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