MemoryDependenceAnalysis.cpp revision 4f8c18c7c757875cfa45383e7cf33d65d2c4d564
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 48/// getCallSiteDependency - Private helper for finding the local dependencies 49/// of a call site. 50MemDepResult MemoryDependenceAnalysis:: 51getCallSiteDependency(CallSite C, BasicBlock::iterator ScanIt, 52 BasicBlock *BB) { 53 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 54 TargetData &TD = getAnalysis<TargetData>(); 55 56 // Walk backwards through the block, looking for dependencies 57 while (ScanIt != BB->begin()) { 58 Instruction *Inst = --ScanIt; 59 60 // If this inst is a memory op, get the pointer it accessed 61 Value *Pointer = 0; 62 uint64_t PointerSize = 0; 63 if (StoreInst *S = dyn_cast<StoreInst>(Inst)) { 64 Pointer = S->getPointerOperand(); 65 PointerSize = TD.getTypeStoreSize(S->getOperand(0)->getType()); 66 } else if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) { 67 Pointer = AI; 68 if (ConstantInt *C = dyn_cast<ConstantInt>(AI->getArraySize())) 69 // Use ABI size (size between elements), not store size (size of one 70 // element without padding). 71 PointerSize = C->getZExtValue() * 72 TD.getABITypeSize(AI->getAllocatedType()); 73 else 74 PointerSize = ~0UL; 75 } else if (VAArgInst *V = dyn_cast<VAArgInst>(Inst)) { 76 Pointer = V->getOperand(0); 77 PointerSize = TD.getTypeStoreSize(V->getType()); 78 } else if (FreeInst *F = dyn_cast<FreeInst>(Inst)) { 79 Pointer = F->getPointerOperand(); 80 81 // FreeInsts erase the entire structure 82 PointerSize = ~0UL; 83 } else if (isa<CallInst>(Inst) || isa<InvokeInst>(Inst)) { 84 if (AA.getModRefBehavior(CallSite::get(Inst)) == 85 AliasAnalysis::DoesNotAccessMemory) 86 continue; 87 return MemDepResult::get(Inst); 88 } else 89 continue; 90 91 if (AA.getModRefInfo(C, Pointer, PointerSize) != AliasAnalysis::NoModRef) 92 return MemDepResult::get(Inst); 93 } 94 95 // No dependence found. 96 return MemDepResult::getNonLocal(); 97} 98 99/// getNonLocalDependency - Perform a full dependency query for the 100/// specified instruction, returning the set of blocks that the value is 101/// potentially live across. The returned set of results will include a 102/// "NonLocal" result for all blocks where the value is live across. 103/// 104/// This method assumes the instruction returns a "nonlocal" dependency 105/// within its own block. 106/// 107void MemoryDependenceAnalysis:: 108getNonLocalDependency(Instruction *QueryInst, 109 SmallVectorImpl<std::pair<BasicBlock*, 110 MemDepResult> > &Result) { 111 assert(getDependency(QueryInst).isNonLocal() && 112 "getNonLocalDependency should only be used on insts with non-local deps!"); 113 DenseMap<BasicBlock*, DepResultTy> &Cache = NonLocalDeps[QueryInst]; 114 115 /// DirtyBlocks - This is the set of blocks that need to be recomputed. In 116 /// the cached case, this can happen due to instructions being deleted etc. In 117 /// the uncached case, this starts out as the set of predecessors we care 118 /// about. 119 SmallVector<BasicBlock*, 32> DirtyBlocks; 120 121 if (!Cache.empty()) { 122 // If we already have a partially computed set of results, scan them to 123 // determine what is dirty, seeding our initial DirtyBlocks worklist. 124 // FIXME: In the "don't need to be updated" case, this is expensive, why not 125 // have a per-"cache" flag saying it is undirty? 