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/SetVector.h"
  26 #include "llvm/ADT/SmallPtrSet.h"
  27 #include "llvm/ADT/Statistic.h"
  28 #include "llvm/Analysis/AliasAnalysis.h"
  29 #include "llvm/Analysis/Dominators.h"
  30 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
  31 #include "llvm/Target/TargetData.h"
  32 #include "llvm/Transforms/Utils/Local.h"
  33 #include "llvm/Support/Compiler.h"
  34 using namespace llvm;
  35
  36 STATISTIC(NumFastStores, "Number of stores deleted");
  37 STATISTIC(NumFastOther , "Number of other instrs removed");
  38
  39 namespace {
  40   struct VISIBILITY_HIDDEN DSE : public FunctionPass {
  41     static char ID; // Pass identification, replacement for typeid
  42     DSE() : FunctionPass(&ID) {}
  43
  44     virtual bool runOnFunction(Function &F) {
  45       bool Changed = false;
  46       for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  47         Changed |= runOnBasicBlock(*I);
  48       return Changed;
  49     }
  50
  51     bool runOnBasicBlock(BasicBlock &BB);
  52     bool handleFreeWithNonTrivialDependency(FreeInst* F,
  53                                             Instruction* dependency,
  54                                         SetVector<Instruction*>& possiblyDead);
  55     bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
  56     bool RemoveUndeadPointers(Value* pointer, uint64_t killPointerSize,
  57                               BasicBlock::iterator& BBI,
  58                               SmallPtrSet<Value*, 64>& deadPointers,
  59                               SetVector<Instruction*>& possiblyDead);
  60     void DeleteDeadInstructionChains(Instruction *I,
  61                                      SetVector<Instruction*> &DeadInsts);
  62
  63
  64     // getAnalysisUsage - We require post dominance frontiers (aka Control
  65     // Dependence Graph)
  66     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  67       AU.setPreservesCFG();
  68       AU.addRequired<DominatorTree>();
  69       AU.addRequired<TargetData>();
  70       AU.addRequired<AliasAnalysis>();
  71       AU.addRequired<MemoryDependenceAnalysis>();
  72       AU.addPreserved<DominatorTree>();
  73       AU.addPreserved<AliasAnalysis>();
  74       AU.addPreserved<MemoryDependenceAnalysis>();
  75     }
  76   };
  77 }
  78
  79 char DSE::ID = 0;
  80 static RegisterPass<DSE> X("dse", "Dead Store Elimination");
  81
  82 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
  83
  84 bool DSE::runOnBasicBlock(BasicBlock &BB) {
  85   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
  86   TargetData &TD = getAnalysis<TargetData>();
  87
  88   // Record the last-seen store to this pointer
  89   DenseMap<Value*, StoreInst*> lastStore;
  90   // Record instructions possibly made dead by deleting a store
  91   SetVector<Instruction*> possiblyDead;
  92
  93   bool MadeChange = false;
  94
  95   // Do a top-down walk on the BB
  96   for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
  97        BBI != BBE; ++BBI) {
  98     // If we find a store or a free...
  99     if (!isa<StoreInst>(BBI) && !isa<FreeInst>(BBI))
 100       continue;
 101
 102     Value* pointer = 0;
 103     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
 104       if (S->isVolatile())
 105         continue;
 106       pointer = S->getPointerOperand();
 107     } else {
 108       pointer = cast<FreeInst>(BBI)->getPointerOperand();
 109     }
 110
 111     pointer = pointer->stripPointerCasts();
 112     StoreInst*& last = lastStore[pointer];
 113     bool deletedStore = false;
 114
 115     // ... to a pointer that has been stored to before...
 116     if (last) {
 117       Instruction* dep = MD.getDependency(BBI);
 118
 119       // ... and no other memory dependencies are between them....
 120       while (dep != MemoryDependenceAnalysis::None &&
 121              dep != MemoryDependenceAnalysis::NonLocal &&
 122              isa<StoreInst>(dep)) {
 123         if (dep != last ||
 124              TD.getTypeStoreSize(last->getOperand(0)->getType()) >
 125              TD.getTypeStoreSize(BBI->getOperand(0)->getType())) {
 126           dep = MD.getDependency(BBI, dep);
 127           continue;
 128         }
 129
 130         // Remove it!
 131         MD.removeInstruction(last);
 132
 133         // DCE instructions only used to calculate that store
 134         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
 135           possiblyDead.insert(D);
 136         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
 137           possiblyDead.insert(D);
 138
 139         last->eraseFromParent();
 140         NumFastStores++;
 141         deletedStore = true;
 142         MadeChange = true;
 143
 144         break;
 145       }
 146     }
 147
 148     // Handle frees whose dependencies are non-trivial.
