1 //===- GVN.h - Eliminate redundant values and loads -------------*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 /// \file 9 /// This file provides the interface for LLVM's Global Value Numbering pass 10 /// which eliminates fully redundant instructions. It also does somewhat Ad-Hoc 11 /// PRE and dead load elimination. 12 /// 13 //===----------------------------------------------------------------------===// 14 15 #ifndef LLVM_TRANSFORMS_SCALAR_GVN_H 16 #define LLVM_TRANSFORMS_SCALAR_GVN_H 17 18 #include "llvm/ADT/DenseMap.h" 19 #include "llvm/ADT/MapVector.h" 20 #include "llvm/ADT/SetVector.h" 21 #include "llvm/ADT/SmallVector.h" 22 #include "llvm/IR/Dominators.h" 23 #include "llvm/IR/InstrTypes.h" 24 #include "llvm/IR/PassManager.h" 25 #include "llvm/IR/ValueHandle.h" 26 #include "llvm/Support/Allocator.h" 27 #include "llvm/Support/Compiler.h" 28 #include <cstdint> 29 #include <optional> 30 #include <utility> 31 #include <vector> 32 33 namespace llvm { 34 35 class AAResults; 36 class AssumeInst; 37 class AssumptionCache; 38 class BasicBlock; 39 class BranchInst; 40 class CallInst; 41 class ExtractValueInst; 42 class Function; 43 class FunctionPass; 44 class GetElementPtrInst; 45 class ImplicitControlFlowTracking; 46 class LoadInst; 47 class LoopInfo; 48 class MemDepResult; 49 class MemoryDependenceResults; 50 class MemorySSA; 51 class MemorySSAUpdater; 52 class NonLocalDepResult; 53 class OptimizationRemarkEmitter; 54 class PHINode; 55 class TargetLibraryInfo; 56 class Value; 57 /// A private "module" namespace for types and utilities used by GVN. These 58 /// are implementation details and should not be used by clients. 59 namespace LLVM_LIBRARY_VISIBILITY gvn { 60 61 struct AvailableValue; 62 struct AvailableValueInBlock; 63 class GVNLegacyPass; 64 65 } // end namespace gvn 66 67 /// A set of parameters to control various transforms performed by GVN pass. 68 // Each of the optional boolean parameters can be set to: 69 /// true - enabling the transformation. 70 /// false - disabling the transformation. 71 /// None - relying on a global default. 72 /// Intended use is to create a default object, modify parameters with 73 /// additional setters and then pass it to GVN. 74 struct GVNOptions { 75 std::optional<bool> AllowPRE; 76 std::optional<bool> AllowLoadPRE; 77 std::optional<bool> AllowLoadInLoopPRE; 78 std::optional<bool> AllowLoadPRESplitBackedge; 79 std::optional<bool> AllowMemDep; 80 81 GVNOptions() = default; 82 83 /// Enables or disables PRE in GVN. setPREGVNOptions84 GVNOptions &setPRE(bool PRE) { 85 AllowPRE = PRE; 86 return *this; 87 } 88 89 /// Enables or disables PRE of loads in GVN. setLoadPREGVNOptions90 GVNOptions &setLoadPRE(bool LoadPRE) { 91 AllowLoadPRE = LoadPRE; 92 return *this; 93 } 94 setLoadInLoopPREGVNOptions95 GVNOptions &setLoadInLoopPRE(bool LoadInLoopPRE) { 96 AllowLoadInLoopPRE = LoadInLoopPRE; 97 return *this; 98 } 99 100 /// Enables or disables PRE of loads in GVN. setLoadPRESplitBackedgeGVNOptions101 GVNOptions &setLoadPRESplitBackedge(bool LoadPRESplitBackedge) { 102 AllowLoadPRESplitBackedge = LoadPRESplitBackedge; 103 return *this; 104 } 105 106 /// Enables or disables use of MemDepAnalysis. setMemDepGVNOptions107 GVNOptions &setMemDep(bool MemDep) { 108 AllowMemDep = MemDep; 109 return *this; 110 } 111 }; 112 113 /// The core GVN pass object. 114 /// 115 /// FIXME: We should have a good summary of the GVN algorithm implemented by 116 /// this particular pass here. 117 class GVNPass : public PassInfoMixin<GVNPass> { 118 GVNOptions Options; 119 120 public: 121 struct Expression; 122 Options(Options)123 GVNPass(GVNOptions Options = {}) : Options(Options) {} 124 125 /// Run the pass over the function. 