MemoryDependenceAnalysis.cpp revision 745291a6ce841f30a8a9e536071bd1b4cf540c55
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 fully cached non-local responses"); 32STATISTIC(NumCacheDirtyNonLocal, "Number of dirty cached non-local responses"); 33STATISTIC(NumUncacheNonLocal, "Number of uncached non-local responses"); 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 55/// getCallSiteDependency - Private helper for finding the local dependencies 56/// of a call site. 57MemDepResult MemoryDependenceAnalysis:: 58getCallSiteDependency(CallSite CS, BasicBlock::iterator ScanIt, BasicBlock *BB) { 59 // Walk backwards through the block, looking for dependencies 60 while (ScanIt != BB->begin()) { 61 Instruction *Inst = --ScanIt; 62 63 // If this inst is a memory op, get the pointer it accessed 64 Value *Pointer = 0; 65 uint64_t PointerSize = 0; 66 if (StoreInst *S = dyn_cast<StoreInst>(Inst)) { 67 Pointer = S->getPointerOperand(); 68 PointerSize = TD->getTypeStoreSize(S->getOperand(0)->getType()); 69 } else if (VAArgInst *V = dyn_cast<VAArgInst>(Inst)) { 70 Pointer = V->getOperand(0); 71 PointerSize = TD->getTypeStoreSize(V->getType()); 72 } else if (FreeInst *F = dyn_cast<FreeInst>(Inst)) { 73 Pointer = F->getPointerOperand(); 74 75 // FreeInsts erase the entire structure 76 PointerSize = ~0UL; 77 } else if (isa<CallInst>(Inst) || isa<InvokeInst>(Inst)) { 78 CallSite InstCS = CallSite::get(Inst); 79 // If these two calls do not interfere, look past it. 80 if (AA->getModRefInfo(CS, InstCS) == AliasAnalysis::NoModRef) 81 continue; 82 83 // FIXME: If this is a ref/ref result, we should ignore it! 84 // X = strlen(P); 85 // Y = strlen(Q); 86 // Z = strlen(P); // Z = X 87 88 // If they interfere, we generally return clobber. However, if they are 89 // calls to the same read-only functions we return Def. 90 if (!AA->onlyReadsMemory(CS) || CS.getCalledFunction() == 0 || 91 CS.getCalledFunction() != InstCS.getCalledFunction()) 92 return MemDepResult::getClobber(Inst); 93 return MemDepResult::getDef(Inst); 94 } else { 95 // Non-memory instruction. 96 continue; 97 } 98 99 if (AA->getModRefInfo(CS, Pointer, PointerSize) != AliasAnalysis::NoModRef) 100 return MemDepResult::getClobber(Inst); 101 } 102 103 // No dependence found. 104 return MemDepResult::getNonLocal(); 105} 106 107/// getDependencyFrom - Return the instruction on which a memory operation 108/// depends. 109MemDepResult MemoryDependenceAnalysis:: 110getDependencyFrom(Instruction *QueryInst, BasicBlock::iterator ScanIt, 111 BasicBlock *BB) { 112 // The first instruction in a block is always non-local. 113 if (ScanIt == BB->begin()) 114 return MemDepResult::getNonLocal(); 115 116 // Get the pointer value for which dependence will be determined 117 Value *MemPtr = 0; 118 uint64_t MemSize = 0; 119 120 if (StoreInst* S = dyn_cast<StoreInst>(QueryInst)) { 121 // If this is a volatile store, don't mess around with it. Just return the 122 // previous instruction as a clobber. 123 if (S->isVolatile()) 124 return MemDepResult::getClobber(--ScanIt); 125 126 MemPtr = S->getPointerOperand(); 127 MemSize = TD->getTypeStoreSize(S->getOperand(0)->getType()); 128 } else if (LoadInst* LI = dyn_cast<LoadInst>(QueryInst)) { 129 // If this is a volatile load, don't mess around with it. Just return the 130 // previous instruction as a clobber. 131 if (S->isVolatile()) 132 return MemDepResult::getClobber(--ScanIt); 133 134 MemPtr = LI->getPointerOperand(); 135 MemSize = TD->getTypeStoreSize(LI->getType()); 136 } else if (FreeInst* F = dyn_cast<FreeInst>(QueryInst)) { 137 MemPtr = F->getPointerOperand(); 138 // FreeInsts erase the entire structure, not just a field. 