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