L);
228
229 return MadeChange;
230 }
231
232 /// A record for a potential prefetch made during the initial scan of the
233 /// loop. This is used to let a single prefetch target multiple memory accesses.
234 struct Prefetch {
235 /// The address formula for this prefetch as returned by ScalarEvolution.
236 const SCEVAddRecExpr *LSCEVAddRec;
237 /// The point of insertion for the prefetch instruction.
238 Instruction *InsertPt = nullptr;
239 /// True if targeting a write memory access.
240 bool Writes = false;
241 /// The (first seen) prefetched instruction.
242 Instruction *MemI = nullptr;
243
244 /// Constructor to create a new Prefetch for \p I.
PrefetchPrefetch245 Prefetch(const SCEVAddRecExpr *L, Instruction *I) : LSCEVAddRec(L) {
246 addInstruction(I);
247 };
248
249 /// Add the instruction \param I to this prefetch. If it's not the first
250 /// one, 'InsertPt' and 'Writes' will be updated as required.
251 /// \param PtrDiff the known constant address difference to the first added
252 /// instruction.
addInstructionPrefetch253 void addInstruction(Instruction *I, DominatorTree *DT = nullptr,
254 int64_t PtrDiff = 0) {
255 if (!InsertPt) {
256 MemI = I;
257 InsertPt = I;
258 Writes = isa<StoreInst>(I);
259 } else {
260 BasicBlock *PrefBB = InsertPt->getParent();
261 BasicBlock *InsBB = I->getParent();
262 if (PrefBB != InsBB) {
263 BasicBlock *DomBB = DT->findNearestCommonDominator(PrefBB, InsBB);
264 if (DomBB != PrefBB)
265 InsertPt = DomBB->getTerminator();
266 }
267
268 if (isa<StoreInst>(I) && PtrDiff == 0)
269 Writes = true;
270 }
271 }
272 };
273
runOnLoop(Loop * L)274 bool LoopDataPrefetch::runOnLoop(Loop *L) {
275 bool MadeChange = false;
276
277 // Only prefetch in the inner-most loop
278 if (!L->isInnermost())
279 return MadeChange;
280
281 SmallPtrSet<const Value *, 32> EphValues;
282 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
283
284 // Calculate the number of iterations ahead to prefetch
285 CodeMetrics Metrics;
286 bool HasCall = false;
287 for (const auto BB : L->blocks()) {
288 // If the loop already has prefetches, then assume that the user knows
289 // what they are doing and don't add any more.
290 for (auto &I : *BB) {
291 if (isa<CallInst>(&I) || isa<InvokeInst>(&I)) {
292 if (const Function *F = cast<CallBase>(I).getCalledFunction()) {
293 if (F->getIntrinsicID() == Intrinsic::prefetch)
294 return MadeChange;
295 if (TTI->isLoweredToCall(F))
296 HasCall = true;
297 } else { // indirect call.
298 HasCall = true;
299 }
300 }
301 }
302 Metrics.analyzeBasicBlock(BB, *TTI, EphValues);
303 }
304
305 if (!Metrics.NumInsts.isValid())
306 return MadeChange;
307
308 unsigned LoopSize = *Metrics.NumInsts.getValue();
309 if (!LoopSize)
310 LoopSize = 1;
311
312 unsigned ItersAhead = getPrefetchDistance() / LoopSize;
313 if (!ItersAhead)
314 ItersAhead = 1;
315
316 if (ItersAhead > getMaxPrefetchIterationsAhead())
317 return MadeChange;
318
319 unsigned ConstantMaxTripCount = SE->getSmallConstantMaxTripCount(L);
320 if (ConstantMaxTripCount && ConstantMaxTripCount < ItersAhead + 1)
321 return MadeChange;
322
323 unsigned NumMemAccesses = 0;
324 unsigned NumStridedMemAccesses = 0;
325 SmallVector<Prefetch, 16> Prefetches;
326 for (const auto BB : L->blocks())
327 for (auto &I : *BB) {
328 Value *PtrValue;
329 Instruction *MemI;
330
331 if (LoadInst *LMemI = dyn_cast<LoadInst>(&I)) {
332 MemI = LMemI;
333 PtrValue = LMemI->getPointerOperand();
334 } else if (StoreInst *SMemI = dyn_cast<StoreInst>(&I)) {
335 if (!doPrefetchWrites()) continue;
336 MemI = SMemI;
337 PtrValue = SMemI->getPointerOperand();
338 } else continue;
339
340 unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
341 if (!TTI->shouldPrefetchAddressSpace(PtrAddrSpace))
342 continue;
343 NumMemAccesses++;
344 if (L->isLoopInvariant(PtrValue))
345 continue;
346
347 const SCEV *LSCEV = SE->getSCEV(PtrValue);
348 const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
349 if (!LSCEVAddRec)
350 continue;
351 NumStridedMemAccesses++;
352
353 // We don't want to double prefetch individual cache lines. If this
354 // access is known to be within one cache line of some other one that
355 // has already been prefetched, then don't prefetch this one as well.
