xref: /llvm-project/llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp (revision 1eca6bc6a7f0c29bb155cfb42775738cdd582e98) 
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/OptimizationDiagnosticInfo.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     // FIXME: For some reason, preserving SE here breaks LSR (even if
124     // this pass changes nothing).
125     // AU.addPreserved<ScalarEvolutionWrapperPass>();
126     AU.addRequired<TargetTransformInfoWrapperPass>();
127   }
128 
129   bool runOnFunction(Function &F) override;
130   };
131 }
132 
133 char LoopDataPrefetchLegacyPass::ID = 0;
134 INITIALIZE_PASS_BEGIN(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
135                       "Loop Data Prefetch", false, false)
136 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
137 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
138 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
139 INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
140 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
141 INITIALIZE_PASS_END(LoopDataPrefetchLegacyPass, "loop-data-prefetch",
142                     "Loop Data Prefetch", false, false)
143 
144 FunctionPass *llvm::createLoopDataPrefetchPass() {
145   return new LoopDataPrefetchLegacyPass();
146 }
147 
148 bool LoopDataPrefetch::isStrideLargeEnough(const SCEVAddRecExpr *AR) {
149   unsigned TargetMinStride = getMinPrefetchStride();
150   // No need to check if any stride goes.
151   if (TargetMinStride <= 1)
152     return true;
153 
154   const auto *ConstStride = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*SE));
155   // If MinStride is set, don't prefetch unless we can ensure that stride is
156   // larger.
157   if (!ConstStride)
158     return false;
159 
160   unsigned AbsStride = std::abs(ConstStride->getAPInt().getSExtValue());
161   return TargetMinStride <= AbsStride;
162 }
163 
164 PreservedAnalyses LoopDataPrefetchPass::run(Function &F,
165                                             FunctionAnalysisManager &AM) {
166   LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
167   ScalarEvolution *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
168   AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
169   OptimizationRemarkEmitter *ORE =
170       &AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
171   const TargetTransformInfo *TTI = &AM.getResult<TargetIRAnalysis>(F);
172 
173   LoopDataPrefetch LDP(AC, LI, SE, TTI, ORE);
174   bool Changed = LDP.run();
175 
176   if (Changed) {
177     PreservedAnalyses PA;
178     PA.preserve<DominatorTreeAnalysis>();
179     PA.preserve<LoopAnalysis>();
180     return PA;
181   }
182 
183   return PreservedAnalyses::all();
184 }
185 
186 bool LoopDataPrefetchLegacyPass::runOnFunction(Function &F) {
187   if (skipFunction(F))
188     return false;
189 
190   LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
191   ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
192   AssumptionCache *AC =
193       &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
194   OptimizationRemarkEmitter *ORE =
195       &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
196   const TargetTransformInfo *TTI =
197       &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
198 
199   LoopDataPrefetch LDP(AC, LI, SE, TTI, ORE);
200   return LDP.run();
201 }
202 
203 bool LoopDataPrefetch::run() {
204   // If PrefetchDistance is not set, don't run the pass.  This gives an
205   // opportunity for targets to run this pass for selected subtargets only
206   // (whose TTI sets PrefetchDistance).
207   if (getPrefetchDistance() == 0)
208     return false;
209   assert(TTI->getCacheLineSize() && "Cache line size is not set for target");
210 
211   bool MadeChange = false;
212 
213   for (Loop *I : *LI)
214     for (auto L = df_begin(I), LE = df_end(I); L != LE; ++L)
215       MadeChange |= runOnLoop(*L);
216 
217   return MadeChange;
218 }
219 
220 bool LoopDataPrefetch::runOnLoop(Loop *L) {
221   bool MadeChange = false;
222 
223   // Only prefetch in the inner-most loop
224   if (!L->empty())
225     return MadeChange;
226 
227   SmallPtrSet<const Value *, 32> EphValues;
228   CodeMetrics::collectEphemeralValues(L, AC, EphValues);
229 
230   // Calculate the number of iterations ahead to prefetch
231   CodeMetrics Metrics;
232   for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
233        I != IE; ++I) {
234 
235     // If the loop already has prefetches, then assume that the user knows
236     // what they are doing and don't add any more.