139 MemSize = ~0UL; 140 } else if (isa<CallInst>(QueryInst) || isa<InvokeInst>(QueryInst)) { 141 return getCallSiteDependency(CallSite::get(QueryInst), ScanIt, BB); 142 } else { 143 // Otherwise, this is a vaarg or non-memory instruction, just return a 144 // clobber dependency on the previous inst. 145 return MemDepResult::getClobber(--ScanIt); 146 } 147 148 // Walk backwards through the basic block, looking for dependencies 149 while (ScanIt != BB->begin()) { 150 Instruction *Inst = --ScanIt; 151 152 // Values depend on loads if the pointers are must aliased. This means that 153 // a load depends on another must aliased load from the same value. 154 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) { 155 Value *Pointer = LI->getPointerOperand(); 156 uint64_t PointerSize = TD->getTypeStoreSize(LI->getType()); 157 158 // If we found a pointer, check if it could be the same as our pointer. 159 AliasAnalysis::AliasResult R = 160 AA->alias(Pointer, PointerSize, MemPtr, MemSize); 161 if (R == AliasAnalysis::NoAlias) 162 continue; 163 164 // May-alias loads don't depend on each other without a dependence. 165 if (isa<LoadInst>(QueryInst) && R == AliasAnalysis::MayAlias) 166 continue; 167 return MemDepResult::getDef(Inst); 168 } 169 170 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 171 Value *Pointer = SI->getPointerOperand(); 172 uint64_t PointerSize = TD->getTypeStoreSize(SI->getOperand(0)->getType()); 173 174 // If we found a pointer, check if it could be the same as our pointer. 175 AliasAnalysis::AliasResult R = 176 AA->alias(Pointer, PointerSize, MemPtr, MemSize); 177 178 if (R == AliasAnalysis::NoAlias) 179 continue; 180 if (R == AliasAnalysis::MayAlias) 181 return MemDepResult::getClobber(Inst); 182 return MemDepResult::getDef(Inst); 183 } 184 185 // If this is an allocation, and if we know that the accessed pointer is to 186 // the allocation, return Def. This means that there is no dependence and 187 // the access can be optimized based on that. For example, a load could 188 // turn into undef. 189 if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) { 190 Value *AccessPtr = MemPtr->getUnderlyingObject(); 191 192 if (AccessPtr == AI || 193 AA->alias(AI, 1, AccessPtr, 1) == AliasAnalysis::MustAlias) 194 return MemDepResult::getDef(AI); 195 continue; 196 } 197 198 // See if this instruction (e.g. a call or vaarg) mod/ref's the pointer. 199 if (AA->getModRefInfo(Inst, MemPtr, MemSize) == AliasAnalysis::NoModRef) 200 continue; 201 202 // Otherwise, there is a dependence. 203 return MemDepResult::getClobber(Inst); 204 } 205 206 // If we found nothing, return the non-local flag. 207 return MemDepResult::getNonLocal(); 208} 209 210/// getDependency - Return the instruction on which a memory operation 211/// depends. 212MemDepResult MemoryDependenceAnalysis::getDependency(Instruction *QueryInst) { 213 Instruction *ScanPos = QueryInst; 214 215 // Check for a cached result 216 MemDepResult &LocalCache = LocalDeps[QueryInst]; 217 218 // If the cached entry is non-dirty, just return it. Note that this depends 219 // on MemDepResult's default constructing to 'dirty'. 220 if (!LocalCache.isDirty()) 221 return LocalCache; 222 223 // Otherwise, if we have a dirty entry, we know we can start the scan at that 224 // instruction, which may save us some work. 225 if (Instruction *Inst = LocalCache.getInst()) { 226 ScanPos = Inst; 227 228 SmallPtrSet<Instruction*, 4> &InstMap = ReverseLocalDeps[Inst]; 229 InstMap.erase(QueryInst); 230 if (InstMap.empty()) 231 ReverseLocalDeps.erase(Inst); 232 } 233 234 // Do the scan. 235 LocalCache = getDependencyFrom(QueryInst, ScanPos, QueryInst->getParent()); 236 237 // Remember the result! 