1//===-------- PPCLoopDataPrefetch.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 "ppc-loop-data-prefetch"
15#include "PPC.h"
16#include "llvm/Transforms/Scalar.h"
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/ScalarEvolution.h"
24#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
25#include "llvm/Analysis/ScalarEvolutionExpander.h"
26#include "llvm/Analysis/ScalarEvolutionExpressions.h"
27#include "llvm/Analysis/TargetTransformInfo.h"
28#include "llvm/Analysis/ValueTracking.h"
29#include "llvm/IR/CFG.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"
39using 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).
43static cl::opt<bool>
44PrefetchWrites("ppc-loop-prefetch-writes", cl::Hidden, cl::init(false),
45               cl::desc("Prefetch write addresses"));
46
47// This seems like a reasonable default for the BG/Q (this pass is enabled, by
48// default, only on the BG/Q).
49static cl::opt<unsigned>
50PrefDist("ppc-loop-prefetch-distance", cl::Hidden, cl::init(300),
51         cl::desc("The loop prefetch distance"));
52
53static cl::opt<unsigned>
54CacheLineSize("ppc-loop-prefetch-cache-line", cl::Hidden, cl::init(64),
55              cl::desc("The loop prefetch cache line size"));
56
57namespace llvm {
58  void initializePPCLoopDataPrefetchPass(PassRegistry&);
59}
60
61namespace {
62
63  class PPCLoopDataPrefetch : public FunctionPass {
64  public:
65    static char ID; // Pass ID, replacement for typeid
66    PPCLoopDataPrefetch() : FunctionPass(ID) {
67      initializePPCLoopDataPrefetchPass(*PassRegistry::getPassRegistry());
68    }
69
70    void getAnalysisUsage(AnalysisUsage &AU) const override {
71      AU.addRequired<AssumptionCacheTracker>();
72      AU.addPreserved<DominatorTreeWrapperPass>();
73      AU.addRequired<LoopInfoWrapperPass>();
74      AU.addPreserved<LoopInfoWrapperPass>();
75      AU.addRequired<ScalarEvolutionWrapperPass>();
76      // FIXME: For some reason, preserving SE here breaks LSR (even if
77      // this pass changes nothing).
78      // AU.addPreserved<ScalarEvolutionWrapperPass>();
79      AU.addRequired<TargetTransformInfoWrapperPass>();
80    }
81
82    bool runOnFunction(Function &F) override;
83    bool runOnLoop(Loop *L);
84
85  private:
86    AssumptionCache *AC;
87    LoopInfo *LI;
88    ScalarEvolution *SE;
89    const TargetTransformInfo *TTI;
90    const DataLayout *DL;
91  };
92}
93
94char PPCLoopDataPrefetch::ID = 0;
95INITIALIZE_PASS_BEGIN(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
96                      "PPC Loop Data Prefetch", false, false)
97INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
98INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
99INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
100INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
101INITIALIZE_PASS_END(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
102                    "PPC Loop Data Prefetch", false, false)
103
104FunctionPass *llvm::createPPCLoopDataPrefetchPass() { return new PPCLoopDataPrefetch(); }
105
106bool PPCLoopDataPrefetch::runOnFunction(Function &F) {
107  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
108  SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
109  DL = &F.getParent()->getDataLayout();
110  AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
111  TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
112
113  bool MadeChange = false;
114
115  for (auto I = LI->begin(), IE = LI->end(); I != IE; ++I)
116    for (auto L = df_begin(*I), LE = df_end(*I); L != LE; ++L)
117      MadeChange |= runOnLoop(*L);
118
119  return MadeChange;
120}
121
122bool PPCLoopDataPrefetch::runOnLoop(Loop *L) {
123  bool MadeChange = false;
124
125  // Only prefetch in the inner-most loop
126  if (!L->empty())
127    return MadeChange;
128
129  SmallPtrSet<const Value *, 32> EphValues;
130  CodeMetrics::collectEphemeralValues(L, AC, EphValues);
131
132  // Calculate the number of iterations ahead to prefetch
133  CodeMetrics Metrics;
134  for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
135       I != IE; ++I) {
136
137    // If the loop already has prefetches, then assume that the user knows
138    // what he or she is doing and don't add any more.
139    for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
140         J != JE; ++J)
141      if (CallInst *CI = dyn_cast<CallInst>(J))
142        if (Function *F = CI->getCalledFunction())
143          if (F->getIntrinsicID() == Intrinsic::prefetch)
144            return MadeChange;
145
146    Metrics.analyzeBasicBlock(*I, *TTI, EphValues);
147  }
148  unsigned LoopSize = Metrics.NumInsts;
149  if (!LoopSize)
150    LoopSize = 1;
151
152  unsigned ItersAhead = PrefDist/LoopSize;
153  if (!ItersAhead)
154    ItersAhead = 1;
155
156  SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
157  for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
158       I != IE; ++I) {
159    for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
160        J != JE; ++J) {
161      Value *PtrValue;
162      Instruction *MemI;
163
164      if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
165        MemI = LMemI;
166        PtrValue = LMemI->getPointerOperand();
167      } else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
168        if (!PrefetchWrites) continue;
169        MemI = SMemI;
170        PtrValue = SMemI->getPointerOperand();
171      } else continue;
172
173      unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
174      if (PtrAddrSpace)
175        continue;
176
177      if (L->isLoopInvariant(PtrValue))
178        continue;
179
180      const SCEV *LSCEV = SE->getSCEV(PtrValue);
181      const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
182      if (!LSCEVAddRec)
183        continue;
184
185      // We don't want to double prefetch individual cache lines. If this load
186      // is known to be within one cache line of some other load that has
187      // already been prefetched, then don't prefetch this one as well.
188      bool DupPref = false;
189      for (SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>,
190             16>::iterator K = PrefLoads.begin(), KE = PrefLoads.end();
191           K != KE; ++K) {
192        const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, K->second);
193        if (const SCEVConstant *ConstPtrDiff =
194            dyn_cast<SCEVConstant>(PtrDiff)) {
195          int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
196          if (PD < (int64_t) CacheLineSize) {
197            DupPref = true;
198            break;
199          }
200        }
201      }
202      if (DupPref)
203        continue;
204
205      const SCEV *NextLSCEV = SE->getAddExpr(LSCEVAddRec, SE->getMulExpr(
206        SE->getConstant(LSCEVAddRec->getType(), ItersAhead),
207        LSCEVAddRec->getStepRecurrence(*SE)));
208      if (!isSafeToExpand(NextLSCEV, *SE))
209        continue;
210
211      PrefLoads.push_back(std::make_pair(MemI, LSCEVAddRec));
212
213      Type *I8Ptr = Type::getInt8PtrTy((*I)->getContext(), PtrAddrSpace);
214      SCEVExpander SCEVE(*SE, J->getModule()->getDataLayout(), "prefaddr");
215      Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
216
217      IRBuilder<> Builder(MemI);
218      Module *M = (*I)->getParent()->getParent();
219      Type *I32 = Type::getInt32Ty((*I)->getContext());
220      Value *PrefetchFunc = Intrinsic::getDeclaration(M, Intrinsic::prefetch);
221      Builder.CreateCall(
222          PrefetchFunc,
223          {PrefPtrValue,
224           ConstantInt::get(I32, MemI->mayReadFromMemory() ? 0 : 1),
225           ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
226
227      MadeChange = true;
228    }
229  }
230
231  return MadeChange;
232}
233
234