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/CFG.h"
  26 #include "llvm/IR/Dominators.h"
  27 #include "llvm/IR/Function.h"
  28 #include "llvm/IR/Module.h"
  29 #include "llvm/Support/CommandLine.h"
  30 #include "llvm/Support/Debug.h"
  31 #include "llvm/Transforms/Scalar.h"
  32 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  33 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
  34 #include "llvm/Transforms/Utils/ValueMapper.h"
  35
  36 #define DEBUG_TYPE "loop-data-prefetch"
  37
  38 using namespace llvm;
  39
  40 // By default, we limit this to creating 16 PHIs (which is a little over half
  41 // of the allocatable register set).
  42 static cl::opt<bool>
  43 PrefetchWrites("loop-prefetch-writes", cl::Hidden, cl::init(false),
  44                cl::desc("Prefetch write addresses"));
  45
  46 static cl::opt<unsigned>
  47     PrefetchDistance("prefetch-distance",
  48                      cl::desc("Number of instructions to prefetch ahead"),
  49                      cl::Hidden);
  50
  51 static cl::opt<unsigned>
  52     MinPrefetchStride("min-prefetch-stride",
  53                       cl::desc("Min stride to add prefetches"), cl::Hidden);
  54
  55 static cl::opt<unsigned> MaxPrefetchIterationsAhead(
  56     "max-prefetch-iters-ahead",
  57     cl::desc("Max number of iterations to prefetch ahead"), cl::Hidden);
  58
  59 STATISTIC(NumPrefetches, "Number of prefetches inserted");
  60
  61 namespace {
  62
  63 /// Loop prefetch implementation class.
  64 class LoopDataPrefetch {
  65 public:
  66   LoopDataPrefetch(AssumptionCache *AC, DominatorTree *DT, LoopInfo *LI,
  67                    ScalarEvolution *SE, const TargetTransformInfo *TTI,
  68                    OptimizationRemarkEmitter *ORE)
  69       : AC(AC), DT(DT), LI(LI), SE(SE), TTI(TTI), ORE(ORE) {}
  70
  71   bool run();
  72
  73 private:
  74   bool runOnLoop(Loop *L);
  75
  76   /// Check if the stride of the accesses is large enough to
  77   /// warrant a prefetch.
  78   bool isStrideLargeEnough(const SCEVAddRecExpr *AR, unsigned TargetMinStride);
  79
  80   unsigned getMinPrefetchStride(unsigned NumMemAccesses,
  81                                 unsigned NumStridedMemAccesses,
  82                                 unsigned NumPrefetches,
  83                                 bool HasCall) {
  84     if (MinPrefetchStride.getNumOccurrences() > 0)
  85       return MinPrefetchStride;
  86     return TTI->getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
  87                                      NumPrefetches, HasCall);
  88   }
  89
  90   unsigned getPrefetchDistance() {
  91     if (PrefetchDistance.getNumOccurrences() > 0)
  92       return PrefetchDistance;
  93     return TTI->getPrefetchDistance();
  94   }
  95
  96   unsigned getMaxPrefetchIterationsAhead() {
  97     if (MaxPrefetchIterationsAhead.getNumOccurrences() > 0)
  98       return MaxPrefetchIterationsAhead;
  99     return TTI->getMaxPrefetchIterationsAhead();
 100   }
 101
 102   bool doPrefetchWrites() {
 103     if (PrefetchWrites.getNumOccurrences() > 0)
 104       return PrefetchWrites;
 105     return TTI->enableWritePrefetching();
 106   }
 107
 108   AssumptionCache *AC;
 109   DominatorTree *DT;
 110   LoopInfo *LI;
 111   ScalarEvolution *SE;
 112   const TargetTransformInfo *TTI;
 113   OptimizationRemarkEmitter *ORE;
 114 };
 115
 116 /// Legacy class for inserting loop data prefetches.
 117 class LoopDataPrefetchLegacyPass : public FunctionPass {
 118 public:
 119   static char ID; // Pass ID, replacement for typeid
 120   LoopDataPrefetchLegacyPass() : FunctionPass(ID) {
 121     initializeLoopDataPrefetchLegacyPassPass(*PassRegistry::getPassRegistry());
 122   }
 123
 124   void getAnalysisUsage(AnalysisUsage &AU) const override {
 125     AU.addRequired<AssumptionCacheTracker>();
 126     AU.addRequired<DominatorTreeWrapperPass>();
 127     AU.addPreserved<DominatorTreeWrapperPass>();
 128     AU.addRequired<LoopInfoWrapperPass>();
 129     AU.addPreserved<LoopInfoWrapperPass>();
 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)
 143 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 144 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 145 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
 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).
