1 //===- LoadCombine.cpp - Combine Adjacent Loads ---------------------------===//
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 /// \file
10 /// This transformation combines adjacent loads.
11 ///
12 //===----------------------------------------------------------------------===//
13
14 #include "llvm/Transforms/Scalar.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/Analysis/AliasAnalysis.h"
18 #include "llvm/Analysis/AliasSetTracker.h"
19 #include "llvm/Analysis/GlobalsModRef.h"
20 #include "llvm/Analysis/TargetFolder.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/Function.h"
23 #include "llvm/IR/IRBuilder.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/Module.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/MathExtras.h"
29 #include "llvm/Support/raw_ostream.h"
30
31 using namespace llvm;
32
33 #define DEBUG_TYPE "load-combine"
34
35 STATISTIC(NumLoadsAnalyzed, "Number of loads analyzed for combining");
36 STATISTIC(NumLoadsCombined, "Number of loads combined");
37
38 namespace {
39 struct PointerOffsetPair {
40 Value *Pointer;
41 uint64_t Offset;
42 };
43
44 struct LoadPOPPair {
45 LoadPOPPair() = default;
LoadPOPPair__anon7821fa570111::LoadPOPPair46 LoadPOPPair(LoadInst *L, PointerOffsetPair P, unsigned O)
47 : Load(L), POP(P), InsertOrder(O) {}
48 LoadInst *Load;
49 PointerOffsetPair POP;
50 /// \brief The new load needs to be created before the first load in IR order.
51 unsigned InsertOrder;
52 };
53
54 class LoadCombine : public BasicBlockPass {
55 LLVMContext *C;
56 AliasAnalysis *AA;
57
58 public:
LoadCombine()59 LoadCombine() : BasicBlockPass(ID), C(nullptr), AA(nullptr) {
60 initializeLoadCombinePass(*PassRegistry::getPassRegistry());
61 }
62
63 using llvm::Pass::doInitialization;
64 bool doInitialization(Function &) override;
65 bool runOnBasicBlock(BasicBlock &BB) override;
66 void getAnalysisUsage(AnalysisUsage &AU) const override;
67
getPassName() const68 const char *getPassName() const override { return "LoadCombine"; }
69 static char ID;
70
71 typedef IRBuilder<true, TargetFolder> BuilderTy;
72
73 private:
74 BuilderTy *Builder;
75
76 PointerOffsetPair getPointerOffsetPair(LoadInst &);
77 bool combineLoads(DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> &);
78 bool aggregateLoads(SmallVectorImpl<LoadPOPPair> &);
79 bool combineLoads(SmallVectorImpl<LoadPOPPair> &);
80 };
81 }
82
doInitialization(Function & F)83 bool LoadCombine::doInitialization(Function &F) {
84 DEBUG(dbgs() << "LoadCombine function: " << F.getName() << "\n");
85 C = &F.getContext();
86 return true;
87 }
88
getPointerOffsetPair(LoadInst & LI)89 PointerOffsetPair LoadCombine::getPointerOffsetPair(LoadInst &LI) {
90 PointerOffsetPair POP;
91 POP.Pointer = LI.getPointerOperand();
92 POP.Offset = 0;
93 while (isa<BitCastInst>(POP.Pointer) || isa<GetElementPtrInst>(POP.Pointer)) {
94 if (auto *GEP = dyn_cast<GetElementPtrInst>(POP.Pointer)) {
95 auto &DL = LI.getModule()->getDataLayout();
96 unsigned BitWidth = DL.getPointerTypeSizeInBits(GEP->getType());
97 APInt Offset(BitWidth, 0);
98 if (GEP->accumulateConstantOffset(DL, Offset))
99 POP.Offset += Offset.getZExtValue();
100 else
101 // Can't handle GEPs with variable indices.
102 return POP;
103 POP.Pointer = GEP->getPointerOperand();
104 } else if (auto *BC = dyn_cast<BitCastInst>(POP.Pointer))
105 POP.Pointer = BC->getOperand(0);
106 }
107 return POP;
108 }
109
combineLoads(DenseMap<const Value *,SmallVector<LoadPOPPair,8>> & LoadMap)110 bool LoadCombine::combineLoads(
111 DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> &LoadMap) {
112 bool Combined = false;
113 for (auto &Loads : LoadMap) {
114 if (Loads.second.size() < 2)
115 continue;
116 std::sort(Loads.second.begin(), Loads.second.end(),
117 [](const LoadPOPPair &A, const LoadPOPPair &B) {
118 return A.POP.Offset < B.POP.Offset;
119 });
120 if (aggregateLoads(Loads.second))
121 Combined = true;
122 }
123 return Combined;
124 }
125
126 /// \brief Try to aggregate loads from a sorted list of loads to be combined.
127 ///
128 /// It is guaranteed that no writes occur between any of the loads. All loads
129 /// have the same base pointer. There are at least two loads.
aggregateLoads(SmallVectorImpl<LoadPOPPair> & Loads)130 bool LoadCombine::aggregateLoads(SmallVectorImpl<LoadPOPPair> &Loads) {
131 assert(Loads.size() >= 2 && "Insufficient loads!");
132 LoadInst *BaseLoad = nullptr;
133 SmallVector<LoadPOPPair, 8> AggregateLoads;
134 bool Combined = false;
135 uint64_t PrevOffset = -1ull;
136 uint64_t PrevSize = 0;
137 for (auto &L : Loads) {
138 if (PrevOffset == -1ull) {
139 BaseLoad = L.Load;
140 PrevOffset = L.POP.Offset;
141 PrevSize = L.Load->getModule()->getDataLayout().getTypeStoreSize(
142 L.Load->getType());
143 AggregateLoads.push_back(L);
144 continue;
145 }
146 if (L.Load->getAlignment() > BaseLoad->getAlignment())
147 continue;
148 if (L.POP.Offset > PrevOffset + PrevSize) {
149 // No other load will be combinable
150 if (combineLoads(AggregateLoads))
151 Combined = true;
152 AggregateLoads.clear();
153 PrevOffset = -1;
154 continue;
155 }
156 if (L.POP.Offset != PrevOffset + PrevSize)
157 // This load is offset less than the size of the last load.
