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     /// Find the base pointer that a pointer came from
  64     /// Because this is used to find pointers that originate
  65     /// from allocas, it is safe to ignore GEP indices, since
  66     /// either the store will be in the alloca, and thus dead,
  67     /// or beyond the end of the alloca, and thus undefined.
  68     void TranslatePointerBitCasts(Value*& v, bool zeroGepsOnly = false) {
  69       assert(isa<PointerType>(v->getType()) &&
  70              "Translating a non-pointer type?");
  71       while (true) {
  72         if (BitCastInst* C = dyn_cast<BitCastInst>(v))
  73           v = C->getOperand(0);
  74         else if (GetElementPtrInst* G = dyn_cast<GetElementPtrInst>(v))
  75           if (!zeroGepsOnly || G->hasAllZeroIndices()) {
  76             v = G->getOperand(0);
  77           } else {
  78             break;
  79           }
  80         else
  81           break;
  82       }
  83     }
  84
  85     // getAnalysisUsage - We require post dominance frontiers (aka Control
  86     // Dependence Graph)
  87     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  88       AU.setPreservesCFG();
  89       AU.addRequired<DominatorTree>();
  90       AU.addRequired<TargetData>();
  91       AU.addRequired<AliasAnalysis>();
  92       AU.addRequired<MemoryDependenceAnalysis>();
  93       AU.addPreserved<DominatorTree>();
  94       AU.addPreserved<AliasAnalysis>();
  95       AU.addPreserved<MemoryDependenceAnalysis>();
  96     }
  97   };
  98 }
  99
 100 char DSE::ID = 0;
 101 static RegisterPass<DSE> X("dse", "Dead Store Elimination");
 102
 103 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
 104
 105 bool DSE::runOnBasicBlock(BasicBlock &BB) {
 106   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 107   TargetData &TD = getAnalysis<TargetData>();
 108
 109   // Record the last-seen store to this pointer
 110   DenseMap<Value*, StoreInst*> lastStore;
 111   // Record instructions possibly made dead by deleting a store
 112   SetVector<Instruction*> possiblyDead;
 113
 114   bool MadeChange = false;
 115
 116   // Do a top-down walk on the BB
 117   for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
 118        BBI != BBE; ++BBI) {
 119     // If we find a store or a free...
 120     if (!isa<StoreInst>(BBI) && !isa<FreeInst>(BBI))
 121       continue;
 122
 123     Value* pointer = 0;
 124     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
 125       if (!S->isVolatile())
 126         pointer = S->getPointerOperand();
 127       else
 128         continue;
 129     } else
 130       pointer = cast<FreeInst>(BBI)->getPointerOperand();
 131
 132     TranslatePointerBitCasts(pointer, true);
 133     StoreInst*& last = lastStore[pointer];
 134     bool deletedStore = false;
 135
 136     // ... to a pointer that has been stored to before...
 137     if (last) {
 138       Instruction* dep = MD.getDependency(BBI);
 139
 140       // ... and no other memory dependencies are between them....
 141       while (dep != MemoryDependenceAnalysis::None &&
 142              dep != MemoryDependenceAnalysis::NonLocal &&
 143              isa<StoreInst>(dep)) {
 144         if (dep != last ||
 145              TD.getTypeStoreSize(last->getOperand(0)->getType()) >
 146              TD.getTypeStoreSize(BBI->getOperand(0)->getType())) {
 147           dep = MD.getDependency(BBI, dep);
 148           continue;
 149         }
 150
 151         // Remove it!
 152         MD.removeInstruction(last);
 153
 154         // DCE instructions only used to calculate that store
 155         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
 156           possiblyDead.insert(D);
 157         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
 158           possiblyDead.insert(D);
 159
 160         last->eraseFromParent();
 161         NumFastStores++;
 162         deletedStore = true;
 163         MadeChange = true;
 164
 165         break;
 166       }
 167     }
 168
 169     // Handle frees whose dependencies are non-trivial.