238 if (Instruction *I = LocalCache.getInst()) 239 ReverseLocalDeps[I].insert(QueryInst); 240 241 return LocalCache; 242} 243 244/// getNonLocalDependency - Perform a full dependency query for the 245/// specified instruction, returning the set of blocks that the value is 246/// potentially live across. The returned set of results will include a 247/// "NonLocal" result for all blocks where the value is live across. 248/// 249/// This method assumes the instruction returns a "nonlocal" dependency 250/// within its own block. 251/// 252const MemoryDependenceAnalysis::NonLocalDepInfo & 253MemoryDependenceAnalysis::getNonLocalDependency(Instruction *QueryInst) { 254 assert(getDependency(QueryInst).isNonLocal() && 255 "getNonLocalDependency should only be used on insts with non-local deps!"); 256 PerInstNLInfo &CacheP = NonLocalDeps[QueryInst]; 257 258 NonLocalDepInfo &Cache = CacheP.first; 259 260 /// DirtyBlocks - This is the set of blocks that need to be recomputed. In 261 /// the cached case, this can happen due to instructions being deleted etc. In 262 /// the uncached case, this starts out as the set of predecessors we care 263 /// about. 264 SmallVector<BasicBlock*, 32> DirtyBlocks; 265 266 if (!Cache.empty()) { 267 // Okay, we have a cache entry. If we know it is not dirty, just return it 268 // with no computation. 269 if (!CacheP.second) { 270 NumCacheNonLocal++; 271 return Cache; 272 } 273 274 // If we already have a partially computed set of results, scan them to 275 // determine what is dirty, seeding our initial DirtyBlocks worklist. 276 for (NonLocalDepInfo::iterator I = Cache.begin(), E = Cache.end(); 277 I != E; ++I) 278 if (I->second.isDirty()) 279 DirtyBlocks.push_back(I->first); 280 281 // Sort the cache so that we can do fast binary search lookups below. 282 std::sort(Cache.begin(), Cache.end()); 283 284 ++NumCacheDirtyNonLocal; 285 //cerr << "CACHED CASE: " << DirtyBlocks.size() << " dirty: " 286 // << Cache.size() << " cached: " << *QueryInst; 287 } else { 288 // Seed DirtyBlocks with each of the preds of QueryInst's block. 289 BasicBlock *QueryBB = QueryInst->getParent(); 290 DirtyBlocks.append(pred_begin(QueryBB), pred_end(QueryBB)); 291 NumUncacheNonLocal++; 292 } 293 294 // Visited checked first, vector in sorted order. 295 SmallPtrSet<BasicBlock*, 64> Visited; 296 297 unsigned NumSortedEntries = Cache.size(); 298 299 // Iterate while we still have blocks to update. 300 while (!DirtyBlocks.empty()) { 301 BasicBlock *DirtyBB = DirtyBlocks.back(); 302 DirtyBlocks.pop_back(); 303 304 // Already processed this block? 305 if (!Visited.insert(DirtyBB)) 306 continue; 307 308 // Do a binary search to see if we already have an entry for this block in 309 // the cache set. If so, find it. 310 NonLocalDepInfo::iterator Entry = 311 std::upper_bound(Cache.begin(), Cache.begin()+NumSortedEntries, 312 std::make_pair(DirtyBB, MemDepResult())); 313 if (Entry != Cache.begin() && (&*Entry)[-1].first == DirtyBB) 314 --Entry; 315 316 MemDepResult *ExistingResult = 0; 317 if (Entry != Cache.begin()+NumSortedEntries && 318 Entry->first == DirtyBB) { 319 // If we already have an entry, and if it isn't already dirty, the block 320 // is done. 321 if (!Entry->second.isDirty()) 322 continue; 323 324 // Otherwise, remember this slot so we can update the value. 325 ExistingResult = &Entry->second; 326 } 327 328 // If the dirty entry has a pointer, start scanning from it so we don't have 329 // to rescan the entire block. 