lib/Transforms/Scalar/DeadStoreElimination.cpp

   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/Instructions.h"
  23 #include "llvm/IntrinsicInst.h"
  24 #include "llvm/Pass.h"
  25 #include "llvm/ADT/SmallPtrSet.h"
  26 #include "llvm/ADT/Statistic.h"
  27 #include "llvm/Analysis/AliasAnalysis.h"
  28 #include "llvm/Analysis/Dominators.h"
  29 #include "llvm/Analysis/MemoryBuiltins.h"
  30 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
  31 #include "llvm/Target/TargetData.h"
  32 #include "llvm/Transforms/Utils/Local.h"
  33 using namespace llvm;
  34
  35 STATISTIC(NumFastStores, "Number of stores deleted");
  36 STATISTIC(NumFastOther , "Number of other instrs removed");
  37
  38 namespace {
  39   struct DSE : public FunctionPass {
  40     TargetData *TD;
  41
  42     static char ID; // Pass identification, replacement for typeid
  43     DSE() : FunctionPass(ID) {
  44       initializeDSEPass(*PassRegistry::getPassRegistry());
  45     }
  46
  47     virtual bool runOnFunction(Function &F) {
  48       bool Changed = false;
  49
  50       DominatorTree &DT = getAnalysis<DominatorTree>();
  51
  52       for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  53         // Only check non-dead blocks.  Dead blocks may have strange pointer
  54         // cycles that will confuse alias analysis.
  55         if (DT.isReachableFromEntry(I))
  56           Changed |= runOnBasicBlock(*I);
  57       return Changed;
  58     }
  59
  60     bool runOnBasicBlock(BasicBlock &BB);
  61     bool handleFreeWithNonTrivialDependency(const CallInst *F,
  62                                             MemDepResult Dep);
  63     bool handleEndBlock(BasicBlock &BB);
  64     bool RemoveUndeadPointers(Value *Ptr, uint64_t killPointerSize,
  65                               BasicBlock::iterator &BBI,
  66                               SmallPtrSet<Value*, 64> &deadPointers);
  67     void DeleteDeadInstruction(Instruction *I,
  68                                SmallPtrSet<Value*, 64> *deadPointers = 0);
  69
  70
  71     // getAnalysisUsage - We require post dominance frontiers (aka Control
  72     // Dependence Graph)
  73     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  74       AU.setPreservesCFG();
  75       AU.addRequired<DominatorTree>();
  76       AU.addRequired<AliasAnalysis>();
  77       AU.addRequired<MemoryDependenceAnalysis>();
  78       AU.addPreserved<DominatorTree>();
  79       AU.addPreserved<MemoryDependenceAnalysis>();
  80     }
  81
  82     uint64_t getPointerSize(Value *V) const;
  83   };
  84 }
  85
  86 char DSE::ID = 0;
  87 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
  88 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  89 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
  90 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
  91 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
  92
  93 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
  94
  95 /// doesClobberMemory - Does this instruction clobber (write without reading)
  96 /// some memory?
  97 static bool doesClobberMemory(Instruction *I) {
  98   if (isa<StoreInst>(I))
  99     return true;
 100   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
 101     switch (II->getIntrinsicID()) {
 102     default:
 103       return false;
 104     case Intrinsic::memset:
 105     case Intrinsic::memmove:
 106     case Intrinsic::memcpy:
 107     case Intrinsic::init_trampoline:
 108     case Intrinsic::lifetime_end:
 109       return true;
 110     }
 111   }
 112   return false;
 113 }
 114
 115 /// isElidable - If the value of this instruction and the memory it writes to is
 116 /// unused, may we delete this instrtction?
 117 static bool isElidable(Instruction *I) {
 118   assert(doesClobberMemory(I));
 119   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
 120     return II->getIntrinsicID() != Intrinsic::lifetime_end;
 121   if (StoreInst *SI = dyn_cast<StoreInst>(I))
 122     return !SI->isVolatile();
 123   return true;
 124 }
 125
 126 /// getPointerOperand - Return the pointer that is being clobbered.
