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