MemoryDependenceAnalysis.cpp revision fd3dcbea06f934572a3ba02821b1485eb7a073aa
1//===- MemoryDependenceAnalysis.cpp - Mem Deps Implementation --*- C++ -*-===// 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 an analysis that determines, for a given memory 11// operation, what preceding memory operations it depends on. It builds on 12// alias analysis information, and tries to provide a lazy, caching interface to 13// a common kind of alias information query. 14// 15//===----------------------------------------------------------------------===// 16 17#define DEBUG_TYPE "memdep" 18#include "llvm/Analysis/MemoryDependenceAnalysis.h" 19#include "llvm/Constants.h" 20#include "llvm/Instructions.h" 21#include "llvm/Function.h" 22#include "llvm/Analysis/AliasAnalysis.h" 23#include "llvm/ADT/Statistic.h" 24#include "llvm/ADT/STLExtras.h" 25#include "llvm/Support/CFG.h" 26#include "llvm/Support/CommandLine.h" 27#include "llvm/Support/Debug.h" 28#include "llvm/Target/TargetData.h" 29using namespace llvm; 30 31STATISTIC(NumCacheNonLocal, "Number of cached non-local responses"); 32STATISTIC(NumUncacheNonLocal, "Number of uncached non-local responses"); 33 34char MemoryDependenceAnalysis::ID = 0; 35 36// Register this pass... 37static RegisterPass<MemoryDependenceAnalysis> X("memdep", 38 "Memory Dependence Analysis", false, true); 39 40/// getAnalysisUsage - Does not modify anything. It uses Alias Analysis. 41/// 42void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 43 AU.setPreservesAll(); 44 AU.addRequiredTransitive<AliasAnalysis>(); 45 AU.addRequiredTransitive<TargetData>(); 46} 47 48bool MemoryDependenceAnalysis::runOnFunction(Function &) { 49 AA = &getAnalysis<AliasAnalysis>(); 50 TD = &getAnalysis<TargetData>(); 51 return false; 52} 53 54/// getCallSiteDependency - Private helper for finding the local dependencies 55/// of a call site. 56MemDepResult MemoryDependenceAnalysis:: 57getCallSiteDependency(CallSite C, BasicBlock::iterator ScanIt, BasicBlock *BB) { 58 // Walk backwards through the block, looking for dependencies 59 while (ScanIt != BB->begin()) { 60 Instruction *Inst = --ScanIt; 61 62 // If this inst is a memory op, get the pointer it accessed 63 Value *Pointer = 0; 64 uint64_t PointerSize = 0; 65 if (StoreInst *S = dyn_cast<StoreInst>(Inst)) { 66 Pointer = S->getPointerOperand(); 67 PointerSize = TD->getTypeStoreSize(S->getOperand(0)->getType()); 68 } else if (VAArgInst *V = dyn_cast<VAArgInst>(Inst)) { 69 Pointer = V->getOperand(0); 70 PointerSize = TD->getTypeStoreSize(V->getType()); 71 } else if (FreeInst *F = dyn_cast<FreeInst>(Inst)) { 72 Pointer = F->getPointerOperand(); 73 74 // FreeInsts erase the entire structure 75 PointerSize = ~0UL; 76 } else if (isa<CallInst>(Inst) || isa<InvokeInst>(Inst)) { 77 if (AA->getModRefBehavior(CallSite::get(Inst)) == 78 AliasAnalysis::DoesNotAccessMemory) 79 continue; 80 return MemDepResult::get(Inst); 81 } else { 82 // Non-memory instruction. 83 continue; 84 } 85 86 if (AA->getModRefInfo(C, Pointer, PointerSize) != AliasAnalysis::NoModRef) 87 return MemDepResult::get(Inst); 88 } 89 90 // No dependence found. 91 return MemDepResult::getNonLocal(); 92} 93 94/// getDependencyFrom - Return the instruction on which a memory operation 95/// depends. 