126 for (DenseMap<BasicBlock*, DepResultTy>::iterator I = Cache.begin(), 127 E = Cache.end(); I != E; ++I) 128 if (I->second.getInt() == Dirty) 129 DirtyBlocks.push_back(I->first); 130 131 NumCacheNonLocal++; 132 133 //cerr << "CACHED CASE: " << DirtyBlocks.size() << " dirty: " 134 // << Cache.size() << " cached: " << *QueryInst; 135 } else { 136 // Seed DirtyBlocks with each of the preds of QueryInst's block. 137 BasicBlock *QueryBB = QueryInst->getParent(); 138 DirtyBlocks.append(pred_begin(QueryBB), pred_end(QueryBB)); 139 NumUncacheNonLocal++; 140 } 141 142 143 // Iterate while we still have blocks to update. 144 while (!DirtyBlocks.empty()) { 145 BasicBlock *DirtyBB = DirtyBlocks.back(); 146 DirtyBlocks.pop_back(); 147 148 // Get the entry for this block. Note that this relies on DepResultTy 149 // default initializing to Dirty. 150 DepResultTy &DirtyBBEntry = Cache[DirtyBB]; 151 152 // If DirtyBBEntry isn't dirty, it ended up on the worklist multiple times. 153 if (DirtyBBEntry.getInt() != Dirty) continue; 154 155 // Find out if this block has a local dependency for QueryInst. 156 // FIXME: If the dirty entry has an instruction pointer, scan from it! 157 // FIXME: Don't convert back and forth for MemDepResult <-> DepResultTy. 158 159 // If the dirty entry has a pointer, start scanning from it so we don't have 160 // to rescan the entire block. 161 BasicBlock::iterator ScanPos = DirtyBB->end(); 162 if (Instruction *Inst = DirtyBBEntry.getPointer()) 163 ScanPos = Inst; 164 165 DirtyBBEntry = ConvFromResult(getDependencyFrom(QueryInst, ScanPos, 166 DirtyBB)); 167 168 // If the block has a dependency (i.e. it isn't completely transparent to 169 // the value), remember it! 170 if (DirtyBBEntry.getInt() != NonLocal) { 171 // Keep the ReverseNonLocalDeps map up to date so we can efficiently 172 // update this when we remove instructions. 173 if (Instruction *Inst = DirtyBBEntry.getPointer()) 174 ReverseNonLocalDeps[Inst].insert(QueryInst); 175 continue; 176 } 177 178 // If the block *is* completely transparent to the load, we need to check 179 // the predecessors of this block. Add them to our worklist. 180 DirtyBlocks.append(pred_begin(DirtyBB), pred_end(DirtyBB)); 181 } 182 183 184 // Copy the result into the output set. 185 for (DenseMap<BasicBlock*, DepResultTy>::iterator I = Cache.begin(), 186 E = Cache.end(); I != E; ++I) 187 Result.push_back(std::make_pair(I->first, ConvToResult(I->second))); 188} 189 190/// getDependency - Return the instruction on which a memory operation 191/// depends. The local parameter indicates if the query should only 192/// evaluate dependencies within the same basic block. 193MemDepResult MemoryDependenceAnalysis:: 194getDependencyFrom(Instruction *QueryInst, BasicBlock::iterator ScanIt, 195 BasicBlock *BB) { 196 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 197 TargetData &TD = getAnalysis<TargetData>(); 198 199 // Get the pointer value for which dependence will be determined 200 Value *MemPtr = 0; 201 uint64_t MemSize = 0; 202 bool MemVolatile = false; 203 204 if (StoreInst* S = dyn_cast<StoreInst>(QueryInst)) { 205 MemPtr = S->getPointerOperand(); 206 MemSize = TD.getTypeStoreSize(S->getOperand(0)->getType()); 207 MemVolatile = S->isVolatile(); 208 } else if (LoadInst* L = dyn_cast<LoadInst>(QueryInst)) { 209 MemPtr = L->getPointerOperand(); 210 MemSize = TD.getTypeStoreSize(L->getType()); 211 MemVolatile = L->isVolatile(); 212 } else if (VAArgInst* V = dyn_cast<VAArgInst>(QueryInst)) { 213 MemPtr = V->getOperand(0); 214 MemSize = TD.getTypeStoreSize(V->getType()); 215 } else if (FreeInst* F = dyn_cast<FreeInst>(QueryInst)) { 216 MemPtr = F->getPointerOperand(); 217 // FreeInsts erase the entire structure, not just a field. 