 149     if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
 150       if (!deletedStore)
 151         MadeChange |= handleFreeWithNonTrivialDependency(F,
 152                                                          MD.getDependency(F),
 153                                                          possiblyDead);
 154       // No known stores after the free
 155       last = 0;
 156     } else {
 157       StoreInst* S = cast<StoreInst>(BBI);
 158
 159       // If we're storing the same value back to a pointer that we just
 160       // loaded from, then the store can be removed;
 161       if (LoadInst* L = dyn_cast<LoadInst>(S->getOperand(0))) {
 162         Instruction* dep = MD.getDependency(S);
 163         DominatorTree& DT = getAnalysis<DominatorTree>();
 164
 165         if (!S->isVolatile() && S->getParent() == L->getParent() &&
 166             S->getPointerOperand() == L->getPointerOperand() &&
 167             ( dep == MemoryDependenceAnalysis::None ||
 168               dep == MemoryDependenceAnalysis::NonLocal ||
 169               DT.dominates(dep, L))) {
 170           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 171             possiblyDead.insert(D);
 172           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 173             possiblyDead.insert(D);
 174
 175           // Avoid iterator invalidation.
 176           BBI--;
 177
 178           MD.removeInstruction(S);
 179           S->eraseFromParent();
 180           NumFastStores++;
 181           MadeChange = true;
 182         } else
 183           // Update our most-recent-store map.
 184           last = S;
 185       } else
 186         // Update our most-recent-store map.
 187         last = S;
 188     }
 189   }
 190
 191   // If this block ends in a return, unwind, unreachable, and eventually
 192   // tailcall, then all allocas are dead at its end.
 193   if (BB.getTerminator()->getNumSuccessors() == 0)
 194     MadeChange |= handleEndBlock(BB, possiblyDead);
 195
 196   // Do a trivial DCE
 197   while (!possiblyDead.empty()) {
 198     Instruction *I = possiblyDead.back();
 199     possiblyDead.pop_back();
 200     DeleteDeadInstructionChains(I, possiblyDead);
 201   }
 202
 203   return MadeChange;
 204 }
 205
 206 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 207 /// dependency is a store to a field of that structure
 208 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
 209                                        SetVector<Instruction*>& possiblyDead) {
 210   TargetData &TD = getAnalysis<TargetData>();
 211   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 212   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 213
 214   if (dep == MemoryDependenceAnalysis::None ||
 215       dep == MemoryDependenceAnalysis::NonLocal)
 216     return false;
 217
 218   StoreInst* dependency = dyn_cast<StoreInst>(dep);
 219   if (!dependency)
 220     return false;
 221   else if (dependency->isVolatile())
 222     return false;
 223
 224   Value* depPointer = dependency->getPointerOperand();
 225   const Type* depType = dependency->getOperand(0)->getType();
 226   unsigned depPointerSize = TD.getTypeStoreSize(depType);
 227
 228   // Check for aliasing
 229   AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0U,
 230                                           depPointer, depPointerSize);
 231
 232   if (A == AliasAnalysis::MustAlias) {
 233     // Remove it!
 234     MD.removeInstruction(dependency);
 235
 236     // DCE instructions only used to calculate that store
 237     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
 238       possiblyDead.insert(D);
 239     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
 240       possiblyDead.insert(D);
 241
 242     dependency->eraseFromParent();
 243     NumFastStores++;
 244     return true;
 245   }
 246
 247   return false;
 248 }
 249
 250 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
 251 /// function end block.  Ex:
 252 /// %A = alloca i32
 253 /// ...
 254 /// store i32 1, i32* %A
 255 /// ret void
 256 bool DSE::handleEndBlock(BasicBlock& BB,
 257                          SetVector<Instruction*>& possiblyDead) {
 258   TargetData &TD = getAnalysis<TargetData>();
 259   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 260   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 261
 262   bool MadeChange = false;
 263
 264   // Pointers alloca'd in this function are dead in the end block
 265   SmallPtrSet<Value*, 64> deadPointers;
 266
 267   // Find all of the alloca'd pointers in the entry block
 268   BasicBlock *Entry = BB.getParent()->begin();
 269   for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
 270     if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
 271       deadPointers.insert(AI);
 272   for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
 273        AE = BB.getParent()->arg_end(); AI != AE; ++AI)
 274     if (AI->hasByValAttr())
 275       deadPointers.insert(AI);
 276
 277   // Scan the basic block backwards
 278   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
 279     --BBI;
 280
 281     // If we find a store whose pointer is dead...
 282     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
 283       if (!S->isVolatile()) {
 284         // See through pointer-to-pointer bitcasts
 285         Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();
 286
 287         // Alloca'd pointers or byval arguments (which are functionally like
 288         // alloca's) are valid candidates for removal.