 216   if (getPrefetchDistance() == 0)
 217     return false;
 218   assert(TTI->getCacheLineSize() && "Cache line size is not set for target");
 219
 220   bool MadeChange = false;
 221
 222   for (Loop *I : *LI)
 223     for (auto L = df_begin(I), LE = df_end(I); L != LE; ++L)
 224       MadeChange |= runOnLoop(*L);
 225
 226   return MadeChange;
 227 }
 228
 229 /// A record for a potential prefetch made during the initial scan of the
 230 /// loop. This is used to let a single prefetch target multiple memory accesses.
 231 struct Prefetch {
 232   /// The address formula for this prefetch as returned by ScalarEvolution.
 233   const SCEVAddRecExpr *LSCEVAddRec;
 234   /// The point of insertion for the prefetch instruction.
 235   Instruction *InsertPt;
 236   /// True if targeting a write memory access.
 237   bool Writes;
 238   /// The (first seen) prefetched instruction.
 239   Instruction *MemI;
 240
 241   /// Constructor to create a new Prefetch for \p I.
 242   Prefetch(const SCEVAddRecExpr *L, Instruction *I)
 243       : LSCEVAddRec(L), InsertPt(nullptr), Writes(false), MemI(nullptr) {
 244     addInstruction(I);
 245   };
 246
 247   /// Add the instruction \param I to this prefetch. If it's not the first
 248   /// one, 'InsertPt' and 'Writes' will be updated as required.
 249   /// \param PtrDiff the known constant address difference to the first added
 250   /// instruction.
 251   void addInstruction(Instruction *I, DominatorTree *DT = nullptr,
 252                       int64_t PtrDiff = 0) {
 253     if (!InsertPt) {
 254       MemI = I;
 255       InsertPt = I;
 256       Writes = isa<StoreInst>(I);
 257     } else {
 258       BasicBlock *PrefBB = InsertPt->getParent();
 259       BasicBlock *InsBB = I->getParent();
 260       if (PrefBB != InsBB) {
 261         BasicBlock *DomBB = DT->findNearestCommonDominator(PrefBB, InsBB);
 262         if (DomBB != PrefBB)
 263           InsertPt = DomBB->getTerminator();
 264       }
 265
 266       if (isa<StoreInst>(I) && PtrDiff == 0)
 267         Writes = true;
 268     }
 269   }
 270 };
 271
 272 bool LoopDataPrefetch::runOnLoop(Loop *L) {
 273   bool MadeChange = false;
 274
 275   // Only prefetch in the inner-most loop
 276   if (!L->isInnermost())
 277     return MadeChange;
 278
 279   SmallPtrSet<const Value *, 32> EphValues;
 280   CodeMetrics::collectEphemeralValues(L, AC, EphValues);
 281
 282   // Calculate the number of iterations ahead to prefetch
 283   CodeMetrics Metrics;
 284   bool HasCall = false;
 285   for (const auto BB : L->blocks()) {
 286     // If the loop already has prefetches, then assume that the user knows
 287     // what they are doing and don't add any more.
 288     for (auto &I : *BB) {
 289       if (isa<CallInst>(&I) || isa<InvokeInst>(&I)) {
 290         if (const Function *F = cast<CallBase>(I).getCalledFunction()) {
 291           if (F->getIntrinsicID() == Intrinsic::prefetch)
 292             return MadeChange;
 293           if (TTI->isLoweredToCall(F))
 294             HasCall = true;
 295         } else { // indirect call.