 127 static Value *getPointerOperand(Instruction *I) {
 128   assert(doesClobberMemory(I));
 129   if (StoreInst *SI = dyn_cast<StoreInst>(I))
 130     return SI->getPointerOperand();
 131   if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
 132     return MI->getArgOperand(0);
 133
 134   IntrinsicInst *II = cast<IntrinsicInst>(I);
 135   switch (II->getIntrinsicID()) {
 136   default: assert(false && "Unexpected intrinsic!");
 137   case Intrinsic::init_trampoline:
 138     return II->getArgOperand(0);
 139   case Intrinsic::lifetime_end:
 140     return II->getArgOperand(1);
 141   }
 142 }
 143
 144 /// getStoreSize - Return the length in bytes of the write by the clobbering
 145 /// instruction. If variable or unknown, returns AliasAnalysis::UnknownSize.
 146 static uint64_t getStoreSize(Instruction *I, const TargetData *TD) {
 147   assert(doesClobberMemory(I));
 148   if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
 149     if (!TD) return AliasAnalysis::UnknownSize;
 150     return TD->getTypeStoreSize(SI->getOperand(0)->getType());
 151   }
 152
 153   Value *Len;
 154   if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
 155     Len = MI->getLength();
 156   } else {
 157     IntrinsicInst *II = cast<IntrinsicInst>(I);
 158     switch (II->getIntrinsicID()) {
 159     default: assert(false && "Unexpected intrinsic!");
 160     case Intrinsic::init_trampoline:
 161       return AliasAnalysis::UnknownSize;
 162     case Intrinsic::lifetime_end:
 163       Len = II->getArgOperand(0);
 164       break;
 165     }
 166   }
 167   if (ConstantInt *LenCI = dyn_cast<ConstantInt>(Len))
 168     if (!LenCI->isAllOnesValue())
 169       return LenCI->getZExtValue();
 170   return AliasAnalysis::UnknownSize;
 171 }
 172
 173 /// isStoreAtLeastAsWideAs - Return true if the size of the store in I1 is
 174 /// greater than or equal to the store in I2.  This returns false if we don't
 175 /// know.
 176 ///
 177 static bool isStoreAtLeastAsWideAs(Instruction *I1, Instruction *I2,
 178                                    const TargetData *TD) {
 179   const Type *I1Ty = getPointerOperand(I1)->getType();
 180   const Type *I2Ty = getPointerOperand(I2)->getType();
 181
 182   // Exactly the same type, must have exactly the same size.
 183   if (I1Ty == I2Ty) return true;
 184
 185   uint64_t I1Size = getStoreSize(I1, TD);
 186   uint64_t I2Size = getStoreSize(I2, TD);
 187
 188   return I1Size != AliasAnalysis::UnknownSize &&
 189          I2Size != AliasAnalysis::UnknownSize &&
 190          I1Size >= I2Size;
 191 }
 192
 193 bool DSE::runOnBasicBlock(BasicBlock &BB) {
 194   MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
 195   TD = getAnalysisIfAvailable<TargetData>();
 196
 197   bool MadeChange = false;
 198
 199   // Do a top-down walk on the BB.
 200   for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
 201     Instruction *Inst = BBI++;
 202
 203     // If we find a store or a free, get its memory dependence.
 204     if (!doesClobberMemory(Inst) && !isFreeCall(Inst))
 205       continue;
 206
 207     MemDepResult InstDep = MD.getDependency(Inst);
 208
 209     // Ignore non-local stores.
 210     // FIXME: cross-block DSE would be fun. :)
 211     if (InstDep.isNonLocal()) continue;
 212
 213     // Handle frees whose dependencies are non-trivial.
 214     if (const CallInst *F = isFreeCall(Inst)) {
 215       MadeChange |= handleFreeWithNonTrivialDependency(F, InstDep);
 216       continue;
 217     }
 218
 219     // If not a definite must-alias dependency, ignore it.
 220     if (!InstDep.isDef())
 221       continue;
 222
 223     // If this is a store-store dependence, then the previous store is dead so
 224     // long as this store is at least as big as it.
 225     if (doesClobberMemory(InstDep.getInst())) {
 226       Instruction *DepStore = InstDep.getInst();
 227       if (isStoreAtLeastAsWideAs(Inst, DepStore, TD) &&
 228           isElidable(DepStore)) {
 229         // Delete the store and now-dead instructions that feed it.