218 MemSize = ~0UL; 219 } else if (isa<CallInst>(QueryInst) || isa<InvokeInst>(QueryInst)) 220 return getCallSiteDependency(CallSite::get(QueryInst), ScanIt, BB); 221 else // Non-memory instructions depend on nothing. 222 return MemDepResult::getNone(); 223 224 // Walk backwards through the basic block, looking for dependencies 225 while (ScanIt != BB->begin()) { 226 Instruction *Inst = --ScanIt; 227 228 // If the access is volatile and this is a volatile load/store, return a 229 // dependence. 230 if (MemVolatile && 231 ((isa<LoadInst>(Inst) && cast<LoadInst>(Inst)->isVolatile()) || 232 (isa<StoreInst>(Inst) && cast<StoreInst>(Inst)->isVolatile()))) 233 return MemDepResult::get(Inst); 234 235 // MemDep is broken w.r.t. loads: it says that two loads of the same pointer 236 // depend on each other. :( 237 // FIXME: ELIMINATE THIS! 238 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 239 Value *Pointer = L->getPointerOperand(); 240 uint64_t PointerSize = TD.getTypeStoreSize(L->getType()); 241 242 // If we found a pointer, check if it could be the same as our pointer 243 AliasAnalysis::AliasResult R = 244 AA.alias(Pointer, PointerSize, MemPtr, MemSize); 245 246 if (R == AliasAnalysis::NoAlias) 247 continue; 248 249 // May-alias loads don't depend on each other without a dependence. 250 if (isa<LoadInst>(QueryInst) && R == AliasAnalysis::MayAlias) 251 continue; 252 return MemDepResult::get(Inst); 253 } 254 255 // FIXME: This claims that an access depends on the allocation. This may 256 // make sense, but is dubious at best. It would be better to fix GVN to 257 // handle a 'None' Query. 258 if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) { 259 Value *Pointer = AI; 260 uint64_t PointerSize; 261 if (ConstantInt *C = dyn_cast<ConstantInt>(AI->getArraySize())) 262 // Use ABI size (size between elements), not store size (size of one 263 // element without padding). 264 PointerSize = C->getZExtValue() * 265 TD.getABITypeSize(AI->getAllocatedType()); 266 else 267 PointerSize = ~0UL; 268 269 AliasAnalysis::AliasResult R = 270 AA.alias(Pointer, PointerSize, MemPtr, MemSize); 271 272 if (R == AliasAnalysis::NoAlias) 273 continue; 274 return MemDepResult::get(Inst); 275 } 276 277 278 // See if this instruction mod/ref's the pointer. 279 AliasAnalysis::ModRefResult MRR = AA.getModRefInfo(Inst, MemPtr, MemSize); 280 281 if (MRR == AliasAnalysis::NoModRef) 282 continue; 283 284 // Loads don't depend on read-only instructions. 285 if (isa<LoadInst>(QueryInst) && MRR == AliasAnalysis::Ref) 286 continue; 287 288 // Otherwise, there is a dependence. 289 return MemDepResult::get(Inst); 290 } 291 292 // If we found nothing, return the non-local flag. 293 return MemDepResult::getNonLocal(); 294} 295 296/// getDependency - Return the instruction on which a memory operation 297/// depends. 298MemDepResult MemoryDependenceAnalysis::getDependency(Instruction *QueryInst) { 299 Instruction *ScanPos = QueryInst; 300 301 // Check for a cached result 302 DepResultTy &LocalCache = LocalDeps[QueryInst]; 303 304 // If the cached entry is non-dirty, just return it. Note that this depends 305 // on DepResultTy's default constructing to 'dirty'. 306 if (LocalCache.getInt() != Dirty) 307 return ConvToResult(LocalCache); 308 309 // Otherwise, if we have a dirty entry, we know we can start the scan at that 310 // instruction, which may save us some work. 311 if (Instruction *Inst = LocalCache.getPointer()) 312 ScanPos = Inst; 313 314 // Do the scan. 315 MemDepResult Res = 316 getDependencyFrom(QueryInst, ScanPos, QueryInst->getParent()); 317 318 // Remember the result! 