DeadStoreElimination.cpp revision a116623e06d0cf833cbefd4921dbca8581f1c806
1//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===// 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 a trivial dead store elimination that only considers 11// basic-block local redundant stores. 12// 13// FIXME: This should eventually be extended to be a post-dominator tree 14// traversal. Doing so would be pretty trivial. 15// 16//===----------------------------------------------------------------------===// 17 18#define DEBUG_TYPE "dse" 19#include "llvm/Transforms/Scalar.h" 20#include "llvm/Constants.h" 21#include "llvm/Function.h" 22#include "llvm/GlobalVariable.h" 23#include "llvm/Instructions.h" 24#include "llvm/IntrinsicInst.h" 25#include "llvm/Pass.h" 26#include "llvm/Analysis/AliasAnalysis.h" 27#include "llvm/Analysis/CaptureTracking.h" 28#include "llvm/Analysis/Dominators.h" 29#include "llvm/Analysis/MemoryBuiltins.h" 30#include "llvm/Analysis/MemoryDependenceAnalysis.h" 31#include "llvm/Analysis/ValueTracking.h" 32#include "llvm/Target/TargetData.h" 33#include "llvm/Transforms/Utils/Local.h" 34#include "llvm/Support/Debug.h" 35#include "llvm/ADT/SmallPtrSet.h" 36#include "llvm/ADT/Statistic.h" 37#include "llvm/ADT/STLExtras.h" 38using namespace llvm; 39 40STATISTIC(NumFastStores, "Number of stores deleted"); 41STATISTIC(NumFastOther , "Number of other instrs removed"); 42 43namespace { 44 struct DSE : public FunctionPass { 45 AliasAnalysis *AA; 46 MemoryDependenceAnalysis *MD; 47 DominatorTree *DT; 48 49 static char ID; // Pass identification, replacement for typeid 50 DSE() : FunctionPass(ID), AA(0), MD(0), DT(0) { 51 initializeDSEPass(*PassRegistry::getPassRegistry()); 52 } 53 54 virtual bool runOnFunction(Function &F) { 55 AA = &getAnalysis<AliasAnalysis>(); 56 MD = &getAnalysis<MemoryDependenceAnalysis>(); 57 DT = &getAnalysis<DominatorTree>(); 58 59 bool Changed = false; 60 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 61 // Only check non-dead blocks. Dead blocks may have strange pointer 62 // cycles that will confuse alias analysis. 63 if (DT->isReachableFromEntry(I)) 64 Changed |= runOnBasicBlock(*I); 65 66 AA = 0; MD = 0; DT = 0; 67 return Changed; 68 } 69 70 bool runOnBasicBlock(BasicBlock &BB); 71 bool HandleFree(CallInst *F); 72 bool handleEndBlock(BasicBlock &BB); 73 void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc, 74 SmallPtrSet<Value*, 16> &DeadStackObjects); 75 76 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 77 AU.setPreservesCFG(); 78 AU.addRequired<DominatorTree>(); 79 AU.addRequired<AliasAnalysis>(); 80 AU.addRequired<MemoryDependenceAnalysis>(); 81 AU.addPreserved<AliasAnalysis>(); 82 AU.addPreserved<DominatorTree>(); 83 AU.addPreserved<MemoryDependenceAnalysis>(); 84 } 85 }; 86} 87 88char DSE::ID = 0; 89INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false) 90INITIALIZE_PASS_DEPENDENCY(DominatorTree) 91INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis) 92INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 93INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false) 94 95FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); } 96 97//===----------------------------------------------------------------------===// 98// Helper functions 99//===----------------------------------------------------------------------===// 100 101/// DeleteDeadInstruction - Delete this instruction. Before we do, go through 102/// and zero out all the operands of this instruction. If any of them become 103/// dead, delete them and the computation tree that feeds them. 104/// 105/// If ValueSet is non-null, remove any deleted instructions from it as well. 106/// 107static void DeleteDeadInstruction(Instruction *I, 108 MemoryDependenceAnalysis &MD, 109 SmallPtrSet<Value*, 16> *ValueSet = 0) { 110 SmallVector<Instruction*, 32> NowDeadInsts; 111 112 NowDeadInsts.push_back(I); 113 --NumFastOther; 114 115 // Before we touch this instruction, remove it from memdep! 116 do { 117 Instruction *DeadInst = NowDeadInsts.pop_back_val(); 118 ++NumFastOther; 119 120 // This instruction is dead, zap it, in stages. Start by removing it from 121 // MemDep, which needs to know the operands and needs it to be in the 122 // function. 