 230         DeleteDeadInstruction(DepStore);
 231         ++NumFastStores;
 232         MadeChange = true;
 233
 234         // DeleteDeadInstruction can delete the current instruction in loop
 235         // cases, reset BBI.
 236         BBI = Inst;
 237         if (BBI != BB.begin())
 238           --BBI;
 239         continue;
 240       }
 241     }
 242
 243     if (!isElidable(Inst))
 244       continue;
 245
 246     // If we're storing the same value back to a pointer that we just
 247     // loaded from, then the store can be removed.
 248     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
 249       if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
 250         if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
 251             SI->getOperand(0) == DepLoad) {
 252           // DeleteDeadInstruction can delete the current instruction.  Save BBI
 253           // in case we need it.
 254           WeakVH NextInst(BBI);
 255
 256           DeleteDeadInstruction(SI);
 257
 258           if (NextInst == 0)  // Next instruction deleted.
 259             BBI = BB.begin();
 260           else if (BBI != BB.begin())  // Revisit this instruction if possible.
 261             --BBI;
 262           ++NumFastStores;
 263           MadeChange = true;
 264           continue;
 265         }
 266       }
 267     }
 268
 269     // If this is a lifetime end marker, we can throw away the store.
 270     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(InstDep.getInst())) {
 271       if (II->getIntrinsicID() == Intrinsic::lifetime_end) {
 272         // Delete the store and now-dead instructions that feed it.
 273         // DeleteDeadInstruction can delete the current instruction.  Save BBI
 274         // in case we need it.
 275         WeakVH NextInst(BBI);
 276
 277         DeleteDeadInstruction(Inst);
 278
 279         if (NextInst == 0)  // Next instruction deleted.
 280           BBI = BB.begin();
 281         else if (BBI != BB.begin())  // Revisit this instruction if possible.
 282           --BBI;
 283         ++NumFastStores;
 284         MadeChange = true;
 285         continue;
 286       }
 287     }
 288   }
 289
 290   // If this block ends in a return, unwind, or unreachable, all allocas are
 291   // dead at its end, which means stores to them are also dead.
 292   if (BB.getTerminator()->getNumSuccessors() == 0)
 293     MadeChange |= handleEndBlock(BB);
 294
 295   return MadeChange;
 296 }
 297
 298 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 299 /// dependency is a store to a field of that structure.
 300 bool DSE::handleFreeWithNonTrivialDependency(const CallInst *F,
 301                                              MemDepResult Dep) {
 302   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 303
 304   Instruction *Dependency = Dep.getInst();
 305   if (!Dependency || !doesClobberMemory(Dependency) || !isElidable(Dependency))
 306     return false;
 307
 308   Value *DepPointer = getPointerOperand(Dependency)->getUnderlyingObject();
 309
 310   // Check for aliasing.
 311   if (AA.alias(F->getArgOperand(0), 1, DepPointer, 1) !=
 312          AliasAnalysis::MustAlias)
 313     return false;
 314
 315   // DCE instructions only used to calculate that store
 316   DeleteDeadInstruction(Dependency);
 317   ++NumFastStores;
 318   return true;
 319 }
 320
 321 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
 322 /// function end block.  Ex:
 323 /// %A = alloca i32
 324 /// ...
 325 /// store i32 1, i32* %A
 326 /// ret void
 327 bool DSE::handleEndBlock(BasicBlock &BB) {
 328   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 329
 330   bool MadeChange = false;
 331
 332   // Pointers alloca'd in this function are dead in the end block
 333   SmallPtrSet<Value*, 64> deadPointers;
 334
 335   // Find all of the alloca'd pointers in the entry block.
 336   BasicBlock *Entry = BB.getParent()->begin();
 337   for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
 338     if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
 339       deadPointers.insert(AI);
 340
 341   // Treat byval arguments the same, stores to them are dead at the end of the
 342   // function.
 343   for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
 344        AE = BB.getParent()->arg_end(); AI != AE; ++AI)
 345     if (AI->hasByValAttr())
 346       deadPointers.insert(AI);
 347
 348   // Scan the basic block backwards
 349   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
 350     --BBI;
 351
 352     // If we find a store whose pointer is dead.
