llvm/lib/Transforms/Scalar/LoopDataPrefetch.cpp

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