123 MD.removeInstruction(DeadInst); 124 125 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) { 126 Value *Op = DeadInst->getOperand(op); 127 DeadInst->setOperand(op, 0); 128 129 // If this operand just became dead, add it to the NowDeadInsts list. 130 if (!Op->use_empty()) continue; 131 132 if (Instruction *OpI = dyn_cast<Instruction>(Op)) 133 if (isInstructionTriviallyDead(OpI)) 134 NowDeadInsts.push_back(OpI); 135 } 136 137 DeadInst->eraseFromParent(); 138 139 if (ValueSet) ValueSet->erase(DeadInst); 140 } while (!NowDeadInsts.empty()); 141} 142 143 144/// hasMemoryWrite - Does this instruction write some memory? This only returns 145/// true for things that we can analyze with other helpers below. 146static bool hasMemoryWrite(Instruction *I) { 147 if (isa<StoreInst>(I)) 148 return true; 149 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { 150 switch (II->getIntrinsicID()) { 151 default: 152 return false; 153 case Intrinsic::memset: 154 case Intrinsic::memmove: 155 case Intrinsic::memcpy: 156 case Intrinsic::init_trampoline: 157 case Intrinsic::lifetime_end: 158 return true; 159 } 160 } 161 return false; 162} 163 164/// getLocForWrite - Return a Location stored to by the specified instruction. 165/// If isRemovable returns true, this function and getLocForRead completely 166/// describe the memory operations for this instruction. 167static AliasAnalysis::Location 168getLocForWrite(Instruction *Inst, AliasAnalysis &AA) { 169 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) 170 return AA.getLocation(SI); 171 172 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) { 173 // memcpy/memmove/memset. 174 AliasAnalysis::Location Loc = AA.getLocationForDest(MI); 175 // If we don't have target data around, an unknown size in Location means 176 // that we should use the size of the pointee type. This isn't valid for 177 // memset/memcpy, which writes more than an i8. 178 if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0) 179 return AliasAnalysis::Location(); 180 return Loc; 181 } 182 183 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst); 184 if (II == 0) return AliasAnalysis::Location(); 185 186 switch (II->getIntrinsicID()) { 187 default: return AliasAnalysis::Location(); // Unhandled intrinsic. 188 case Intrinsic::init_trampoline: 189 // If we don't have target data around, an unknown size in Location means 190 // that we should use the size of the pointee type. This isn't valid for 191 // init.trampoline, which writes more than an i8. 192 if (AA.getTargetData() == 0) return AliasAnalysis::Location(); 193 194 // FIXME: We don't know the size of the trampoline, so we can't really 195 // handle it here. 196 return AliasAnalysis::Location(II->getArgOperand(0)); 197 case Intrinsic::lifetime_end: { 198 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); 199 return AliasAnalysis::Location(II->getArgOperand(1), Len); 200 } 201 } 202} 203 204/// getLocForRead - Return the location read by the specified "hasMemoryWrite" 205/// instruction if any. 206static AliasAnalysis::Location 207getLocForRead(Instruction *Inst, AliasAnalysis &AA) { 208 assert(hasMemoryWrite(Inst) && "Unknown instruction case"); 209 210 // The only instructions that both read and write are the mem transfer 211 // instructions (memcpy/memmove). 212 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst)) 213 return AA.getLocationForSource(MTI); 214 return AliasAnalysis::Location(); 215} 216 217 218/// isRemovable - If the value of this instruction and the memory it writes to 219/// is unused, may we delete this instruction? 220static bool isRemovable(Instruction *I) { 221 // Don't remove volatile/atomic stores. 222 if (StoreInst *SI = dyn_cast<StoreInst>(I)) 223 return SI->isUnordered(); 224 225 IntrinsicInst *II = cast<IntrinsicInst>(I); 226 switch (II->getIntrinsicID()) { 227 default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate"); 228 case Intrinsic::lifetime_end: 229 // Never remove dead lifetime_end's, e.g. because it is followed by a 230 // free. 231 return false; 232 case Intrinsic::init_trampoline: 233 // Always safe to remove init_trampoline. 