xref: /llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp (revision 9590658fb88d40b88ad5bb34c66c946513579384)
1 //===-------- LoopDataPrefetch.cpp - Loop Data Prefetching Pass -----------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements a Loop Data Prefetching Pass.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/Scalar/LoopDataPrefetch.h"
15 
16 #define DEBUG_TYPE "loop-data-prefetch"
17 #include "llvm/ADT/DepthFirstIterator.h"
18 #include "llvm/ADT/Statistic.h"
19 #include "llvm/Analysis/AssumptionCache.h"
20 #include "llvm/Analysis/CodeMetrics.h"
21 #include "llvm/Analysis/InstructionSimplify.h"
22 #include "llvm/Analysis/LoopInfo.h"
23 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
24 #include "llvm/Analysis/ScalarEvolution.h"
25 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
26 #include "llvm/Analysis/ScalarEvolutionExpander.h"
27 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
28 #include "llvm/Analysis/TargetTransformInfo.h"
29 #include "llvm/Analysis/ValueTracking.h"
30 #include "llvm/IR/CFG.h"
31 #include "llvm/IR/Dominators.h"
32 #include "llvm/IR/Function.h"
33 #include "llvm/IR/IntrinsicInst.h"
34 #include "llvm/IR/Module.h"
35 #include "llvm/Support/CommandLine.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Transforms/Scalar.h"
38 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
39 #include "llvm/Transforms/Utils/Local.h"
40 #include "llvm/Transforms/Utils/ValueMapper.h"
41 using namespace llvm;
42 
43 // By default, we limit this to creating 16 PHIs (which is a little over half
44 // of the allocatable register set).
45 static cl::opt<bool>
46 PrefetchWrites("loop-prefetch-writes", cl::Hidden, cl::init(false),
47                cl::desc("Prefetch write addresses"));
48 
49 static cl::opt<unsigned>
50     PrefetchDistance("prefetch-distance",
51                      cl::desc("Number of instructions to prefetch ahead"),
52                      cl::Hidden);
53 
54 static cl::opt<unsigned>
55     MinPrefetchStride("min-prefetch-stride",
56                       cl::desc("Min stride to add prefetches"), cl::Hidden);
57 
58 static cl::opt<unsigned> MaxPrefetchIterationsAhead(
59     "max-prefetch-iters-ahead",
60     cl::desc("Max number of iterations to prefetch ahead"), cl::Hidden);
61 
62 STATISTIC(NumPrefetches, "Number of prefetches inserted");
63 
64 namespace {
65 
66 /// Loop prefetch implementation class.
67 class LoopDataPrefetch {
68 public:
69   LoopDataPrefetch(AssumptionCache *AC, LoopInfo *LI, ScalarEvolution *SE,
70                    const TargetTransformInfo *TTI,
71                    OptimizationRemarkEmitter *ORE)
72       : AC(AC), LI(LI), SE(SE), TTI(TTI), ORE(ORE) {}
73 
74   bool run();
75 
76 private:
77   bool runOnLoop(Loop *L);
78 
79   /// \brief Check if the the stride of the accesses is large enough to
80   /// warrant a prefetch.
81   bool isStrideLargeEnough(const SCEVAddRecExpr *AR);
82 
83   unsigned getMinPrefetchStride() {
84     if (MinPrefetchStride.getNumOccurrences() > 0)
85       return MinPrefetchStride;
86     return TTI->getMinPrefetchStride();
87   }
88 
89   unsigned getPrefetchDistance() {
90     if (PrefetchDistance.getNumOccurrences() > 0)
91       return PrefetchDistance;
92     return TTI->getPrefetchDistance();
93   }
94 
95   unsigned getMaxPrefetchIterationsAhead() {
96     if (MaxPrefetchIterationsAhead.getNumOccurrences() > 0)
97       return MaxPrefetchIterationsAhead;
98     return TTI->getMaxPrefetchIterationsAhead();
99   }
100 
101   AssumptionCache *AC;
102   LoopInfo *LI;
103   ScalarEvolution *SE;
104   const TargetTransformInfo *TTI;
105   OptimizationRemarkEmitter *ORE;
106 };
107 
108 /// Legacy class for inserting loop data prefetches.