 289         if (deadPointers.count(pointerOperand)) {
 290           // Remove it!
 291           MD.removeInstruction(S);
 292
 293           // DCE instructions only used to calculate that store
 294           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 295             possiblyDead.insert(D);
 296           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 297             possiblyDead.insert(D);
 298
 299           BBI++;
 300           MD.removeInstruction(S);
 301           S->eraseFromParent();
 302           NumFastStores++;
 303           MadeChange = true;
 304         }
 305       }
 306
 307       continue;
 308
 309     // We can also remove memcpy's to local variables at the end of a function
 310     } else if (MemCpyInst* M = dyn_cast<MemCpyInst>(BBI)) {
 311       Value* dest = M->getDest()->getUnderlyingObject();
 312
 313       if (deadPointers.count(dest)) {
 314         MD.removeInstruction(M);
 315
 316         // DCE instructions only used to calculate that memcpy
 317         if (Instruction* D = dyn_cast<Instruction>(M->getRawSource()))
 318           possiblyDead.insert(D);
 319         if (Instruction* D = dyn_cast<Instruction>(M->getLength()))
 320           possiblyDead.insert(D);
 321         if (Instruction* D = dyn_cast<Instruction>(M->getRawDest()))
 322           possiblyDead.insert(D);
 323
 324         BBI++;
 325         M->eraseFromParent();
 326         NumFastOther++;
 327         MadeChange = true;
 328
 329         continue;
 330       }
 331
 332       // Because a memcpy is also a load, we can't skip it if we didn't remove it
 333     }
 334
 335     Value* killPointer = 0;
 336     uint64_t killPointerSize = ~0UL;
 337
 338     // If we encounter a use of the pointer, it is no longer considered dead
 339     if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
 340       // However, if this load is unused and not volatile, we can go ahead and
 341       // remove it, and not have to worry about it making our pointer undead!
 342       if (L->use_empty() && !L->isVolatile()) {
 343         MD.removeInstruction(L);
 344
 345         // DCE instructions only used to calculate that load
 346         if (Instruction* D = dyn_cast<Instruction>(L->getPointerOperand()))
 347           possiblyDead.insert(D);
 348
 349         BBI++;
 350         L->eraseFromParent();
 351         NumFastOther++;
 352         MadeChange = true;
 353         possiblyDead.remove(L);
 354
 355         continue;
 356       }
 357
 358       killPointer = L->getPointerOperand();
 359     } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
 360       killPointer = V->getOperand(0);
 361     } else if (isa<MemCpyInst>(BBI) &&
 362                isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
 363       killPointer = cast<MemCpyInst>(BBI)->getSource();
 364       killPointerSize = cast<ConstantInt>(
 365                             cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
 366     } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
 367       deadPointers.erase(A);
 368
 369       // Dead alloca's can be DCE'd when we reach them
 370       if (A->use_empty()) {
 371         MD.removeInstruction(A);
 372
 373         // DCE instructions only used to calculate that load
 374         if (Instruction* D = dyn_cast<Instruction>(A->getArraySize()))
 375           possiblyDead.insert(D);
 376
 377         BBI++;
 378         A->eraseFromParent();
 379         NumFastOther++;
 380         MadeChange = true;
 381         possiblyDead.remove(A);
 382       }
 383
 384       continue;
 385     } else if (CallSite::get(BBI).getInstruction() != 0) {
 386       // If this call does not access memory, it can't
 387       // be undeadifying any of our pointers.
 388       CallSite CS = CallSite::get(BBI);
 389       if (AA.doesNotAccessMemory(CS))
 390         continue;
 391
 392       unsigned modRef = 0;
 393       unsigned other = 0;
 394
 395       // Remove any pointers made undead by the call from the dead set
 396       std::vector<Value*> dead;
 397       for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
 398            E = deadPointers.end(); I != E; ++I) {
 399         // HACK: if we detect that our AA is imprecise, it's not
 400         // worth it to scan the rest of the deadPointers set.  Just
 401         // assume that the AA will return ModRef for everything, and
 402         // go ahead and bail.