234 return true; 235 236 case Intrinsic::memset: 237 case Intrinsic::memmove: 238 case Intrinsic::memcpy: 239 // Don't remove volatile memory intrinsics. 240 return !cast<MemIntrinsic>(II)->isVolatile(); 241 } 242} 243 244 245/// isShortenable - Returns true if this instruction can be safely shortened in 246/// length. 247static bool isShortenable(Instruction *I) { 248 // Don't shorten stores for now 249 if (isa<StoreInst>(I)) 250 return false; 251 252 IntrinsicInst *II = cast<IntrinsicInst>(I); 253 switch (II->getIntrinsicID()) { 254 default: return false; 255 case Intrinsic::memset: 256 case Intrinsic::memcpy: 257 // Do shorten memory intrinsics. 258 return true; 259 } 260} 261 262 263/// isMemset - Returns true if this instruction is an intrinsic memset 264static bool isMemset(Instruction *I) { 265 IntrinsicInst *II = dyn_cast<IntrinsicInst>(I); 266 return II && II->getIntrinsicID() == Intrinsic::memset; 267} 268 269/// getStoredPointerOperand - Return the pointer that is being written to. 270static Value *getStoredPointerOperand(Instruction *I) { 271 if (StoreInst *SI = dyn_cast<StoreInst>(I)) 272 return SI->getPointerOperand(); 273 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) 274 return MI->getDest(); 275 276 IntrinsicInst *II = cast<IntrinsicInst>(I); 277 switch (II->getIntrinsicID()) { 278 default: llvm_unreachable("Unexpected intrinsic!"); 279 case Intrinsic::init_trampoline: 280 return II->getArgOperand(0); 281 } 282} 283 284static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) { 285 const TargetData *TD = AA.getTargetData(); 286 287 if (const CallInst *CI = extractMallocCall(V)) { 288 if (const ConstantInt *C = dyn_cast<ConstantInt>(CI->getArgOperand(0))) 289 return C->getZExtValue(); 290 } 291 292 if (TD == 0) 293 return AliasAnalysis::UnknownSize; 294 295 if (const AllocaInst *A = dyn_cast<AllocaInst>(V)) { 296 // Get size information for the alloca 297 if (const ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize())) 298 return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType()); 299 } 300 301 if (const Argument *A = dyn_cast<Argument>(V)) { 302 if (A->hasByValAttr()) 303 if (PointerType *PT = dyn_cast<PointerType>(A->getType())) 304 return TD->getTypeAllocSize(PT->getElementType()); 305 } 306 307 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 308 if (!GV->mayBeOverridden()) 309 return TD->getTypeAllocSize(GV->getType()->getElementType()); 310 } 311 312 return AliasAnalysis::UnknownSize; 313} 314 315namespace { 316 enum OverwriteResult 317 { 318 OverwriteComplete, 319 OverwriteEnd, 320 OverwriteStart, 321 OverwriteUnknown 322 }; 323} 324 325/// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location 326/// completely overwrites a store to the 'Earlier' location. 327/// 'OverwriteEnd' if the end of the 'Earlier' location is completely 328/// overwritten by 'Later', 'OverWriteStart' if the start of 'Earlier' 329/// is completely overwritten by 'Later' or 'OverwriteUnknown' if nothing 330/// can be determined 331static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later, 332 const AliasAnalysis::Location &Earlier, 333 AliasAnalysis &AA, 334 int64_t &EarlierOff, 335 int64_t &LaterOff) { 336 const Value *P1 = Earlier.Ptr->stripPointerCasts(); 337 const Value *P2 = Later.Ptr->stripPointerCasts(); 338 339 // If the start pointers are the same, we just have to compare sizes to see if 340 // the later store was larger than the earlier store. 341 if (P1 == P2) { 342 // If we don't know the sizes of either access, then we can't do a 343 // comparison. 