330 BasicBlock::iterator ScanPos = DirtyBB->end(); 331 if (ExistingResult) { 332 if (Instruction *Inst = ExistingResult->getInst()) { 333 ScanPos = Inst; 334 335 // We're removing QueryInst's use of Inst. 336 SmallPtrSet<Instruction*, 4> &InstMap = ReverseNonLocalDeps[Inst]; 337 InstMap.erase(QueryInst); 338 if (InstMap.empty()) ReverseNonLocalDeps.erase(Inst); 339 } 340 } 341 342 // Find out if this block has a local dependency for QueryInst. 343 MemDepResult Dep = getDependencyFrom(QueryInst, ScanPos, DirtyBB); 344 345 // If we had a dirty entry for the block, update it. Otherwise, just add 346 // a new entry. 347 if (ExistingResult) 348 *ExistingResult = Dep; 349 else 350 Cache.push_back(std::make_pair(DirtyBB, Dep)); 351 352 // If the block has a dependency (i.e. it isn't completely transparent to 353 // the value), remember the association! 354 if (!Dep.isNonLocal()) { 355 // Keep the ReverseNonLocalDeps map up to date so we can efficiently 356 // update this when we remove instructions. 357 if (Instruction *Inst = Dep.getInst()) 358 ReverseNonLocalDeps[Inst].insert(QueryInst); 359 } else { 360 361 // If the block *is* completely transparent to the load, we need to check 362 // the predecessors of this block. Add them to our worklist. 363 DirtyBlocks.append(pred_begin(DirtyBB), pred_end(DirtyBB)); 364 } 365 } 366 367 return Cache; 368} 369 370/// removeInstruction - Remove an instruction from the dependence analysis, 371/// updating the dependence of instructions that previously depended on it. 372/// This method attempts to keep the cache coherent using the reverse map. 373void MemoryDependenceAnalysis::removeInstruction(Instruction *RemInst) { 374 // Walk through the Non-local dependencies, removing this one as the value 375 // for any cached queries. 376 NonLocalDepMapType::iterator NLDI = NonLocalDeps.find(RemInst); 377 if (NLDI != NonLocalDeps.end()) { 378 NonLocalDepInfo &BlockMap = NLDI->second.first; 379 for (NonLocalDepInfo::iterator DI = BlockMap.begin(), DE = BlockMap.end(); 380 DI != DE; ++DI) 381 if (Instruction *Inst = DI->second.getInst()) 382 ReverseNonLocalDeps[Inst].erase(RemInst); 383 NonLocalDeps.erase(NLDI); 384 } 385 386 // If we have a cached local dependence query for this instruction, remove it. 387 // 388 LocalDepMapType::iterator LocalDepEntry = LocalDeps.find(RemInst); 389 if (LocalDepEntry != LocalDeps.end()) { 390 // Remove us from DepInst's reverse set now that the local dep info is gone. 391 if (Instruction *Inst = LocalDepEntry->second.getInst()) { 392 SmallPtrSet<Instruction*, 4> &RLD = ReverseLocalDeps[Inst]; 393 RLD.erase(RemInst); 394 if (RLD.empty()) 395 ReverseLocalDeps.erase(Inst); 396 } 397 398 // Remove this local dependency info. 399 LocalDeps.erase(LocalDepEntry); 400 } 401 402 // Loop over all of the things that depend on the instruction we're removing. 403 // 404 SmallVector<std::pair<Instruction*, Instruction*>, 8> ReverseDepsToAdd; 405 406 ReverseDepMapType::iterator ReverseDepIt = ReverseLocalDeps.find(RemInst); 407 if (ReverseDepIt != ReverseLocalDeps.end()) { 408 SmallPtrSet<Instruction*, 4> &ReverseDeps = ReverseDepIt->second; 409 // RemInst can't be the terminator if it has stuff depending on it. 410 assert(!ReverseDeps.empty() && !isa<TerminatorInst>(RemInst) && 411 "Nothing can locally depend on a terminator"); 412 413 // Anything that was locally dependent on RemInst is now going to be 414 // dependent on the instruction after RemInst. It will have the dirty flag 415 // set so it will rescan. This saves having to scan the entire block to get 416 // to this point. 