 403         if (modRef >= 16 && other == 0) {
 404           deadPointers.clear();
 405           return MadeChange;
 406         }
 407
 408         // Get size information for the alloca
 409         unsigned pointerSize = ~0U;
 410         if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
 411           if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
 412             pointerSize = C->getZExtValue() * \
 413                           TD.getABITypeSize(A->getAllocatedType());
 414         } else {
 415           const PointerType* PT = cast<PointerType>(
 416                                                  cast<Argument>(*I)->getType());
 417           pointerSize = TD.getABITypeSize(PT->getElementType());
 418         }
 419
 420         // See if the call site touches it
 421         AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
 422
 423         if (A == AliasAnalysis::ModRef)
 424           modRef++;
 425         else
 426           other++;
 427
 428         if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
 429           dead.push_back(*I);
 430       }
 431
 432       for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
 433            I != E; ++I)
 434         deadPointers.erase(*I);
 435
 436       continue;
 437     } else {
 438       // For any non-memory-affecting non-terminators, DCE them as we reach them
 439       Instruction *CI = BBI;
 440       if (!CI->isTerminator() && CI->use_empty() && !isa<FreeInst>(CI)) {
 441
 442         // DCE instructions only used to calculate that load
 443         for (Instruction::op_iterator OI = CI->op_begin(), OE = CI->op_end();
 444              OI != OE; ++OI)
 445           if (Instruction* D = dyn_cast<Instruction>(OI))
 446             possiblyDead.insert(D);
 447
 448         BBI++;
 449         CI->eraseFromParent();
 450         NumFastOther++;
 451         MadeChange = true;
 452         possiblyDead.remove(CI);
 453
 454         continue;
 455       }
 456     }
 457
 458     if (!killPointer)
 459       continue;
 460
 461     killPointer = killPointer->getUnderlyingObject();
 462
 463     // Deal with undead pointers
 464     MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
 465                                        deadPointers, possiblyDead);
 466   }
 467
 468   return MadeChange;
 469 }
 470
 471 /// RemoveUndeadPointers - check for uses of a pointer that make it
 472 /// undead when scanning for dead stores to alloca's.
 473 bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
 474                                 BasicBlock::iterator& BBI,
 475                                 SmallPtrSet<Value*, 64>& deadPointers,
 476                                 SetVector<Instruction*>& possiblyDead) {
 477   TargetData &TD = getAnalysis<TargetData>();
 478   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 479   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 480
 481   // If the kill pointer can be easily reduced to an alloca,
 482   // don't bother doing extraneous AA queries
 483   if (deadPointers.count(killPointer)) {
 484     deadPointers.erase(killPointer);
 485     return false;
 486   } else if (isa<GlobalValue>(killPointer)) {
 487     // A global can't be in the dead pointer set
 488     return false;
 489   }
 490
 491   bool MadeChange = false;
 492
 493   std::vector<Value*> undead;
 494
 495   for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
 496       E = deadPointers.end(); I != E; ++I) {
 497     // Get size information for the alloca
 498     unsigned pointerSize = ~0U;
 499     if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
 500       if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
 501         pointerSize = C->getZExtValue() * \
 502                       TD.getABITypeSize(A->getAllocatedType());
 503     } else {
 504       const PointerType* PT = cast<PointerType>(
 505                                                 cast<Argument>(*I)->getType());
 506       pointerSize = TD.getABITypeSize(PT->getElementType());
 507     }
 508
 509     // See if this pointer could alias it
 510     AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
 511                                             killPointer, killPointerSize);
 512
 513     // If it must-alias and a store, we can delete it
 514     if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
 515       StoreInst* S = cast<StoreInst>(BBI);
 516
 517       // Remove it!
 518       MD.removeInstruction(S);
 519
 520       // DCE instructions only used to calculate that store
 521       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 522         possiblyDead.insert(D);
 523       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 524         possiblyDead.insert(D);
 525
 526       BBI++;
 527       S->eraseFromParent();
 528       NumFastStores++;
 529       MadeChange = true;
 530
 531       continue;
 532
 533       // Otherwise, it is undead
 534       } else if (A != AliasAnalysis::NoAlias)
 535         undead.push_back(*I);
 536   }
 537
 538   for (std::vector<Value*>::iterator I = undead.begin(), E = undead.end();
 539        I != E; ++I)
 540       deadPointers.erase(*I);
 541
 542   return MadeChange;
 543 }
 544
 545 /// DeleteDeadInstructionChains - takes an instruction and a setvector of
 546 /// dead instructions.  If I is dead, it is erased, and its operands are
 547 /// checked for deadness.  If they are dead, they are added to the dead
 548 /// setvector.
 549 void DSE::DeleteDeadInstructionChains(Instruction *I,
 550                                       SetVector<Instruction*> &DeadInsts) {
 551   // Instruction must be dead.
 552   if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
 553
 554   // Let the memory dependence know
 555   getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);
 556
 557   // See if this made any operands dead.  We do it this way in case the
 558   // instruction uses the same operand twice.  We don't want to delete a
 559   // value then reference it.
 560   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
 561     if (I->getOperand(i)->hasOneUse())
 562       if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
 563         DeadInsts.insert(Op);      // Attempt to nuke it later.
 564
 565     I->setOperand(i, 0);         // Drop from the operand list.
 566   }
 567
 568   I->eraseFromParent();
 569   ++NumFastOther;
 570 }