344 if (Later.Size == AliasAnalysis::UnknownSize || 345 Earlier.Size == AliasAnalysis::UnknownSize) { 346 // If we have no TargetData information around, then the size of the store 347 // is inferrable from the pointee type. If they are the same type, then 348 // we know that the store is safe. 349 if (AA.getTargetData() == 0 && 350 Later.Ptr->getType() == Earlier.Ptr->getType()) 351 return OverwriteComplete; 352 353 return OverwriteUnknown; 354 } 355 356 // Make sure that the Later size is >= the Earlier size. 357 if (Later.Size >= Earlier.Size) 358 return OverwriteComplete; 359 } 360 361 // Otherwise, we have to have size information, and the later store has to be 362 // larger than the earlier one. 363 if (Later.Size == AliasAnalysis::UnknownSize || 364 Earlier.Size == AliasAnalysis::UnknownSize || 365 AA.getTargetData() == 0) 366 return OverwriteUnknown; 367 368 // Check to see if the later store is to the entire object (either a global, 369 // an alloca, or a byval argument). If so, then it clearly overwrites any 370 // other store to the same object. 371 const TargetData &TD = *AA.getTargetData(); 372 373 const Value *UO1 = GetUnderlyingObject(P1, &TD), 374 *UO2 = GetUnderlyingObject(P2, &TD); 375 376 // If we can't resolve the same pointers to the same object, then we can't 377 // analyze them at all. 378 if (UO1 != UO2) 379 return OverwriteUnknown; 380 381 // If the "Later" store is to a recognizable object, get its size. 382 uint64_t ObjectSize = getPointerSize(UO2, AA); 383 if (ObjectSize != AliasAnalysis::UnknownSize) 384 if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size) 385 return OverwriteComplete; 386 387 // Okay, we have stores to two completely different pointers. Try to 388 // decompose the pointer into a "base + constant_offset" form. If the base 389 // pointers are equal, then we can reason about the two stores. 390 EarlierOff = 0; 391 LaterOff = 0; 392 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD); 393 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD); 394 395 // If the base pointers still differ, we have two completely different stores. 396 if (BP1 != BP2) 397 return OverwriteUnknown; 398 399 // The later store completely overlaps the earlier store if: 400 // 401 // 1. Both start at the same offset and the later one's size is greater than 402 // or equal to the earlier one's, or 403 // 404 // |--earlier--| 405 // |-- later --| 406 // 407 // 2. The earlier store has an offset greater than the later offset, but which 408 // still lies completely within the later store. 409 // 410 // |--earlier--| 411 // |----- later ------| 412 // 413 // We have to be careful here as *Off is signed while *.Size is unsigned. 414 if (EarlierOff >= LaterOff && 415 Later.Size > Earlier.Size && 416 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size) 417 return OverwriteComplete; 418 419 // The other interesting case is if the later store overwrites the end of 420 // the earlier store 421 // 422 // |--earlier--| 423 // |-- later --| 424 // 425 // In this case we may want to trim the size of earlier to avoid generating 426 // writes to addresses which will definitely be overwritten later 427 if (LaterOff > EarlierOff && 428 LaterOff < int64_t(EarlierOff + Earlier.Size) && 429 int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size)) 430 return OverwriteEnd; 431 432 // The other interesting case is if the later store overwrites the end of 433 // the earlier store 434 // 435 // |--earlier--| 436 // |-- later --| 437 // 438 // In this case we may want to trim the size of earlier to avoid generating 439 // writes to addresses which will definitely be overwritten later 440 if (EarlierOff >= LaterOff && 441 EarlierOff < int64_t(LaterOff + Later.Size) && 442 int64_t(EarlierOff + Earlier.Size) >= int64_t(LaterOff + Later.Size)) { 443 LaterOff = LaterOff + Later.Size; 444 return OverwriteStart; 445 } 446 447 // Otherwise, they don't completely overlap. 448 return OverwriteUnknown; 449} 450 451/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a 452/// memory region into an identical pointer) then it doesn't actually make its 453/// input dead in the traditional sense. Consider this case: 454/// 455/// memcpy(A <- B) 456/// memcpy(A <- A) 457/// 458/// In this case, the second store to A does not make the first store to A dead. 459/// The usual situation isn't an explicit A<-A store like this (which can be 460/// trivially removed) but a case where two pointers may alias. 