 170     if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
 171       if (!deletedStore)
 172         MadeChange |= handleFreeWithNonTrivialDependency(F,
 173                                                          MD.getDependency(F),
 174                                                          possiblyDead);
 175       // No known stores after the free
 176       last = 0;
 177     } else {
 178       StoreInst* S = cast<StoreInst>(BBI);
 179
 180       // If we're storing the same value back to a pointer that we just
 181       // loaded from, then the store can be removed;
 182       if (LoadInst* L = dyn_cast<LoadInst>(S->getOperand(0))) {
 183         Instruction* dep = MD.getDependency(S);
 184         DominatorTree& DT = getAnalysis<DominatorTree>();
 185
 186         if (!S->isVolatile() && S->getParent() == L->getParent() &&
 187             S->getPointerOperand() == L->getPointerOperand() &&
 188             ( dep == MemoryDependenceAnalysis::None ||
 189               dep == MemoryDependenceAnalysis::NonLocal ||
 190               DT.dominates(dep, L))) {
 191           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 192             possiblyDead.insert(D);
 193           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 194             possiblyDead.insert(D);
 195
 196           // Avoid iterator invalidation.
 197           BBI--;
 198
 199           MD.removeInstruction(S);
 200           S->eraseFromParent();
 201           NumFastStores++;
 202           MadeChange = true;
 203         } else
 204           // Update our most-recent-store map.
 205           last = S;
 206       } else
 207         // Update our most-recent-store map.
 208         last = S;
 209     }
 210   }
 211
 212   // If this block ends in a return, unwind, unreachable, and eventually
 213   // tailcall, then all allocas are dead at its end.
 214   if (BB.getTerminator()->getNumSuccessors() == 0)
 215     MadeChange |= handleEndBlock(BB, possiblyDead);
 216
 217   // Do a trivial DCE
 218   while (!possiblyDead.empty()) {
 219     Instruction *I = possiblyDead.back();
 220     possiblyDead.pop_back();
 221     DeleteDeadInstructionChains(I, possiblyDead);
 222   }
 223
 224   return MadeChange;
 225 }
 226
 227 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 228 /// dependency is a store to a field of that structure
 229 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
 230                                        SetVector<Instruction*>& possiblyDead) {
 231   TargetData &TD = getAnalysis<TargetData>();
 232   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 233   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 234
 235   if (dep == MemoryDependenceAnalysis::None ||
 236       dep == MemoryDependenceAnalysis::NonLocal)
 237     return false;
 238
 239   StoreInst* dependency = dyn_cast<StoreInst>(dep);
 240   if (!dependency)
 241     return false;
 242   else if (dependency->isVolatile())
 243     return false;
 244
 245   Value* depPointer = dependency->getPointerOperand();
 246   const Type* depType = dependency->getOperand(0)->getType();
 247   unsigned depPointerSize = TD.getTypeStoreSize(depType);
 248
 249   // Check for aliasing
 250   AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0U,
 251                                           depPointer, depPointerSize);
 252
 253   if (A == AliasAnalysis::MustAlias) {
 254     // Remove it!
 255     MD.removeInstruction(dependency);
 256
 257     // DCE instructions only used to calculate that store
 258     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
 259       possiblyDead.insert(D);
 260     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
 261       possiblyDead.insert(D);
 262
 263     dependency->eraseFromParent();
 264     NumFastStores++;
 265     return true;
 266   }
 267
 268   return false;
 269 }
 270
 271 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
 272 /// function end block.  Ex:
 273 /// %A = alloca i32
 274 /// ...
 275 /// store i32 1, i32* %A
 276 /// ret void
 277 bool DSE::handleEndBlock(BasicBlock& BB,
 278                          SetVector<Instruction*>& possiblyDead) {
 279   TargetData &TD = getAnalysis<TargetData>();
 280   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 281   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 282
 283   bool MadeChange = false;
 284
 285   // Pointers alloca'd in this function are dead in the end block
 286   SmallPtrSet<Value*, 64> deadPointers;
 287
 288   // Find all of the alloca'd pointers in the entry block
 289   BasicBlock *Entry = BB.getParent()->begin();
 290   for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
 291     if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
 292       deadPointers.insert(AI);
 293   for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
 294        AE = BB.getParent()->arg_end(); AI != AE; ++AI)
 295     if (AI->hasByValAttr())
 296       deadPointers.insert(AI);
 297
 298   // Scan the basic block backwards
 299   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
 300     --BBI;
 301
 302     // If we find a store whose pointer is dead...