319 // FIXME: Don't convert back and forth! Make a shared helper function. 320 LocalCache = ConvFromResult(Res); 321 if (Instruction *I = Res.getInst()) 322 ReverseLocalDeps[I].insert(QueryInst); 323 324 return Res; 325} 326 327/// removeInstruction - Remove an instruction from the dependence analysis, 328/// updating the dependence of instructions that previously depended on it. 329/// This method attempts to keep the cache coherent using the reverse map. 330void MemoryDependenceAnalysis::removeInstruction(Instruction *RemInst) { 331 // Walk through the Non-local dependencies, removing this one as the value 332 // for any cached queries. 333 for (DenseMap<BasicBlock*, DepResultTy>::iterator DI = 334 NonLocalDeps[RemInst].begin(), DE = NonLocalDeps[RemInst].end(); 335 DI != DE; ++DI) 336 if (Instruction *Inst = DI->second.getPointer()) 337 ReverseNonLocalDeps[Inst].erase(RemInst); 338 339 // Shortly after this, we will look for things that depend on RemInst. In 340 // order to update these, we'll need a new dependency to base them on. We 341 // could completely delete any entries that depend on this, but it is better 342 // to make a more accurate approximation where possible. Compute that better 343 // approximation if we can. 344 DepResultTy NewDependency; 345 346 // If we have a cached local dependence query for this instruction, remove it. 347 // 348 LocalDepMapType::iterator LocalDepEntry = LocalDeps.find(RemInst); 349 if (LocalDepEntry != LocalDeps.end()) { 350 DepResultTy LocalDep = LocalDepEntry->second; 351 352 // Remove this local dependency info. 353 LocalDeps.erase(LocalDepEntry); 354 355 // Remove us from DepInst's reverse set now that the local dep info is gone. 356 if (Instruction *Inst = LocalDep.getPointer()) 357 ReverseLocalDeps[Inst].erase(RemInst); 358 359 // If we have unconfirmed info, don't trust it. 360 if (LocalDep.getInt() != Dirty) { 361 // If we have a confirmed non-local flag, use it. 362 if (LocalDep.getInt() == NonLocal || LocalDep.getInt() == None) { 363 // The only time this dependency is confirmed is if it is non-local. 364 NewDependency = LocalDep; 365 } else { 366 // If we have dep info for RemInst, set them to it. 367 Instruction *NDI = next(BasicBlock::iterator(LocalDep.getPointer())); 368 if (NDI != RemInst) // Don't use RemInst for the new dependency! 369 NewDependency = DepResultTy(NDI, Dirty); 370 } 371 } 372 } 373 374 // If we don't already have a local dependency answer for this instruction, 375 // use the immediate successor of RemInst. We use the successor because 376 // getDependence starts by checking the immediate predecessor of what is in 377 // the cache. 378 if (NewDependency == DepResultTy(0, Dirty)) 379 NewDependency = DepResultTy(next(BasicBlock::iterator(RemInst)), Dirty); 380 381 // Loop over all of the things that depend on the instruction we're removing. 382 // 383 SmallVector<std::pair<Instruction*, Instruction*>, 8> ReverseDepsToAdd; 384 385 ReverseDepMapType::iterator ReverseDepIt = ReverseLocalDeps.find(RemInst); 386 if (ReverseDepIt != ReverseLocalDeps.end()) { 387 SmallPtrSet<Instruction*, 4> &ReverseDeps = ReverseDepIt->second; 388 for (SmallPtrSet<Instruction*, 4>::iterator I = ReverseDeps.begin(), 389 E = ReverseDeps.end(); I != E; ++I) { 390 Instruction *InstDependingOnRemInst = *I; 391 392 // If we thought the instruction depended on itself (possible for 393 // unconfirmed dependencies) ignore the update. 