461/// 462/// This function detects when it is unsafe to remove a dependent instruction 463/// because the DSE inducing instruction may be a self-read. 464static bool isPossibleSelfRead(Instruction *Inst, 465 const AliasAnalysis::Location &InstStoreLoc, 466 Instruction *DepWrite, AliasAnalysis &AA) { 467 // Self reads can only happen for instructions that read memory. Get the 468 // location read. 469 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA); 470 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction. 471 472 // If the read and written loc obviously don't alias, it isn't a read. 473 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false; 474 475 // Okay, 'Inst' may copy over itself. However, we can still remove a the 476 // DepWrite instruction if we can prove that it reads from the same location 477 // as Inst. This handles useful cases like: 478 // memcpy(A <- B) 479 // memcpy(A <- B) 480 // Here we don't know if A/B may alias, but we do know that B/B are must 481 // aliases, so removing the first memcpy is safe (assuming it writes <= # 482 // bytes as the second one. 483 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA); 484 485 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr)) 486 return false; 487 488 // If DepWrite doesn't read memory or if we can't prove it is a must alias, 489 // then it can't be considered dead. 490 return true; 491} 492 493 494//===----------------------------------------------------------------------===// 495// DSE Pass 496//===----------------------------------------------------------------------===// 497 498bool DSE::runOnBasicBlock(BasicBlock &BB) { 499 bool MadeChange = false; 500 501 // Do a top-down walk on the BB. 502 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) { 503 Instruction *Inst = BBI++; 504 505 // Handle 'free' calls specially. 506 if (CallInst *F = isFreeCall(Inst)) { 507 MadeChange |= HandleFree(F); 508 continue; 509 } 510 511 // If we find something that writes memory, get its memory dependence. 512 if (!hasMemoryWrite(Inst)) 513 continue; 514 515 MemDepResult InstDep = MD->getDependency(Inst); 516 517 // Ignore any store where we can't find a local dependence. 518 // FIXME: cross-block DSE would be fun. :) 519 if (!InstDep.isDef() && !InstDep.isClobber()) 520 continue; 521 522 // If we're storing the same value back to a pointer that we just 523 // loaded from, then the store can be removed. 524 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 525 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) { 526 if (SI->getPointerOperand() == DepLoad->getPointerOperand() && 527 SI->getOperand(0) == DepLoad && isRemovable(SI)) { 528 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n " 529 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n'); 530 531 // DeleteDeadInstruction can delete the current instruction. Save BBI 532 // in case we need it. 533 WeakVH NextInst(BBI); 534 535 DeleteDeadInstruction(SI, *MD); 536 537 if (NextInst == 0) // Next instruction deleted. 538 BBI = BB.begin(); 539 else if (BBI != BB.begin()) // Revisit this instruction if possible. 540 --BBI; 541 ++NumFastStores; 542 MadeChange = true; 543 continue; 544 } 545 } 546 } 547 548 // Figure out what location is being stored to. 549 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA); 550 551 // If we didn't get a useful location, fail. 552 if (Loc.Ptr == 0) 553 continue; 554 555 while (InstDep.isDef() || InstDep.isClobber()) { 556 // Get the memory clobbered by the instruction we depend on. MemDep will 557 // skip any instructions that 'Loc' clearly doesn't interact with. If we 558 // end up depending on a may- or must-aliased load, then we can't optimize 559 // away the store and we bail out. However, if we depend on on something 560 // that overwrites the memory location we *can* potentially optimize it. 561 // 562 // Find out what memory location the dependent instruction stores. 