96MemDepResult MemoryDependenceAnalysis:: 97getDependencyFrom(Instruction *QueryInst, BasicBlock::iterator ScanIt, 98 BasicBlock *BB) { 99 // Get the pointer value for which dependence will be determined 100 Value *MemPtr = 0; 101 uint64_t MemSize = 0; 102 bool MemVolatile = false; 103 104 if (StoreInst* S = dyn_cast<StoreInst>(QueryInst)) { 105 MemPtr = S->getPointerOperand(); 106 MemSize = TD->getTypeStoreSize(S->getOperand(0)->getType()); 107 MemVolatile = S->isVolatile(); 108 } else if (LoadInst* L = dyn_cast<LoadInst>(QueryInst)) { 109 MemPtr = L->getPointerOperand(); 110 MemSize = TD->getTypeStoreSize(L->getType()); 111 MemVolatile = L->isVolatile(); 112 } else if (VAArgInst* V = dyn_cast<VAArgInst>(QueryInst)) { 113 MemPtr = V->getOperand(0); 114 MemSize = TD->getTypeStoreSize(V->getType()); 115 } else if (FreeInst* F = dyn_cast<FreeInst>(QueryInst)) { 116 MemPtr = F->getPointerOperand(); 117 // FreeInsts erase the entire structure, not just a field. 118 MemSize = ~0UL; 119 } else if (isa<CallInst>(QueryInst) || isa<InvokeInst>(QueryInst)) 120 return getCallSiteDependency(CallSite::get(QueryInst), ScanIt, BB); 121 else // Non-memory instructions depend on nothing. 122 return MemDepResult::getNone(); 123 124 // Walk backwards through the basic block, looking for dependencies 125 while (ScanIt != BB->begin()) { 126 Instruction *Inst = --ScanIt; 127 128 // If the access is volatile and this is a volatile load/store, return a 129 // dependence. 130 if (MemVolatile && 131 ((isa<LoadInst>(Inst) && cast<LoadInst>(Inst)->isVolatile()) || 132 (isa<StoreInst>(Inst) && cast<StoreInst>(Inst)->isVolatile()))) 133 return MemDepResult::get(Inst); 134 135 // Values depend on loads if the pointers are must aliased. This means that 136 // a load depends on another must aliased load from the same value. 137 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 138 Value *Pointer = L->getPointerOperand(); 139 uint64_t PointerSize = TD->getTypeStoreSize(L->getType()); 140 141 // If we found a pointer, check if it could be the same as our pointer 142 AliasAnalysis::AliasResult R = 143 AA->alias(Pointer, PointerSize, MemPtr, MemSize); 144 145 if (R == AliasAnalysis::NoAlias) 146 continue; 147 148 // May-alias loads don't depend on each other without a dependence. 149 if (isa<LoadInst>(QueryInst) && R == AliasAnalysis::MayAlias) 150 continue; 151 return MemDepResult::get(Inst); 152 } 153 154 // If this is an allocation, and if we know that the accessed pointer is to 155 // the allocation, return None. This means that there is no dependence and 156 // the access can be optimized based on that. For example, a load could 157 // turn into undef. 158 if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) { 159 Value *AccessPtr = MemPtr->getUnderlyingObject(); 160 161 if (AccessPtr == AI || 162 AA->alias(AI, 1, AccessPtr, 1) == AliasAnalysis::MustAlias) 163 return MemDepResult::getNone(); 164 continue; 165 } 166 167 // See if this instruction mod/ref's the pointer. 168 AliasAnalysis::ModRefResult MRR = AA->getModRefInfo(Inst, MemPtr, MemSize); 169 170 if (MRR == AliasAnalysis::NoModRef) 171 continue; 172 173 // Loads don't depend on read-only instructions. 174 if (isa<LoadInst>(QueryInst) && MRR == AliasAnalysis::Ref) 175 continue; 176 177 // Otherwise, there is a dependence. 178 return MemDepResult::get(Inst); 179 } 180 181 // If we found nothing, return the non-local flag. 182 return MemDepResult::getNonLocal(); 183} 184 185/// getDependency - Return the instruction on which a memory operation 186/// depends. 187MemDepResult MemoryDependenceAnalysis::getDependency(Instruction *QueryInst) { 188 Instruction *ScanPos = QueryInst; 189 190 // Check for a cached result 191 MemDepResult &LocalCache = LocalDeps[QueryInst]; 192 193 // If the cached entry is non-dirty, just return it. Note that this depends 194 // on MemDepResult's default constructing to 'dirty'. 