 353     if (doesClobberMemory(BBI)) {
 354       if (isElidable(BBI)) {
 355         // See through pointer-to-pointer bitcasts
 356         Value *pointerOperand = getPointerOperand(BBI)->getUnderlyingObject();
 357
 358         // Alloca'd pointers or byval arguments (which are functionally like
 359         // alloca's) are valid candidates for removal.
 360         if (deadPointers.count(pointerOperand)) {
 361           // DCE instructions only used to calculate that store.
 362           Instruction *Dead = BBI;
 363           ++BBI;
 364           DeleteDeadInstruction(Dead, &deadPointers);
 365           ++NumFastStores;
 366           MadeChange = true;
 367           continue;
 368         }
 369       }
 370
 371       // Because a memcpy or memmove is also a load, we can't skip it if we
 372       // didn't remove it.
 373       if (!isa<MemTransferInst>(BBI))
 374         continue;
 375     }
 376
 377     Value *killPointer = 0;
 378     uint64_t killPointerSize = AliasAnalysis::UnknownSize;
 379
 380     // If we encounter a use of the pointer, it is no longer considered dead
 381     if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
 382       // However, if this load is unused and not volatile, we can go ahead and
 383       // remove it, and not have to worry about it making our pointer undead!
 384       if (L->use_empty() && !L->isVolatile()) {
 385         ++BBI;
 386         DeleteDeadInstruction(L, &deadPointers);
 387         ++NumFastOther;
 388         MadeChange = true;
 389         continue;
 390       }
 391
 392       killPointer = L->getPointerOperand();
 393     } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
 394       killPointer = V->getOperand(0);
 395     } else if (isa<MemTransferInst>(BBI) &&
 396                isa<ConstantInt>(cast<MemTransferInst>(BBI)->getLength())) {
 397       killPointer = cast<MemTransferInst>(BBI)->getSource();
 398       killPointerSize = cast<ConstantInt>(
 399                        cast<MemTransferInst>(BBI)->getLength())->getZExtValue();
 400     } else if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
 401       deadPointers.erase(A);
 402
 403       // Dead alloca's can be DCE'd when we reach them
 404       if (A->use_empty()) {
 405         ++BBI;
 406         DeleteDeadInstruction(A, &deadPointers);
 407         ++NumFastOther;
 408         MadeChange = true;
 409       }
 410
 411       continue;
 412     } else if (CallSite CS = cast<Value>(BBI)) {
 413       // If this call does not access memory, it can't
 414       // be undeadifying any of our pointers.
 415       if (AA.doesNotAccessMemory(CS))
 416         continue;
 417
 418       unsigned modRef = 0;
 419       unsigned other = 0;
 420
 421       // Remove any pointers made undead by the call from the dead set
 422       std::vector<Value*> dead;
 423       for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
 424            E = deadPointers.end(); I != E; ++I) {
 425         // HACK: if we detect that our AA is imprecise, it's not
 426         // worth it to scan the rest of the deadPointers set.  Just
 427         // assume that the AA will return ModRef for everything, and
 428         // go ahead and bail.
 429         if (modRef >= 16 && other == 0) {
 430           deadPointers.clear();
 431           return MadeChange;
 432         }
 433
 434         // See if the call site touches it
 435         AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I,
 436                                                          getPointerSize(*I));
 437
 438         if (A == AliasAnalysis::ModRef)
 439           ++modRef;
 440         else
 441           ++other;
 442
 443         if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
 444           dead.push_back(*I);
 445       }
 446
 447       for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
 448            I != E; ++I)
 449         deadPointers.erase(*I);
 450
 451       continue;
 452     } else if (isInstructionTriviallyDead(BBI)) {
 453       // For any non-memory-affecting non-terminators, DCE them as we reach them
 454       Instruction *Inst = BBI;
 455       ++BBI;
 456       DeleteDeadInstruction(Inst, &deadPointers);
 457       ++NumFastOther;
 458       MadeChange = true;
 459       continue;
 460     }
 461
 462     if (!killPointer)
 463       continue;
 464
 465     killPointer = killPointer->getUnderlyingObject();
 466
 467     // Deal with undead pointers
 468     MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
 469                                        deadPointers);
 470   }
 471
 472   return MadeChange;
 473 }
 474
 475 /// RemoveUndeadPointers - check for uses of a pointer that make it
 476 /// undead when scanning for dead stores to alloca's.