563 Instruction *DepWrite = InstDep.getInst(); 564 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA); 565 // If we didn't get a useful location, or if it isn't a size, bail out. 566 if (DepLoc.Ptr == 0) 567 break; 568 569 // If we find a write that is a) removable (i.e., non-volatile), b) is 570 // completely obliterated by the store to 'Loc', and c) which we know that 571 // 'Inst' doesn't load from, then we can remove it. 572 if (isRemovable(DepWrite) && 573 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) { 574 int64_t InstWriteOffset, DepWriteOffset; 575 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA, 576 DepWriteOffset, InstWriteOffset); 577 if (OR == OverwriteComplete) { 578 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " 579 << *DepWrite << "\n KILLER: " << *Inst << '\n'); 580 581 // Delete the store and now-dead instructions that feed it. 582 DeleteDeadInstruction(DepWrite, *MD); 583 ++NumFastStores; 584 MadeChange = true; 585 586 // DeleteDeadInstruction can delete the current instruction in loop 587 // cases, reset BBI. 588 BBI = Inst; 589 if (BBI != BB.begin()) 590 --BBI; 591 break; 592 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) { 593 // TODO: base this on the target vector size so that if the earlier 594 // store was too small to get vector writes anyway then its likely 595 // a good idea to shorten it 596 // Power of 2 vector writes are probably always a bad idea to optimize 597 // as any store/memset/memcpy is likely using vector instructions so 598 // shortening it to not vector size is likely to be slower 599 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite); 600 unsigned DepWriteAlign = DepIntrinsic->getAlignment(); 601 if (llvm::isPowerOf2_64(InstWriteOffset) || 602 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) { 603 604 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: " 605 << *DepWrite << "\n KILLER (offset " 606 << InstWriteOffset << ", " 607 << DepLoc.Size << ")" 608 << *Inst << '\n'); 609 610 Value* DepWriteLength = DepIntrinsic->getLength(); 611 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(), 612 InstWriteOffset - 613 DepWriteOffset); 614 DepIntrinsic->setLength(TrimmedLength); 615 MadeChange = true; 616 } 617 } else if (OR == OverwriteStart && isMemset(DepWrite)) { 618 // TODO: base this on the target vector size so that if the earlier 619 // store was too small to get vector writes anyway then its likely 620 // a good idea to shorten it 621 // Power of 2 vector writes are probably always a bad idea to optimize 622 // as any store/memset/memcpy is likely using vector instructions so 623 // shortening it to not vector size is likely to be slower 624 // TODO: shorten memcpy and memmove by offsetting the source address. 625 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite); 626 unsigned DepWriteAlign = DepIntrinsic->getAlignment(); 627 if (llvm::isPowerOf2_64(InstWriteOffset) || 628 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) { 629 630 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW START: " 631 << *DepWrite << "\n KILLER (offset " 632 << InstWriteOffset << ", " 633 << DepWriteOffset << ", " 634 << DepLoc.Size << ")" 635 << *Inst << '\n'); 636 637 Value* DepWriteLength = DepIntrinsic->getLength(); 638 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(), 639 DepLoc.Size - 640 (InstWriteOffset - 641 DepWriteOffset)); 642 DepIntrinsic->setLength(TrimmedLength); 643 const TargetData *TD = AA->getTargetData(); 644 Type *IntPtrTy = TD->getIntPtrType(BB.getContext()); 645 Value* Offset = ConstantInt::get(IntPtrTy, 646 InstWriteOffset - DepWriteOffset); 647 // Offset the start of the memset with a GEP. As the memset type is 648 // i8* a GEP will do this without needing to use ptrtoint, etc. 649 Value *Dest = GetElementPtrInst::Create(DepIntrinsic->getRawDest(), 650 Offset, 651 "", 652 DepWrite); 653 DepIntrinsic->setDest(Dest); 654 MadeChange = true; 655 } 656 } 657 } 658 659 // If this is a may-aliased store that is clobbering the store value, we 660 // can keep searching past it for another must-aliased pointer that stores 661 // to the same location. For example, in: 662 // store -> P 663 // store -> Q 664 // store -> P 665 // we can remove the first store to P even though we don't know if P and Q 666 // alias. 667 if (DepWrite == &BB.front()) break; 668 669 // Can't look past this instruction if it might read 'Loc'. 