158 // FIXME: We may want to handle this case.
159 continue;
160 PrevOffset = L.POP.Offset;
161 PrevSize = L.Load->getModule()->getDataLayout().getTypeStoreSize(
162 L.Load->getType());
163 AggregateLoads.push_back(L);
164 }
165 if (combineLoads(AggregateLoads))
166 Combined = true;
167 return Combined;
168 }
169
170 /// \brief Given a list of combinable load. Combine the maximum number of them.
combineLoads(SmallVectorImpl<LoadPOPPair> & Loads)171 bool LoadCombine::combineLoads(SmallVectorImpl<LoadPOPPair> &Loads) {
172 // Remove loads from the end while the size is not a power of 2.
173 unsigned TotalSize = 0;
174 for (const auto &L : Loads)
175 TotalSize += L.Load->getType()->getPrimitiveSizeInBits();
176 while (TotalSize != 0 && !isPowerOf2_32(TotalSize))
177 TotalSize -= Loads.pop_back_val().Load->getType()->getPrimitiveSizeInBits();
178 if (Loads.size() < 2)
179 return false;
180
181 DEBUG({
182 dbgs() << "***** Combining Loads ******\n";
183 for (const auto &L : Loads) {
184 dbgs() << L.POP.Offset << ": " << *L.Load << "\n";
185 }
186 });
187
188 // Find first load. This is where we put the new load.
189 LoadPOPPair FirstLP;
190 FirstLP.InsertOrder = -1u;
191 for (const auto &L : Loads)
192 if (L.InsertOrder < FirstLP.InsertOrder)
193 FirstLP = L;
194
195 unsigned AddressSpace =
196 FirstLP.POP.Pointer->getType()->getPointerAddressSpace();
197
198 Builder->SetInsertPoint(FirstLP.Load);
199 Value *Ptr = Builder->CreateConstGEP1_64(
200 Builder->CreatePointerCast(Loads[0].POP.Pointer,
201 Builder->getInt8PtrTy(AddressSpace)),
202 Loads[0].POP.Offset);
203 LoadInst *NewLoad = new LoadInst(
204 Builder->CreatePointerCast(
205 Ptr, PointerType::get(IntegerType::get(Ptr->getContext(), TotalSize),
206 Ptr->getType()->getPointerAddressSpace())),
207 Twine(Loads[0].Load->getName()) + ".combined", false,
208 Loads[0].Load->getAlignment(), FirstLP.Load);
209
210 for (const auto &L : Loads) {
211 Builder->SetInsertPoint(L.Load);
212 Value *V = Builder->CreateExtractInteger(
213 L.Load->getModule()->getDataLayout(), NewLoad,
214 cast<IntegerType>(L.Load->getType()),
215 L.POP.Offset - Loads[0].POP.Offset, "combine.extract");
216 L.Load->replaceAllUsesWith(V);
217 }
218
219 NumLoadsCombined = NumLoadsCombined + Loads.size();
220 return true;
221 }
222
runOnBasicBlock(BasicBlock & BB)223 bool LoadCombine::runOnBasicBlock(BasicBlock &BB) {
224 if (skipOptnoneFunction(BB))
225 return false;
226
227 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
228
229 IRBuilder<true, TargetFolder> TheBuilder(
230 BB.getContext(), TargetFolder(BB.getModule()->getDataLayout()));
231 Builder = &TheBuilder;
232
233 DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> LoadMap;
234 AliasSetTracker AST(*AA);
235
236 bool Combined = false;
237 unsigned Index = 0;
238 for (auto &I : BB) {
239 if (I.mayThrow() || (I.mayWriteToMemory() && AST.containsUnknown(&I))) {
240 if (combineLoads(LoadMap))
241 Combined = true;
242 LoadMap.clear();
243 AST.clear();
244 continue;
245 }
246 LoadInst *LI = dyn_cast<LoadInst>(&I);
247 if (!LI)
248 continue;
249 ++NumLoadsAnalyzed;
250 if (!LI->isSimple() || !LI->getType()->isIntegerTy())
251 continue;
252 auto POP = getPointerOffsetPair(*LI);
253 if (!POP.Pointer)
254 continue;
255 LoadMap[POP.Pointer].push_back(LoadPOPPair(LI, POP, Index++));
256 AST.add(LI);
257 }
258 if (combineLoads(LoadMap))
259 Combined = true;
260 return Combined;
261 }
262
getAnalysisUsage(AnalysisUsage & AU) const263 void LoadCombine::getAnalysisUsage(AnalysisUsage &AU) const {
264 AU.setPreservesCFG();
265
266 AU.addRequired<AAResultsWrapperPass>();
267 AU.addPreserved<GlobalsAAWrapperPass>();
268 }
269
270 char LoadCombine::ID = 0;
271
createLoadCombinePass()272 BasicBlockPass *llvm::createLoadCombinePass() {
273 return new LoadCombine();
274 }
275
276 INITIALIZE_PASS_BEGIN(LoadCombine, "load-combine", "Combine Adjacent Loads",
277 false, false)
278 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
279 INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
280 INITIALIZE_PASS_END(LoadCombine, "load-combine", "Combine Adjacent Loads",
281 false, false)
282
283