417 Instruction *NewDepInst = next(BasicBlock::iterator(RemInst)); 418 419 for (SmallPtrSet<Instruction*, 4>::iterator I = ReverseDeps.begin(), 420 E = ReverseDeps.end(); I != E; ++I) { 421 Instruction *InstDependingOnRemInst = *I; 422 assert(InstDependingOnRemInst != RemInst && 423 "Already removed our local dep info"); 424 425 LocalDeps[InstDependingOnRemInst] = MemDepResult::getDirty(NewDepInst); 426 427 // Make sure to remember that new things depend on NewDepInst. 428 ReverseDepsToAdd.push_back(std::make_pair(NewDepInst, 429 InstDependingOnRemInst)); 430 } 431 432 ReverseLocalDeps.erase(ReverseDepIt); 433 434 // Add new reverse deps after scanning the set, to avoid invalidating the 435 // 'ReverseDeps' reference. 436 while (!ReverseDepsToAdd.empty()) { 437 ReverseLocalDeps[ReverseDepsToAdd.back().first] 438 .insert(ReverseDepsToAdd.back().second); 439 ReverseDepsToAdd.pop_back(); 440 } 441 } 442 443 ReverseDepIt = ReverseNonLocalDeps.find(RemInst); 444 if (ReverseDepIt != ReverseNonLocalDeps.end()) { 445 SmallPtrSet<Instruction*, 4>& set = ReverseDepIt->second; 446 for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end(); 447 I != E; ++I) { 448 assert(*I != RemInst && "Already removed NonLocalDep info for RemInst"); 449 450 PerInstNLInfo &INLD = NonLocalDeps[*I]; 451 // The information is now dirty! 452 INLD.second = true; 453 454 for (NonLocalDepInfo::iterator DI = INLD.first.begin(), 455 DE = INLD.first.end(); DI != DE; ++DI) { 456 if (DI->second.getInst() != RemInst) continue; 457 458 // Convert to a dirty entry for the subsequent instruction. 459 Instruction *NextI = 0; 460 if (!RemInst->isTerminator()) { 461 NextI = next(BasicBlock::iterator(RemInst)); 462 ReverseDepsToAdd.push_back(std::make_pair(NextI, *I)); 463 } 464 DI->second = MemDepResult::getDirty(NextI); 465 } 466 } 467 468 ReverseNonLocalDeps.erase(ReverseDepIt); 469 470 // Add new reverse deps after scanning the set, to avoid invalidating 'Set' 471 while (!ReverseDepsToAdd.empty()) { 472 ReverseNonLocalDeps[ReverseDepsToAdd.back().first] 473 .insert(ReverseDepsToAdd.back().second); 474 ReverseDepsToAdd.pop_back(); 475 } 476 } 477 478 assert(!NonLocalDeps.count(RemInst) && "RemInst got reinserted?"); 479 AA->deleteValue(RemInst); 480 DEBUG(verifyRemoved(RemInst)); 481} 482 483/// verifyRemoved - Verify that the specified instruction does not occur 484/// in our internal data structures. 485void MemoryDependenceAnalysis::verifyRemoved(Instruction *D) const { 486 for (LocalDepMapType::const_iterator I = LocalDeps.begin(), 487 E = LocalDeps.end(); I != E; ++I) { 488 assert(I->first != D && "Inst occurs in data structures"); 489 assert(I->second.getInst() != D && 490 "Inst occurs in data structures"); 491 } 492 493 for (NonLocalDepMapType::const_iterator I = NonLocalDeps.begin(), 494 E = NonLocalDeps.end(); I != E; ++I) { 495 assert(I->first != D && "Inst occurs in data structures"); 496 const PerInstNLInfo &INLD = I->second; 497 for (NonLocalDepInfo::const_iterator II = INLD.first.begin(), 498 EE = INLD.first.end(); II != EE; ++II) 499 assert(II->second.getInst() != D && "Inst occurs in data structures"); 500 } 501 502 for (ReverseDepMapType::const_iterator I = ReverseLocalDeps.begin(), 503 E = ReverseLocalDeps.end(); I != E; ++I) { 504 assert(I->first != D && "Inst occurs in data structures"); 505 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(), 506 EE = I->second.end(); II != EE; ++II) 507 assert(*II != D && "Inst occurs in data structures"); 508 } 509 510 for (ReverseDepMapType::const_iterator I = ReverseNonLocalDeps.begin(), 511 E = ReverseNonLocalDeps.end(); 512 I != E; ++I) { 513 assert(I->first != D && "Inst occurs in data structures"); 514 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(), 515 EE = I->second.end(); II != EE; ++II) 516 assert(*II != D && "Inst occurs in data structures"); 517 } 518} 519