670 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref) 671 break; 672 673 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB); 674 } 675 } 676 677 // If this block ends in a return, unwind, or unreachable, all allocas are 678 // dead at its end, which means stores to them are also dead. 679 if (BB.getTerminator()->getNumSuccessors() == 0) 680 MadeChange |= handleEndBlock(BB); 681 682 return MadeChange; 683} 684 685/// Find all blocks that will unconditionally lead to the block BB and append 686/// them to F. 687static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks, 688 BasicBlock *BB, DominatorTree *DT) { 689 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { 690 BasicBlock *Pred = *I; 691 if (Pred == BB) continue; 692 TerminatorInst *PredTI = Pred->getTerminator(); 693 if (PredTI->getNumSuccessors() != 1) 694 continue; 695 696 if (DT->isReachableFromEntry(Pred)) 697 Blocks.push_back(Pred); 698 } 699} 700 701/// HandleFree - Handle frees of entire structures whose dependency is a store 702/// to a field of that structure. 703bool DSE::HandleFree(CallInst *F) { 704 bool MadeChange = false; 705 706 AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0)); 707 SmallVector<BasicBlock *, 16> Blocks; 708 Blocks.push_back(F->getParent()); 709 710 while (!Blocks.empty()) { 711 BasicBlock *BB = Blocks.pop_back_val(); 712 Instruction *InstPt = BB->getTerminator(); 713 if (BB == F->getParent()) InstPt = F; 714 715 MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB); 716 while (Dep.isDef() || Dep.isClobber()) { 717 Instruction *Dependency = Dep.getInst(); 718 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency)) 719 break; 720 721 Value *DepPointer = 722 GetUnderlyingObject(getStoredPointerOperand(Dependency)); 723 724 // Check for aliasing. 725 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer)) 726 break; 727 728 Instruction *Next = llvm::next(BasicBlock::iterator(Dependency)); 729 730 // DCE instructions only used to calculate that store 731 DeleteDeadInstruction(Dependency, *MD); 732 ++NumFastStores; 733 MadeChange = true; 734 735 // Inst's old Dependency is now deleted. Compute the next dependency, 736 // which may also be dead, as in 737 // s[0] = 0; 738 // s[1] = 0; // This has just been deleted. 739 // free(s); 740 Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB); 741 } 742 743 if (Dep.isNonLocal()) 744 FindUnconditionalPreds(Blocks, BB, DT); 745 } 746 747 return MadeChange; 748} 749 750/// handleEndBlock - Remove dead stores to stack-allocated locations in the 751/// function end block. Ex: 752/// %A = alloca i32 753/// ... 754/// store i32 1, i32* %A 755/// ret void 756bool DSE::handleEndBlock(BasicBlock &BB) { 757 bool MadeChange = false; 758 759 // Keep track of all of the stack objects that are dead at the end of the 760 // function. 761 SmallPtrSet<Value*, 16> DeadStackObjects; 762 763 // Find all of the alloca'd pointers in the entry block. 764 BasicBlock *Entry = BB.getParent()->begin(); 765 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) { 766 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) 767 DeadStackObjects.insert(AI); 768 769 // Okay, so these are dead heap objects, but if the pointer never escapes 770 // then it's leaked by this function anyways. 771 if (CallInst *CI = extractMallocCall(I)) 772 if (!PointerMayBeCaptured(CI, true, true)) 773 DeadStackObjects.insert(CI); 774 } 775 776 // Treat byval arguments the same, stores to them are dead at the end of the 777 // function. 778 for (Function::arg_iterator AI = BB.getParent()->arg_begin(), 779 AE = BB.getParent()->arg_end(); AI != AE; ++AI) 780 if (AI->hasByValAttr()) 781 DeadStackObjects.insert(AI); 782 783 // Scan the basic block backwards 784 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){ 785 --BBI; 786 787 // If we find a store, check to see if it points into a dead stack value. 788 if (hasMemoryWrite(BBI) && isRemovable(BBI)) { 789 // See through pointer-to-pointer bitcasts 790 Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI)); 791 792 // Stores to stack values are valid candidates for removal. 793 if (DeadStackObjects.count(Pointer)) { 794 Instruction *Dead = BBI++; 795 796 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: " 797 << *Dead << "\n Object: " << *Pointer << '\n'); 798 799 // DCE instructions only used to calculate that store. 