109 class LoopDataPrefetchLegacyPass : public FunctionPass {
110 public:
111   static char ID; // Pass ID, replacement for typeid
112   LoopDataPrefetchLegacyPass() : FunctionPass(ID) {
113     initializeLoopDataPrefetchLegacyPassPass(*PassRegistry::getPassRegistry());
114   }
115 
116   void getAnalysisUsage(AnalysisUsage &AU) const override {
117     AU.addRequired<AssumptionCacheTracker>();
118     AU.addPreserved<DominatorTreeWrapperPass>();
119     AU.addRequired<LoopInfoWrapperPass>();
120     AU.addPreserved<LoopInfoWrapperPass>();
121     AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
122     AU.addRequired<ScalarEvolutionWrapperPass>();
123     AU.addPreserved<ScalarEvolutionWrapperPass>();
124     AU.addRequired<TargetTransformInfoWrapperPass>();
125   }
126 
127   bool runOnFunction(Function &F) override;
128   };
129 }
130 
131 char LoopDataPrefetchLegacyPass::ID = 0;
132 INITIALIZE_PASS_BEGIN(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
133                       "Loop Data Prefetch", false, false)
134 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
135 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
136 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
137 INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
138 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
139 INITIALIZE_PASS_END(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
140                     "Loop Data Prefetch", false, false)
141 
142 FunctionPass *llvm::createLoopDataPrefetchPass() {
143   return new LoopDataPrefetchLegacyPass();
144 }
145 
146 bool LoopDataPrefetch::isStrideLargeEnough(const SCEVAddRecExpr *AR) {
147   unsigned TargetMinStride = getMinPrefetchStride();
148   // No need to check if any stride goes.
149   if (TargetMinStride <= 1)
150     return true;
151 
152   const auto *ConstStride = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*SE));
153   // If MinStride is set, don't prefetch unless we can ensure that stride is
154   // larger.
155   if (!ConstStride)
156     return false;
157 
158   unsigned AbsStride = std::abs(ConstStride->getAPInt().getSExtValue());
159   return TargetMinStride <= AbsStride;
160 }
161 
162 PreservedAnalyses LoopDataPrefetchPass::run(Function &F,
163                                             FunctionAnalysisManager &AM) {
164   LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
165   ScalarEvolution *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
166   AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
167   OptimizationRemarkEmitter *ORE =
168       &AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
169   const TargetTransformInfo *TTI = &AM.getResult<TargetIRAnalysis>(F);
170 
171   LoopDataPrefetch LDP(AC, LI, SE, TTI, ORE);
172   bool Changed = LDP.run();
173 
174   if (Changed) {
175     PreservedAnalyses PA;
176     PA.preserve<DominatorTreeAnalysis>();
177     PA.preserve<LoopAnalysis>();
178     return PA;
179   }
180 
181   return PreservedAnalyses::all();
182 }
183 
184 bool LoopDataPrefetchLegacyPass::runOnFunction(Function &F) {
185   if (skipFunction(F))
186     return false;
187 
188   LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
189   ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
190   AssumptionCache *AC =
191       &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
192   OptimizationRemarkEmitter *ORE =
193       &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
194   const TargetTransformInfo *TTI =
195       &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
196 
197   LoopDataPrefetch LDP(AC, LI, SE, TTI, ORE);
198   return LDP.run();
199 }
200 
201 bool LoopDataPrefetch::run() {
202   // If PrefetchDistance is not set, don't run the pass.  This gives an
203   // opportunity for targets to run this pass for selected subtargets only
204   // (whose TTI sets PrefetchDistance).
205   if (getPrefetchDistance() == 0)
206     return false;
207   assert(TTI->getCacheLineSize() && "Cache line size is not set for target");
208 
209   bool MadeChange = false;
210 
211   for (Loop *I : *LI)
212     for (auto L = df_begin(I), LE = df_end(I); L != LE; ++L)
213       MadeChange |= runOnLoop(*L);
214 
215   return MadeChange;
216 }
217 
218 bool LoopDataPrefetch::runOnLoop(Loop *L) {
219   bool MadeChange = false;
220 
221   // Only prefetch in the inner-most loop
222   if (!L->empty())
223     return MadeChange;
224 
225   SmallPtrSet<const Value *, 32> EphValues;
226   CodeMetrics::collectEphemeralValues(L, AC, EphValues);
227 
228   // Calculate the number of iterations ahead to prefetch
229   CodeMetrics Metrics;
230   for (const auto BB : L->blocks()) {
231     // If the loop already has prefetches, then assume that the user knows
232     // what they are doing and don't add any more.