 477 bool DSE::RemoveUndeadPointers(Value *killPointer, uint64_t killPointerSize,
 478                                BasicBlock::iterator &BBI,
 479                                SmallPtrSet<Value*, 64> &deadPointers) {
 480   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 481
 482   // If the kill pointer can be easily reduced to an alloca,
 483   // don't bother doing extraneous AA queries.
 484   if (deadPointers.count(killPointer)) {
 485     deadPointers.erase(killPointer);
 486     return false;
 487   }
 488
 489   // A global can't be in the dead pointer set.
 490   if (isa<GlobalValue>(killPointer))
 491     return false;
 492
 493   bool MadeChange = false;
 494
 495   SmallVector<Value*, 16> undead;
 496
 497   for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
 498        E = deadPointers.end(); I != E; ++I) {
 499     // See if this pointer could alias it
 500     AliasAnalysis::AliasResult A = AA.alias(*I, getPointerSize(*I),
 501                                             killPointer, killPointerSize);
 502
 503     // If it must-alias and a store, we can delete it
 504     if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
 505       StoreInst *S = cast<StoreInst>(BBI);
 506
 507       // Remove it!
 508       ++BBI;
 509       DeleteDeadInstruction(S, &deadPointers);
 510       ++NumFastStores;
 511       MadeChange = true;
 512
 513       continue;
 514
 515       // Otherwise, it is undead
 516     } else if (A != AliasAnalysis::NoAlias)
 517       undead.push_back(*I);
 518   }
 519
 520   for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
 521        I != E; ++I)
 522       deadPointers.erase(*I);
 523
 524   return MadeChange;
 525 }
 526
 527 /// DeleteDeadInstruction - Delete this instruction.  Before we do, go through
 528 /// and zero out all the operands of this instruction.  If any of them become
 529 /// dead, delete them and the computation tree that feeds them.
 530 ///
 531 /// If ValueSet is non-null, remove any deleted instructions from it as well.
 532 ///
 533 void DSE::DeleteDeadInstruction(Instruction *I,
 534                                 SmallPtrSet<Value*, 64> *ValueSet) {
 535   SmallVector<Instruction*, 32> NowDeadInsts;
 536
 537   NowDeadInsts.push_back(I);
 538   --NumFastOther;
 539
 540   // Before we touch this instruction, remove it from memdep!
 541   MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
 542   do {
 543     Instruction *DeadInst = NowDeadInsts.pop_back_val();
 544
 545     ++NumFastOther;
 546
 547     // This instruction is dead, zap it, in stages.  Start by removing it from
 548     // MemDep, which needs to know the operands and needs it to be in the
 549     // function.
 550     MDA.removeInstruction(DeadInst);
 551
 552     for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
 553       Value *Op = DeadInst->getOperand(op);
 554       DeadInst->setOperand(op, 0);
 555
 556       // If this operand just became dead, add it to the NowDeadInsts list.
 557       if (!Op->use_empty()) continue;
 558
 559       if (Instruction *OpI = dyn_cast<Instruction>(Op))
 560         if (isInstructionTriviallyDead(OpI))
 561           NowDeadInsts.push_back(OpI);
 562     }
 563
 564     DeadInst->eraseFromParent();
 565
 566     if (ValueSet) ValueSet->erase(DeadInst);
 567   } while (!NowDeadInsts.empty());
 568 }
 569
 570 uint64_t DSE::getPointerSize(Value *V) const {
 571   if (TD) {
 572     if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
 573       // Get size information for the alloca
 574       if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
 575         return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
 576     } else {
 577       assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
 578       const PointerType *PT = cast<PointerType>(V->getType());
 579       return TD->getTypeAllocSize(PT->getElementType());
 580     }
 581   }
 582   return AliasAnalysis::UnknownSize;
 583 }