800 DeleteDeadInstruction(Dead, *MD, &DeadStackObjects); 801 ++NumFastStores; 802 MadeChange = true; 803 continue; 804 } 805 } 806 807 // Remove any dead non-memory-mutating instructions. 808 if (isInstructionTriviallyDead(BBI)) { 809 Instruction *Inst = BBI++; 810 DeleteDeadInstruction(Inst, *MD, &DeadStackObjects); 811 ++NumFastOther; 812 MadeChange = true; 813 continue; 814 } 815 816 if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) { 817 DeadStackObjects.erase(A); 818 continue; 819 } 820 821 if (CallInst *CI = extractMallocCall(BBI)) { 822 DeadStackObjects.erase(CI); 823 continue; 824 } 825 826 if (CallSite CS = cast<Value>(BBI)) { 827 // If this call does not access memory, it can't be loading any of our 828 // pointers. 829 if (AA->doesNotAccessMemory(CS)) 830 continue; 831 832 // If the call might load from any of our allocas, then any store above 833 // the call is live. 834 SmallVector<Value*, 8> LiveAllocas; 835 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(), 836 E = DeadStackObjects.end(); I != E; ++I) { 837 // See if the call site touches it. 838 AliasAnalysis::ModRefResult A = 839 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA)); 840 841 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref) 842 LiveAllocas.push_back(*I); 843 } 844 845 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(), 846 E = LiveAllocas.end(); I != E; ++I) 847 DeadStackObjects.erase(*I); 848 849 // If all of the allocas were clobbered by the call then we're not going 850 // to find anything else to process. 851 if (DeadStackObjects.empty()) 852 return MadeChange; 853 854 continue; 855 } 856 857 AliasAnalysis::Location LoadedLoc; 858 859 // If we encounter a use of the pointer, it is no longer considered dead 860 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) { 861 if (!L->isUnordered()) // Be conservative with atomic/volatile load 862 break; 863 LoadedLoc = AA->getLocation(L); 864 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) { 865 LoadedLoc = AA->getLocation(V); 866 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) { 867 LoadedLoc = AA->getLocationForSource(MTI); 868 } else if (!BBI->mayReadFromMemory()) { 869 // Instruction doesn't read memory. Note that stores that weren't removed 870 // above will hit this case. 871 continue; 872 } else { 873 // Unknown inst; assume it clobbers everything. 874 break; 875 } 876 877 // Remove any allocas from the DeadPointer set that are loaded, as this 878 // makes any stores above the access live. 879 RemoveAccessedObjects(LoadedLoc, DeadStackObjects); 880 881 // If all of the allocas were clobbered by the access then we're not going 882 // to find anything else to process. 883 if (DeadStackObjects.empty()) 884 break; 885 } 886 887 return MadeChange; 888} 889 890/// RemoveAccessedObjects - Check to see if the specified location may alias any 891/// of the stack objects in the DeadStackObjects set. If so, they become live 892/// because the location is being loaded. 893void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc, 894 SmallPtrSet<Value*, 16> &DeadStackObjects) { 895 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr); 896 897 // A constant can't be in the dead pointer set. 898 if (isa<Constant>(UnderlyingPointer)) 899 return; 900 901 // If the kill pointer can be easily reduced to an alloca, don't bother doing 902 // extraneous AA queries. 903 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) { 904 DeadStackObjects.erase(const_cast<Value*>(UnderlyingPointer)); 905 return; 906 } 907 908 SmallVector<Value*, 16> NowLive; 909 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(), 910 E = DeadStackObjects.end(); I != E; ++I) { 911 // See if the loaded location could alias the stack location. 912 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA)); 913 if (!AA->isNoAlias(StackLoc, LoadedLoc)) 914 NowLive.push_back(*I); 915 } 916 917 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end(); 918 I != E; ++I) 919 DeadStackObjects.erase(*I); 920} 921