233     for (auto &I : *BB)
234       if (CallInst *CI = dyn_cast<CallInst>(&I))
235         if (Function *F = CI->getCalledFunction())
236           if (F->getIntrinsicID() == Intrinsic::prefetch)
237             return MadeChange;
238 
239     Metrics.analyzeBasicBlock(BB, *TTI, EphValues);
240   }
241   unsigned LoopSize = Metrics.NumInsts;
242   if (!LoopSize)
243     LoopSize = 1;
244 
245   unsigned ItersAhead = getPrefetchDistance() / LoopSize;
246   if (!ItersAhead)
247     ItersAhead = 1;
248 
249   if (ItersAhead > getMaxPrefetchIterationsAhead())
250     return MadeChange;
251 
252   DEBUG(dbgs() << "Prefetching " << ItersAhead
253                << " iterations ahead (loop size: " << LoopSize << ") in "
254                << L->getHeader()->getParent()->getName() << ": " << *L);
255 
256   SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
257   for (const auto BB : L->blocks()) {
258     for (auto &I : *BB) {
259       Value *PtrValue;
260       Instruction *MemI;
261 
262       if (LoadInst *LMemI = dyn_cast<LoadInst>(&I)) {
263         MemI = LMemI;
264         PtrValue = LMemI->getPointerOperand();
265       } else if (StoreInst *SMemI = dyn_cast<StoreInst>(&I)) {
266         if (!PrefetchWrites) continue;
267         MemI = SMemI;
268         PtrValue = SMemI->getPointerOperand();
269       } else continue;
270 
271       unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
272       if (PtrAddrSpace)
273         continue;
274 
275       if (L->isLoopInvariant(PtrValue))
276         continue;
277 
278       const SCEV *LSCEV = SE->getSCEV(PtrValue);
279       const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
280       if (!LSCEVAddRec)
281         continue;
282 
283       // Check if the the stride of the accesses is large enough to warrant a
284       // prefetch.
285       if (!isStrideLargeEnough(LSCEVAddRec))
286         continue;
287 
288       // We don't want to double prefetch individual cache lines. If this load
289       // is known to be within one cache line of some other load that has
290       // already been prefetched, then don't prefetch this one as well.
291       bool DupPref = false;
292       for (const auto &PrefLoad : PrefLoads) {
293         const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, PrefLoad.second);
294         if (const SCEVConstant *ConstPtrDiff =
295             dyn_cast<SCEVConstant>(PtrDiff)) {
296           int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
297           if (PD < (int64_t) TTI->getCacheLineSize()) {
298             DupPref = true;
299             break;
300           }
301         }
302       }
303       if (DupPref)
304         continue;
305 
306       const SCEV *NextLSCEV = SE->getAddExpr(LSCEVAddRec, SE->getMulExpr(
307         SE->getConstant(LSCEVAddRec->getType(), ItersAhead),
308         LSCEVAddRec->getStepRecurrence(*SE)));
309       if (!isSafeToExpand(NextLSCEV, *SE))
310         continue;
311 
312       PrefLoads.push_back(std::make_pair(MemI, LSCEVAddRec));
313 
314       Type *I8Ptr = Type::getInt8PtrTy(BB->getContext(), PtrAddrSpace);
315       SCEVExpander SCEVE(*SE, I.getModule()->getDataLayout(), "prefaddr");
316       Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
317 
318       IRBuilder<> Builder(MemI);
319       Module *M = BB->getParent()->getParent();
320       Type *I32 = Type::getInt32Ty(BB->getContext());
321       Value *PrefetchFunc = Intrinsic::getDeclaration(M, Intrinsic::prefetch);
322       Builder.CreateCall(
323           PrefetchFunc,
324           {PrefPtrValue,
325            ConstantInt::get(I32, MemI->mayReadFromMemory() ? 0 : 1),
326            ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
327       ++NumPrefetches;
328       DEBUG(dbgs() << "  Access: " << *PtrValue << ", SCEV: " << *LSCEV
329                    << "\n");
330       ORE->emit([&]() {
331         return OptimizationRemark(DEBUG_TYPE, "Prefetched", MemI)
332                << "prefetched memory access";
333       });
334 
335       MadeChange = true;
336     }
337   }
338 
339   return MadeChange;
340 }
341 
342