237     for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
238          J != JE; ++J)
239       if (CallInst *CI = dyn_cast<CallInst>(J))
240         if (Function *F = CI->getCalledFunction())
241           if (F->getIntrinsicID() == Intrinsic::prefetch)
242             return MadeChange;
243 
244     Metrics.analyzeBasicBlock(*I, *TTI, EphValues);
245   }
246   unsigned LoopSize = Metrics.NumInsts;
247   if (!LoopSize)
248     LoopSize = 1;
249 
250   unsigned ItersAhead = getPrefetchDistance() / LoopSize;
251   if (!ItersAhead)
252     ItersAhead = 1;
253 
254   if (ItersAhead > getMaxPrefetchIterationsAhead())
255     return MadeChange;
256 
257   DEBUG(dbgs() << "Prefetching " << ItersAhead
258                << " iterations ahead (loop size: " << LoopSize << ") in "
259                << L->getHeader()->getParent()->getName() << ": " << *L);
260 
261   SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
262   for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
263        I != IE; ++I) {
264     for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
265         J != JE; ++J) {
266       Value *PtrValue;
267       Instruction *MemI;
268 
269       if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
270         MemI = LMemI;
271         PtrValue = LMemI->getPointerOperand();
272       } else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
273         if (!PrefetchWrites) continue;
274         MemI = SMemI;
275         PtrValue = SMemI->getPointerOperand();
276       } else continue;
277 
278       unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
279       if (PtrAddrSpace)
280         continue;
281 
282       if (L->isLoopInvariant(PtrValue))
283         continue;
284 
285       const SCEV *LSCEV = SE->getSCEV(PtrValue);
286       const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
287       if (!LSCEVAddRec)
288         continue;
289 
290       // Check if the the stride of the accesses is large enough to warrant a
291       // prefetch.
292       if (!isStrideLargeEnough(LSCEVAddRec))
293         continue;
294 
295       // We don't want to double prefetch individual cache lines. If this load
296       // is known to be within one cache line of some other load that has
297       // already been prefetched, then don't prefetch this one as well.
298       bool DupPref = false;
299       for (const auto &PrefLoad : PrefLoads) {
300         const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, PrefLoad.second);
301         if (const SCEVConstant *ConstPtrDiff =
302             dyn_cast<SCEVConstant>(PtrDiff)) {
303           int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
304           if (PD < (int64_t) TTI->getCacheLineSize()) {
305             DupPref = true;
306             break;
307           }
308         }
309       }
310       if (DupPref)
311         continue;
312 
313       const SCEV *NextLSCEV = SE->getAddExpr(LSCEVAddRec, SE->getMulExpr(
314         SE->getConstant(LSCEVAddRec->getType(), ItersAhead),
315         LSCEVAddRec->getStepRecurrence(*SE)));
316       if (!isSafeToExpand(NextLSCEV, *SE))
317         continue;
318 
319       PrefLoads.push_back(std::make_pair(MemI, LSCEVAddRec));
320 
321       Type *I8Ptr = Type::getInt8PtrTy((*I)->getContext(), PtrAddrSpace);
322       SCEVExpander SCEVE(*SE, J->getModule()->getDataLayout(), "prefaddr");
323       Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
324 
325       IRBuilder<> Builder(MemI);
326       Module *M = (*I)->getParent()->getParent();
327       Type *I32 = Type::getInt32Ty((*I)->getContext());
328       Value *PrefetchFunc = Intrinsic::getDeclaration(M, Intrinsic::prefetch);
329       Builder.CreateCall(
330           PrefetchFunc,
331           {PrefPtrValue,
332            ConstantInt::get(I32, MemI->mayReadFromMemory() ? 0 : 1),
333            ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
334       ++NumPrefetches;
335       DEBUG(dbgs() << "  Access: " << *PtrValue << ", SCEV: " << *LSCEV
336                    << "\n");
337       ORE->emitOptimizationRemark(DEBUG_TYPE, MemI, "prefetched memory access");
338 
339       MadeChange = true;
340     }
341   }
342 
343   return MadeChange;
344 }
345 
346