126 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 127 128 void printPipeline(raw_ostream &OS, 129 function_ref<StringRef(StringRef)> MapClassName2PassName); 130 131 /// This removes the specified instruction from 132 /// our various maps and marks it for deletion. markInstructionForDeletion(Instruction * I)133 void markInstructionForDeletion(Instruction *I) { 134 VN.erase(I); 135 InstrsToErase.push_back(I); 136 } 137 getDominatorTree()138 DominatorTree &getDominatorTree() const { return *DT; } getAliasAnalysis()139 AAResults *getAliasAnalysis() const { return VN.getAliasAnalysis(); } getMemDep()140 MemoryDependenceResults &getMemDep() const { return *MD; } 141 142 bool isPREEnabled() const; 143 bool isLoadPREEnabled() const; 144 bool isLoadInLoopPREEnabled() const; 145 bool isLoadPRESplitBackedgeEnabled() const; 146 bool isMemDepEnabled() const; 147 148 /// This class holds the mapping between values and value numbers. It is used 149 /// as an efficient mechanism to determine the expression-wise equivalence of 150 /// two values. 151 class ValueTable { 152 DenseMap<Value *, uint32_t> valueNumbering; 153 DenseMap<Expression, uint32_t> expressionNumbering; 154 155 // Expressions is the vector of Expression. ExprIdx is the mapping from 156 // value number to the index of Expression in Expressions. We use it 157 // instead of a DenseMap because filling such mapping is faster than 158 // filling a DenseMap and the compile time is a little better. 159 uint32_t nextExprNumber = 0; 160 161 std::vector<Expression> Expressions; 162 std::vector<uint32_t> ExprIdx; 163 164 // Value number to PHINode mapping. Used for phi-translate in scalarpre. 165 DenseMap<uint32_t, PHINode *> NumberingPhi; 166 167 // Cache for phi-translate in scalarpre. 168 using PhiTranslateMap = 169 DenseMap<std::pair<uint32_t, const BasicBlock *>, uint32_t>; 170 PhiTranslateMap PhiTranslateTable; 171 172 AAResults *AA = nullptr; 173 MemoryDependenceResults *MD = nullptr; 174 DominatorTree *DT = nullptr; 175 176 uint32_t nextValueNumber = 1; 177 178 Expression createExpr(Instruction *I); 179 Expression createCmpExpr(unsigned Opcode, CmpInst::Predicate Predicate, 180 Value *LHS, Value *RHS); 181 Expression createExtractvalueExpr(ExtractValueInst *EI); 182 Expression createGEPExpr(GetElementPtrInst *GEP); 183 uint32_t lookupOrAddCall(CallInst *C); 184 uint32_t phiTranslateImpl(const BasicBlock *BB, const BasicBlock *PhiBlock, 185 uint32_t Num, GVNPass &Gvn); 186 bool areCallValsEqual(uint32_t Num, uint32_t NewNum, const BasicBlock *Pred, 187 const BasicBlock *PhiBlock, GVNPass &Gvn); 188 std::pair<uint32_t, bool> assignExpNewValueNum(Expression &exp); 189 bool areAllValsInBB(uint32_t num, const BasicBlock *BB, GVNPass &Gvn); 190 191 public: 192 ValueTable(); 193 ValueTable(const ValueTable &Arg); 194 ValueTable(ValueTable &&Arg); 195 ~ValueTable(); 196 ValueTable &operator=(const ValueTable &Arg); 197 198 uint32_t lookupOrAdd(Value *V); 199 uint32_t lookup(Value *V, bool Verify = true) const; 200 uint32_t lookupOrAddCmp(unsigned Opcode, CmpInst::Predicate Pred, 201 Value *LHS, Value *RHS); 202 uint32_t phiTranslate(const BasicBlock *BB, const BasicBlock *PhiBlock, 203 uint32_t Num, GVNPass &Gvn); 204 void eraseTranslateCacheEntry(uint32_t Num, const BasicBlock &CurrBlock); 205 bool exists(Value *V) const; 206 void add(Value *V, uint32_t num); 207 void clear(); 208 void erase(Value *v); setAliasAnalysis(AAResults * A)209 void setAliasAnalysis(AAResults *A) { AA = A; } getAliasAnalysis()210 AAResults *getAliasAnalysis() const { return AA; } setMemDep(MemoryDependenceResults * M)211 void setMemDep(MemoryDependenceResults *M) { MD = M; } setDomTree(DominatorTree * D)212 void setDomTree(DominatorTree *D) { DT = D; } getNextUnusedValueNumber()213 uint32_t getNextUnusedValueNumber() { return nextValueNumber; } 214 void verifyRemoved(const Value *) const; 215 }; 216 217 private: 218 friend class gvn::GVNLegacyPass; 219 friend struct DenseMapInfo<Expression>; 220 221 MemoryDependenceResults *MD = nullptr; 222 DominatorTree *DT = nullptr; 223 const TargetLibraryInfo *TLI = nullptr; 224 AssumptionCache *AC = nullptr; 225 SetVector<BasicBlock *> DeadBlocks; 226 OptimizationRemarkEmitter *ORE = nullptr; 227 ImplicitControlFlowTracking *ICF = nullptr; 228 LoopInfo *LI = nullptr; 229 MemorySSAUpdater *MSSAU = nullptr; 230 231 ValueTable VN; 232 233 /// A mapping from value numbers to lists of Value*'s that 234 /// have that value number. Use findLeader to query it. 235 class LeaderMap { 236 public: 237 struct LeaderTableEntry { 238 Value *Val; 239 const BasicBlock *BB; 240 }; 241 242 private: 243 struct LeaderListNode { 244 LeaderTableEntry Entry; 245 LeaderListNode *Next; 246 }; 247 DenseMap<uint32_t, LeaderListNode> NumToLeaders; 248 BumpPtrAllocator TableAllocator; 249 250 public: 251 class leader_iterator { 252 const LeaderListNode *Current; 253 254 public: 255 using iterator_category = std::forward_iterator_tag; 256 using value_type = const LeaderTableEntry; 257 using difference_type = std::ptrdiff_t; 258 using pointer = value_type *; 259 using reference = value_type &; 260 261 leader_iterator(const LeaderListNode *C) : Current(C) {} 262 leader_iterator &operator++() { 263 assert(Current && "Dereferenced end of leader list!"); 264 Current = Current->Next; 265 return *this; 266 } 267 bool operator==(const leader_iterator &Other) const { 268 return Current == Other.Current; 269 } 270 bool operator!=(const leader_iterator &Other) const { 271 return Current != Other.Current; 272 } 273 reference operator*() const { return Current->Entry; } 274 }; 275 276 iterator_range<leader_iterator> getLeaders(uint32_t N) { 277 auto I = NumToLeaders.find(N); 278 if (I == NumToLeaders.end()) { 279 return iterator_range(leader_iterator(nullptr), 280 leader_iterator(nullptr)); 281 } 282 283 return iterator_range(leader_iterator(&I->second), 284 leader_iterator(nullptr)); 285 } 286 287 void insert(uint32_t N, Value *V, const BasicBlock *BB); 288 void erase(uint32_t N, Instruction *I, const BasicBlock *BB); 289 void verifyRemoved(const Value *Inst) const; 290 void clear() { 291 NumToLeaders.clear(); 292 TableAllocator.Reset(); 293 } 294 }; 295 LeaderMap LeaderTable; 296 297 // Block-local map of equivalent values to their leader, does not 298 // propagate to any successors. Entries added mid-block are applied 299 // to the remaining instructions in the block. 300 SmallMapVector<Value *, Value *, 4> ReplaceOperandsWithMap; 301 SmallVector<Instruction *, 8> InstrsToErase; 302 303 // Map the block to reversed postorder traversal number. It is used to 304 // find back edge easily. 305 DenseMap<AssertingVH<BasicBlock>, uint32_t> BlockRPONumber; 306 307 // This is set 'true' initially and also when new blocks have been added to 308 // the function being analyzed. This boolean is used to control the updating 309 // of BlockRPONumber prior to accessing the contents of BlockRPONumber. 310 bool InvalidBlockRPONumbers = true; 311 312 using LoadDepVect = SmallVector<NonLocalDepResult, 64>; 313 using AvailValInBlkVect = SmallVector<gvn::AvailableValueInBlock, 64>; 314 using UnavailBlkVect = SmallVector<BasicBlock *, 64>; 315 316 bool runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT, 317 const TargetLibraryInfo &RunTLI, AAResults &RunAA, 318 MemoryDependenceResults *RunMD, LoopInfo &LI, 319 OptimizationRemarkEmitter *ORE, MemorySSA *MSSA = nullptr); 320 321 // List of critical edges to be split between iterations. 