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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"
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("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).
49 static cl::opt<unsigned>
50 PrefDist("ppc-loop-prefetch-distance", cl::Hidden, cl::init(300),
51          cl::desc("The loop prefetch distance"));
52 
53 static cl::opt<unsigned>
54 CacheLineSize("ppc-loop-prefetch-cache-line", cl::Hidden, cl::init(64),
55               cl::desc("The loop prefetch cache line size"));
56 
57 namespace llvm {
58   void initializePPCLoopDataPrefetchPass(PassRegistry&);
59 }
60 
61 namespace {
62 
63   class PPCLoopDataPrefetch : public FunctionPass {
64   public:
65     static char ID; // Pass ID, replacement for typeid
PPCLoopDataPrefetch()66     PPCLoopDataPrefetch() : FunctionPass(ID) {
67       initializePPCLoopDataPrefetchPass(*PassRegistry::getPassRegistry());
68     }
69 
getAnalysisUsage(AnalysisUsage & AU) const70     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 
94 char PPCLoopDataPrefetch::ID = 0;
95 INITIALIZE_PASS_BEGIN(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
96                       "PPC Loop Data Prefetch", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)97 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
98 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
99 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
100 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
101 INITIALIZE_PASS_END(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
102                     "PPC Loop Data Prefetch", false, false)
103 
104 FunctionPass *llvm::createPPCLoopDataPrefetchPass() { return new PPCLoopDataPrefetch(); }
105 
runOnFunction(Function & F)106 bool 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 
runOnLoop(Loop * L)122 bool 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