356 bool DupPref = false;
357 for (auto &Pref : Prefetches) {
358 const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, Pref.LSCEVAddRec);
359 if (const SCEVConstant *ConstPtrDiff =
360 dyn_cast<SCEVConstant>(PtrDiff)) {
361 int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
362 if (PD < (int64_t) TTI->getCacheLineSize()) {
363 Pref.addInstruction(MemI, DT, PD);
364 DupPref = true;
365 break;
366 }
367 }
368 }
369 if (!DupPref)
370 Prefetches.push_back(Prefetch(LSCEVAddRec, MemI));
371 }
372
373 unsigned TargetMinStride =
374 getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
375 Prefetches.size(), HasCall);
376
377 LLVM_DEBUG(dbgs() << "Prefetching " << ItersAhead
378 << " iterations ahead (loop size: " << LoopSize << ") in "
379 << L->getHeader()->getParent()->getName() << ": " << *L);
380 LLVM_DEBUG(dbgs() << "Loop has: "
381 << NumMemAccesses << " memory accesses, "
382 << NumStridedMemAccesses << " strided memory accesses, "
383 << Prefetches.size() << " potential prefetch(es), "
384 << "a minimum stride of " << TargetMinStride << ", "
385 << (HasCall ? "calls" : "no calls") << ".\n");
386
387 for (auto &P : Prefetches) {
388 // Check if the stride of the accesses is large enough to warrant a
389 // prefetch.
390 if (!isStrideLargeEnough(P.LSCEVAddRec, TargetMinStride))
391 continue;
392
393 BasicBlock *BB = P.InsertPt->getParent();
394 SCEVExpander SCEVE(*SE, BB->getModule()->getDataLayout(), "prefaddr");
395 const SCEV *NextLSCEV = SE->getAddExpr(P.LSCEVAddRec, SE->getMulExpr(
396 SE->getConstant(P.LSCEVAddRec->getType(), ItersAhead),
397 P.LSCEVAddRec->getStepRecurrence(*SE)));
398 if (!SCEVE.isSafeToExpand(NextLSCEV))
399 continue;
400
401 unsigned PtrAddrSpace = NextLSCEV->getType()->getPointerAddressSpace();
402 Type *I8Ptr = PointerType::get(BB->getContext(), PtrAddrSpace);
403 Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, P.InsertPt);
404
405 IRBuilder<> Builder(P.InsertPt);
406 Module *M = BB->getParent()->getParent();
407 Type *I32 = Type::getInt32Ty(BB->getContext());
408 Function *PrefetchFunc = Intrinsic::getDeclaration(
409 M, Intrinsic::prefetch, PrefPtrValue->getType());
410 Builder.CreateCall(
411 PrefetchFunc,
412 {PrefPtrValue,
413 ConstantInt::get(I32, P.Writes),
414 ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
415 ++NumPrefetches;
416 LLVM_DEBUG(dbgs() << " Access: "
417 << *P.MemI->getOperand(isa<LoadInst>(P.MemI) ? 0 : 1)
418 << ", SCEV: " << *P.LSCEVAddRec << "\n");
419 ORE->emit([&]() {
420 return OptimizationRemark(DEBUG_TYPE, "Prefetched", P.MemI)
421 << "prefetched memory access";
422 });
423
424 MadeChange = true;
425 }
426
427 return MadeChange;
428 }
429