195 if (!LocalCache.isDirty()) 196 return LocalCache; 197 198 // Otherwise, if we have a dirty entry, we know we can start the scan at that 199 // instruction, which may save us some work. 200 if (Instruction *Inst = LocalCache.getInst()) { 201 ScanPos = Inst; 202 203 SmallPtrSet<Instruction*, 4> &InstMap = ReverseLocalDeps[Inst]; 204 InstMap.erase(QueryInst); 205 if (InstMap.empty()) 206 ReverseLocalDeps.erase(Inst); 207 } 208 209 // Do the scan. 210 LocalCache = getDependencyFrom(QueryInst, ScanPos, QueryInst->getParent()); 211 212 // Remember the result! 213 if (Instruction *I = LocalCache.getInst()) 214 ReverseLocalDeps[I].insert(QueryInst); 215 216 return LocalCache; 217} 218 219/// getNonLocalDependency - Perform a full dependency query for the 220/// specified instruction, returning the set of blocks that the value is 221/// potentially live across. The returned set of results will include a 222/// "NonLocal" result for all blocks where the value is live across. 223/// 224/// This method assumes the instruction returns a "nonlocal" dependency 225/// within its own block. 226/// 227void MemoryDependenceAnalysis:: 228getNonLocalDependency(Instruction *QueryInst, 229 SmallVectorImpl<std::pair<BasicBlock*, 230 MemDepResult> > &Result) { 231 assert(getDependency(QueryInst).isNonLocal() && 232 "getNonLocalDependency should only be used on insts with non-local deps!"); 233 PerInstNLInfo &CacheP = NonLocalDeps[QueryInst]; 234 if (CacheP.getPointer() == 0) CacheP.setPointer(new NonLocalDepInfo()); 235 236 NonLocalDepInfo &Cache = *CacheP.getPointer(); 237 238 /// DirtyBlocks - This is the set of blocks that need to be recomputed. In 239 /// the cached case, this can happen due to instructions being deleted etc. In 240 /// the uncached case, this starts out as the set of predecessors we care 241 /// about. 242 SmallVector<BasicBlock*, 32> DirtyBlocks; 243 244 if (!Cache.empty()) { 245 // If we already have a partially computed set of results, scan them to 246 // determine what is dirty, seeding our initial DirtyBlocks worklist. The 247 // Int bit of CacheP tells us if we have anything dirty. 248 if (CacheP.getInt()) 249 for (NonLocalDepInfo::iterator I = Cache.begin(), E = Cache.end(); 250 I != E; ++I) 251 if (I->second.isDirty()) 252 DirtyBlocks.push_back(I->first); 253 254 NumCacheNonLocal++; 255 256 //cerr << "CACHED CASE: " << DirtyBlocks.size() << " dirty: " 257 // << Cache.size() << " cached: " << *QueryInst; 258 } else { 259 // Seed DirtyBlocks with each of the preds of QueryInst's block. 260 BasicBlock *QueryBB = QueryInst->getParent(); 261 DirtyBlocks.append(pred_begin(QueryBB), pred_end(QueryBB)); 262 NumUncacheNonLocal++; 263 } 264 265 // Iterate while we still have blocks to update. 266 while (!DirtyBlocks.empty()) { 267 BasicBlock *DirtyBB = DirtyBlocks.back(); 268 DirtyBlocks.pop_back(); 269 270 // Get the entry for this block. Note that this relies on MemDepResult 271 // default initializing to Dirty. 272 MemDepResult &DirtyBBEntry = Cache[DirtyBB]; 273 274 // If DirtyBBEntry isn't dirty, it ended up on the worklist multiple times. 275 if (!DirtyBBEntry.isDirty()) continue; 276 277 // If the dirty entry has a pointer, start scanning from it so we don't have 278 // to rescan the entire block. 279 BasicBlock::iterator ScanPos = DirtyBB->end(); 280 if (Instruction *Inst = DirtyBBEntry.getInst()) { 281 ScanPos = Inst; 282 283 // We're removing QueryInst's dependence on Inst. 284 SmallPtrSet<Instruction*, 4> &InstMap = ReverseNonLocalDeps[Inst]; 285 InstMap.erase(QueryInst); 286 if (InstMap.empty()) ReverseNonLocalDeps.erase(Inst); 287 } 288 289 // Find out if this block has a local dependency for QueryInst. 