394 if (InstDependingOnRemInst == RemInst) continue; 395 396 // Insert the new dependencies. 397 // FIXME: DEPENDENCIES ARE NOT TRANSITIVE! 398 //cerr << "FOO:\n"; 399 //RemInst->dump(); 400 //InstDependingOnRemInst->dump(); 401 LocalDeps[InstDependingOnRemInst] = NewDependency; 402 403 // If our NewDependency is an instruction, make sure to remember that new 404 // things depend on it. 405 if (Instruction *Inst = NewDependency.getPointer()) { 406 assert(Inst != RemInst); 407 ReverseDepsToAdd.push_back(std::make_pair(Inst, 408 InstDependingOnRemInst)); 409 } 410 } 411 412 ReverseLocalDeps.erase(ReverseDepIt); 413 414 // Add new reverse deps after scanning the set, to avoid invalidating the 415 // 'ReverseDeps' reference. 416 while (!ReverseDepsToAdd.empty()) { 417 ReverseLocalDeps[ReverseDepsToAdd.back().first] 418 .insert(ReverseDepsToAdd.back().second); 419 ReverseDepsToAdd.pop_back(); 420 } 421 } 422 423 ReverseDepIt = ReverseNonLocalDeps.find(RemInst); 424 if (ReverseDepIt != ReverseNonLocalDeps.end()) { 425 SmallPtrSet<Instruction*, 4>& set = ReverseDepIt->second; 426 for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end(); 427 I != E; ++I) 428 for (DenseMap<BasicBlock*, DepResultTy>::iterator 429 DI = NonLocalDeps[*I].begin(), DE = NonLocalDeps[*I].end(); 430 DI != DE; ++DI) 431 if (DI->second.getPointer() == RemInst) { 432 // Convert to a dirty entry for the subsequent instruction. 433 DI->second.setInt(Dirty); 434 if (RemInst->isTerminator()) 435 DI->second.setPointer(0); 436 else { 437 Instruction *NextI = next(BasicBlock::iterator(RemInst)); 438 DI->second.setPointer(NextI); 439 assert(NextI != RemInst); 440 ReverseDepsToAdd.push_back(std::make_pair(NextI, *I)); 441 } 442 } 443 444 ReverseNonLocalDeps.erase(ReverseDepIt); 445 446 // Add new reverse deps after scanning the set, to avoid invalidating 'Set' 447 while (!ReverseDepsToAdd.empty()) { 448 ReverseNonLocalDeps[ReverseDepsToAdd.back().first] 449 .insert(ReverseDepsToAdd.back().second); 450 ReverseDepsToAdd.pop_back(); 451 } 452 } 453 454 NonLocalDeps.erase(RemInst); 455 getAnalysis<AliasAnalysis>().deleteValue(RemInst); 456 DEBUG(verifyRemoved(RemInst)); 457} 458 459/// verifyRemoved - Verify that the specified instruction does not occur 460/// in our internal data structures. 461void MemoryDependenceAnalysis::verifyRemoved(Instruction *D) const { 462 for (LocalDepMapType::const_iterator I = LocalDeps.begin(), 463 E = LocalDeps.end(); I != E; ++I) { 464 assert(I->first != D && "Inst occurs in data structures"); 465 assert(I->second.getPointer() != D && 466 "Inst occurs in data structures"); 467 } 468 469 for (NonLocalDepMapType::const_iterator I = NonLocalDeps.begin(), 470 E = NonLocalDeps.end(); I != E; ++I) { 471 assert(I->first != D && "Inst occurs in data structures"); 472 for (DenseMap<BasicBlock*, DepResultTy>::iterator II = I->second.begin(), 473 EE = I->second.end(); II != EE; ++II) 474 assert(II->second.getPointer() != D && "Inst occurs in data structures"); 475 } 476 477 for (ReverseDepMapType::const_iterator I = ReverseLocalDeps.begin(), 478 E = ReverseLocalDeps.end(); I != E; ++I) 479 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(), 480 EE = I->second.end(); II != EE; ++II) 481 assert(*II != D && "Inst occurs in data structures"); 482 483 for (ReverseDepMapType::const_iterator I = ReverseNonLocalDeps.begin(), 484 E = ReverseNonLocalDeps.end(); 485 I != E; ++I) 486 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(), 487 EE = I->second.end(); II != EE; ++II) 488 assert(*II != D && "Inst occurs in data structures"); 489} 490