322 SmallVector<std::pair<Instruction *, unsigned>, 4> toSplit; 323 324 // Helper functions of redundant load elimination 325 bool processLoad(LoadInst *L); 326 bool processNonLocalLoad(LoadInst *L); 327 bool processAssumeIntrinsic(AssumeInst *II); 328 329 /// Given a local dependency (Def or Clobber) determine if a value is 330 /// available for the load. 331 std::optional<gvn::AvailableValue> 332 AnalyzeLoadAvailability(LoadInst *Load, MemDepResult DepInfo, Value *Address); 333 334 /// Given a list of non-local dependencies, determine if a value is 335 /// available for the load in each specified block. If it is, add it to 336 /// ValuesPerBlock. If not, add it to UnavailableBlocks. 337 void AnalyzeLoadAvailability(LoadInst *Load, LoadDepVect &Deps, 338 AvailValInBlkVect &ValuesPerBlock, 339 UnavailBlkVect &UnavailableBlocks); 340 341 /// Given a critical edge from Pred to LoadBB, find a load instruction 342 /// which is identical to Load from another successor of Pred. 343 LoadInst *findLoadToHoistIntoPred(BasicBlock *Pred, BasicBlock *LoadBB, 344 LoadInst *Load); 345 346 bool PerformLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock, 347 UnavailBlkVect &UnavailableBlocks); 348 349 /// Try to replace a load which executes on each loop iteraiton with Phi 350 /// translation of load in preheader and load(s) in conditionally executed 351 /// paths. 352 bool performLoopLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock, 353 UnavailBlkVect &UnavailableBlocks); 354 355 /// Eliminates partially redundant \p Load, replacing it with \p 356 /// AvailableLoads (connected by Phis if needed). 357 void eliminatePartiallyRedundantLoad( 358 LoadInst *Load, AvailValInBlkVect &ValuesPerBlock, 359 MapVector<BasicBlock *, Value *> &AvailableLoads, 360 MapVector<BasicBlock *, LoadInst *> *CriticalEdgePredAndLoad); 361 362 // Other helper routines 363 bool processInstruction(Instruction *I); 364 bool processBlock(BasicBlock *BB); 365 void dump(DenseMap<uint32_t, Value *> &d) const; 366 bool iterateOnFunction(Function &F); 367 bool performPRE(Function &F); 368 bool performScalarPRE(Instruction *I); 369 bool performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred, 370 BasicBlock *Curr, unsigned int ValNo); 371 Value *findLeader(const BasicBlock *BB, uint32_t num); 372 void cleanupGlobalSets(); 373 void removeInstruction(Instruction *I); 374 void verifyRemoved(const Instruction *I) const; 375 bool splitCriticalEdges(); 376 BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ); 377 bool replaceOperandsForInBlockEquality(Instruction *I) const; 378 bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root, 379 bool DominatesByEdge); 380 bool processFoldableCondBr(BranchInst *BI); 381 void addDeadBlock(BasicBlock *BB); 382 void assignValNumForDeadCode(); 383 void assignBlockRPONumber(Function &F); 384 }; 385 386 /// Create a legacy GVN pass. This also allows parameterizing whether or not 387 /// MemDep is enabled. 388 FunctionPass *createGVNPass(bool NoMemDepAnalysis = false); 389 390 /// A simple and fast domtree-based GVN pass to hoist common expressions 391 /// from sibling branches. 392 struct GVNHoistPass : PassInfoMixin<GVNHoistPass> { 393 /// Run the pass over the function. 394 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 395 }; 396 397 /// Uses an "inverted" value numbering to decide the similarity of 398 /// expressions and sinks similar expressions into successors. 399 struct GVNSinkPass : PassInfoMixin<GVNSinkPass> { 400 /// Run the pass over the function. 401 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 402 }; 403 404 } // end namespace llvm 405 406 #endif // LLVM_TRANSFORMS_SCALAR_GVN_H 407