290 DirtyBBEntry = getDependencyFrom(QueryInst, ScanPos, DirtyBB); 291 292 // If the block has a dependency (i.e. it isn't completely transparent to 293 // the value), remember it! 294 if (!DirtyBBEntry.isNonLocal()) { 295 // Keep the ReverseNonLocalDeps map up to date so we can efficiently 296 // update this when we remove instructions. 297 if (Instruction *Inst = DirtyBBEntry.getInst()) 298 ReverseNonLocalDeps[Inst].insert(QueryInst); 299 continue; 300 } 301 302 // If the block *is* completely transparent to the load, we need to check 303 // the predecessors of this block. Add them to our worklist. 304 DirtyBlocks.append(pred_begin(DirtyBB), pred_end(DirtyBB)); 305 } 306 307 308 // Copy the result into the output set. 309 for (NonLocalDepInfo::iterator I = Cache.begin(), E = Cache.end(); I != E;++I) 310 Result.push_back(std::make_pair(I->first, I->second)); 311} 312 313/// removeInstruction - Remove an instruction from the dependence analysis, 314/// updating the dependence of instructions that previously depended on it. 315/// This method attempts to keep the cache coherent using the reverse map. 316void MemoryDependenceAnalysis::removeInstruction(Instruction *RemInst) { 317 // Walk through the Non-local dependencies, removing this one as the value 318 // for any cached queries. 319 NonLocalDepMapType::iterator NLDI = NonLocalDeps.find(RemInst); 320 if (NLDI != NonLocalDeps.end()) { 321 NonLocalDepInfo &BlockMap = *NLDI->second.getPointer(); 322 for (NonLocalDepInfo::iterator DI = BlockMap.begin(), DE = BlockMap.end(); 323 DI != DE; ++DI) 324 if (Instruction *Inst = DI->second.getInst()) 325 ReverseNonLocalDeps[Inst].erase(RemInst); 326 delete &BlockMap; 327 NonLocalDeps.erase(NLDI); 328 } 329 330 // If we have a cached local dependence query for this instruction, remove it. 331 // 332 LocalDepMapType::iterator LocalDepEntry = LocalDeps.find(RemInst); 333 if (LocalDepEntry != LocalDeps.end()) { 334 // Remove us from DepInst's reverse set now that the local dep info is gone. 335 if (Instruction *Inst = LocalDepEntry->second.getInst()) { 336 SmallPtrSet<Instruction*, 4> &RLD = ReverseLocalDeps[Inst]; 337 RLD.erase(RemInst); 338 if (RLD.empty()) 339 ReverseLocalDeps.erase(Inst); 340 } 341 342 // Remove this local dependency info. 343 LocalDeps.erase(LocalDepEntry); 344 } 345 346 // Loop over all of the things that depend on the instruction we're removing. 347 // 348 SmallVector<std::pair<Instruction*, Instruction*>, 8> ReverseDepsToAdd; 349 350 ReverseDepMapType::iterator ReverseDepIt = ReverseLocalDeps.find(RemInst); 351 if (ReverseDepIt != ReverseLocalDeps.end()) { 352 SmallPtrSet<Instruction*, 4> &ReverseDeps = ReverseDepIt->second; 353 // RemInst can't be the terminator if it has stuff depending on it. 354 assert(!ReverseDeps.empty() && !isa<TerminatorInst>(RemInst) && 355 "Nothing can locally depend on a terminator"); 356 357 // Anything that was locally dependent on RemInst is now going to be 358 // dependent on the instruction after RemInst. It will have the dirty flag 359 // set so it will rescan. This saves having to scan the entire block to get 360 // to this point. 361 Instruction *NewDepInst = next(BasicBlock::iterator(RemInst)); 362 363 for (SmallPtrSet<Instruction*, 4>::iterator I = ReverseDeps.begin(), 364 E = ReverseDeps.end(); I != E; ++I) { 365 Instruction *InstDependingOnRemInst = *I; 366 assert(InstDependingOnRemInst != RemInst && 367 "Already removed our local dep info"); 368 369 LocalDeps[InstDependingOnRemInst] = MemDepResult::getDirty(NewDepInst); 370 371 // Make sure to remember that new things depend on NewDepInst. 