 296           HasCall = true;
 297         }
 298       }
 299     }
 300     Metrics.analyzeBasicBlock(BB, *TTI, EphValues);
 301   }
 302   unsigned LoopSize = Metrics.NumInsts;
 303   if (!LoopSize)
 304     LoopSize = 1;
 305
 306   unsigned ItersAhead = getPrefetchDistance() / LoopSize;
 307   if (!ItersAhead)
 308     ItersAhead = 1;
 309
 310   if (ItersAhead > getMaxPrefetchIterationsAhead())
 311     return MadeChange;
 312
 313   unsigned ConstantMaxTripCount = SE->getSmallConstantMaxTripCount(L);
 314   if (ConstantMaxTripCount && ConstantMaxTripCount < ItersAhead + 1)
 315     return MadeChange;
 316
 317   unsigned NumMemAccesses = 0;
 318   unsigned NumStridedMemAccesses = 0;
 319   SmallVector<Prefetch, 16> Prefetches;
 320   for (const auto BB : L->blocks())
 321     for (auto &I : *BB) {
 322       Value *PtrValue;
 323       Instruction *MemI;
 324
 325       if (LoadInst *LMemI = dyn_cast<LoadInst>(&I)) {
 326         MemI = LMemI;
 327         PtrValue = LMemI->getPointerOperand();
 328       } else if (StoreInst *SMemI = dyn_cast<StoreInst>(&I)) {
 329         if (!doPrefetchWrites()) continue;
 330         MemI = SMemI;
 331         PtrValue = SMemI->getPointerOperand();
 332       } else continue;
 333
 334       unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
 335       if (PtrAddrSpace)
 336         continue;
 337       NumMemAccesses++;
 338       if (L->isLoopInvariant(PtrValue))
 339         continue;
 340
 341       const SCEV *LSCEV = SE->getSCEV(PtrValue);
 342       const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
 343       if (!LSCEVAddRec)
 344         continue;
 345       NumStridedMemAccesses++;
 346
 347       // We don't want to double prefetch individual cache lines. If this
 348       // access is known to be within one cache line of some other one that
 349       // has already been prefetched, then don't prefetch this one as well.
 350       bool DupPref = false;
 351       for (auto &Pref : Prefetches) {
 352         const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, Pref.LSCEVAddRec);
 353         if (const SCEVConstant *ConstPtrDiff =
 354             dyn_cast<SCEVConstant>(PtrDiff)) {
 355           int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
 356           if (PD < (int64_t) TTI->getCacheLineSize()) {
 357             Pref.addInstruction(MemI, DT, PD);
 358             DupPref = true;
 359             break;
 360           }
 361         }
 362       }
 363       if (!DupPref)
 364         Prefetches.push_back(Prefetch(LSCEVAddRec, MemI));
 365     }
 366
 367   unsigned TargetMinStride =
 368     getMinPrefetchStride(NumMemAccesses, NumStridedMemAccesses,
 369                          Prefetches.size(), HasCall);
 370
 371   LLVM_DEBUG(dbgs() << "Prefetching " << ItersAhead
 372              << " iterations ahead (loop size: " << LoopSize << ") in "
 373              << L->getHeader()->getParent()->getName() << ": " << *L);
 374   LLVM_DEBUG(dbgs() << "Loop has: "
 375              << NumMemAccesses << " memory accesses, "
 376              << NumStridedMemAccesses << " strided memory accesses, "
 377              << Prefetches.size() << " potential prefetch(es), "
 378              << "a minimum stride of " << TargetMinStride << ", "
 379              << (HasCall ? "calls" : "no calls") << ".\n");
 380
 381   for (auto &P : Prefetches) {
 382     // Check if the stride of the accesses is large enough to warrant a
 383     // prefetch.
 384     if (!isStrideLargeEnough(P.LSCEVAddRec, TargetMinStride))
 385       continue;
 386
 387     const SCEV *NextLSCEV = SE->getAddExpr(P.LSCEVAddRec, SE->getMulExpr(
 388       SE->getConstant(P.LSCEVAddRec->getType(), ItersAhead),
 389       P.LSCEVAddRec->getStepRecurrence(*SE)));
 390     if (!isSafeToExpand(NextLSCEV, *SE))
 391       continue;
 392
 393     BasicBlock *BB = P.InsertPt->getParent();
 394     Type *I8Ptr = Type::getInt8PtrTy(BB->getContext(), 0/*PtrAddrSpace*/);
 395     SCEVExpander SCEVE(*SE, BB->getModule()->getDataLayout(), "prefaddr");
 396     Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, P.InsertPt);
 397
 398     IRBuilder<> Builder(P.InsertPt);
 399     Module *M = BB->getParent()->getParent();
 400     Type *I32 = Type::getInt32Ty(BB->getContext());
 401     Function *PrefetchFunc = Intrinsic::getDeclaration(
 402         M, Intrinsic::prefetch, PrefPtrValue->getType());
 403     Builder.CreateCall(
 404         PrefetchFunc,
 405         {PrefPtrValue,
 406          ConstantInt::get(I32, P.Writes),
 407          ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
 408     ++NumPrefetches;
 409     LLVM_DEBUG(dbgs() << "  Access: "
 410                << *P.MemI->getOperand(isa<LoadInst>(P.MemI) ? 0 : 1)
 411                << ", SCEV: " << *P.LSCEVAddRec << "\n");
 412     ORE->emit([&]() {
 413         return OptimizationRemark(DEBUG_TYPE, "Prefetched", P.MemI)
 414           << "prefetched memory access";
 415       });
 416
 417     MadeChange = true;
 418   }
 419
 420   return MadeChange;
 421 }