372 ReverseDepsToAdd.push_back(std::make_pair(NewDepInst, 373 InstDependingOnRemInst)); 374 } 375 376 ReverseLocalDeps.erase(ReverseDepIt); 377 378 // Add new reverse deps after scanning the set, to avoid invalidating the 379 // 'ReverseDeps' reference. 380 while (!ReverseDepsToAdd.empty()) { 381 ReverseLocalDeps[ReverseDepsToAdd.back().first] 382 .insert(ReverseDepsToAdd.back().second); 383 ReverseDepsToAdd.pop_back(); 384 } 385 } 386 387 ReverseDepIt = ReverseNonLocalDeps.find(RemInst); 388 if (ReverseDepIt != ReverseNonLocalDeps.end()) { 389 SmallPtrSet<Instruction*, 4>& set = ReverseDepIt->second; 390 for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end(); 391 I != E; ++I) { 392 assert(*I != RemInst && "Already removed NonLocalDep info for RemInst"); 393 394 PerInstNLInfo &INLD = NonLocalDeps[*I]; 395 assert(INLD.getPointer() != 0 && "Reverse mapping out of date?"); 396 // The information is now dirty! 397 INLD.setInt(true); 398 399 for (NonLocalDepInfo::iterator DI = INLD.getPointer()->begin(), 400 DE = INLD.getPointer()->end(); DI != DE; ++DI) { 401 if (DI->second.getInst() != RemInst) continue; 402 403 // Convert to a dirty entry for the subsequent instruction. 404 Instruction *NextI = 0; 405 if (!RemInst->isTerminator()) { 406 NextI = next(BasicBlock::iterator(RemInst)); 407 ReverseDepsToAdd.push_back(std::make_pair(NextI, *I)); 408 } 409 DI->second = MemDepResult::getDirty(NextI); 410 } 411 } 412 413 ReverseNonLocalDeps.erase(ReverseDepIt); 414 415 // Add new reverse deps after scanning the set, to avoid invalidating 'Set' 416 while (!ReverseDepsToAdd.empty()) { 417 ReverseNonLocalDeps[ReverseDepsToAdd.back().first] 418 .insert(ReverseDepsToAdd.back().second); 419 ReverseDepsToAdd.pop_back(); 420 } 421 } 422 423 assert(!NonLocalDeps.count(RemInst) && "RemInst got reinserted?"); 424 AA->deleteValue(RemInst); 425 DEBUG(verifyRemoved(RemInst)); 426} 427 428/// verifyRemoved - Verify that the specified instruction does not occur 429/// in our internal data structures. 430void MemoryDependenceAnalysis::verifyRemoved(Instruction *D) const { 431 for (LocalDepMapType::const_iterator I = LocalDeps.begin(), 432 E = LocalDeps.end(); I != E; ++I) { 433 assert(I->first != D && "Inst occurs in data structures"); 434 assert(I->second.getInst() != D && 435 "Inst occurs in data structures"); 436 } 437 438 for (NonLocalDepMapType::const_iterator I = NonLocalDeps.begin(), 439 E = NonLocalDeps.end(); I != E; ++I) { 440 assert(I->first != D && "Inst occurs in data structures"); 441 const PerInstNLInfo &INLD = I->second; 442 for (NonLocalDepInfo::iterator II = INLD.getPointer()->begin(), 443 EE = INLD.getPointer()->end(); II != EE; ++II) 444 assert(II->second.getInst() != D && "Inst occurs in data structures"); 445 } 446 447 for (ReverseDepMapType::const_iterator I = ReverseLocalDeps.begin(), 448 E = ReverseLocalDeps.end(); I != E; ++I) { 449 assert(I->first != D && "Inst occurs in data structures"); 450 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(), 451 EE = I->second.end(); II != EE; ++II) 452 assert(*II != D && "Inst occurs in data structures"); 453 } 454 455 for (ReverseDepMapType::const_iterator I = ReverseNonLocalDeps.begin(), 456 E = ReverseNonLocalDeps.end(); 457 I != E; ++I) { 458 assert(I->first != D && "Inst occurs in data structures"); 459 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(), 460 EE = I->second.end(); II != EE; ++II) 461 assert(*II != D && "Inst occurs in data structures"); 462 } 463} 464