 303     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
 304       if (!S->isVolatile()) {
 305         Value* pointerOperand = S->getPointerOperand();
 306         // See through pointer-to-pointer bitcasts
 307         TranslatePointerBitCasts(pointerOperand);
 308
 309         // Alloca'd pointers or byval arguments (which are functionally like
 310         // alloca's) are valid candidates for removal.
 311         if (deadPointers.count(pointerOperand)) {
 312           // Remove it!
 313           MD.removeInstruction(S);
 314
 315           // DCE instructions only used to calculate that store
 316           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 317             possiblyDead.insert(D);
 318           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 319             possiblyDead.insert(D);
 320
 321           BBI++;
 322           MD.removeInstruction(S);
 323           S->eraseFromParent();
 324           NumFastStores++;
 325           MadeChange = true;
 326         }
 327       }
 328
 329       continue;
 330
 331     // We can also remove memcpy's to local variables at the end of a function
 332     } else if (MemCpyInst* M = dyn_cast<MemCpyInst>(BBI)) {
 333       Value* dest = M->getDest();
 334       TranslatePointerBitCasts(dest);
 335
 336       if (deadPointers.count(dest)) {
 337         MD.removeInstruction(M);
 338
 339         // DCE instructions only used to calculate that memcpy
 340         if (Instruction* D = dyn_cast<Instruction>(M->getRawSource()))
 341           possiblyDead.insert(D);
 342         if (Instruction* D = dyn_cast<Instruction>(M->getLength()))
 343           possiblyDead.insert(D);
 344         if (Instruction* D = dyn_cast<Instruction>(M->getRawDest()))
 345           possiblyDead.insert(D);
 346
 347         BBI++;
 348         M->eraseFromParent();
 349         NumFastOther++;
 350         MadeChange = true;
 351
 352         continue;
 353       }
 354
 355       // Because a memcpy is also a load, we can't skip it if we didn't remove it
 356     }
 357
 358     Value* killPointer = 0;
 359     uint64_t killPointerSize = ~0UL;
 360
 361     // If we encounter a use of the pointer, it is no longer considered dead
 362     if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
 363       // However, if this load is unused and not volatile, we can go ahead and
 364       // remove it, and not have to worry about it making our pointer undead!
 365       if (L->use_empty() && !L->isVolatile()) {
 366         MD.removeInstruction(L);
 367
 368         // DCE instructions only used to calculate that load
 369         if (Instruction* D = dyn_cast<Instruction>(L->getPointerOperand()))
 370           possiblyDead.insert(D);
 371
 372         BBI++;
 373         L->eraseFromParent();
 374         NumFastOther++;
 375         MadeChange = true;
 376         possiblyDead.remove(L);
 377
 378         continue;
 379       }
 380
 381       killPointer = L->getPointerOperand();
 382     } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
 383       killPointer = V->getOperand(0);
 384     } else if (isa<MemCpyInst>(BBI) &&
 385                isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
 386       killPointer = cast<MemCpyInst>(BBI)->getSource();
 387       killPointerSize = cast<ConstantInt>(
 388                             cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
 389     } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
 390       deadPointers.erase(A);
 391
 392       // Dead alloca's can be DCE'd when we reach them
 393       if (A->use_empty()) {
 394         MD.removeInstruction(A);
 395
 396         // DCE instructions only used to calculate that load
 397         if (Instruction* D = dyn_cast<Instruction>(A->getArraySize()))
 398           possiblyDead.insert(D);
 399
 400         BBI++;
 401         A->eraseFromParent();
 402         NumFastOther++;
 403         MadeChange = true;
 404         possiblyDead.remove(A);
 405       }
 406
 407       continue;
 408     } else if (CallSite::get(BBI).getInstruction() != 0) {
 409       // If this call does not access memory, it can't
 410       // be undeadifying any of our pointers.
 411       CallSite CS = CallSite::get(BBI);
 412       if (AA.doesNotAccessMemory(CS))
 413         continue;
 414
 415       unsigned modRef = 0;
 416       unsigned other = 0;
 417
 418       // Remove any pointers made undead by the call from the dead set
 419       std::vector<Value*> dead;
 420       for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
 421            E = deadPointers.end(); I != E; ++I) {
 422         // HACK: if we detect that our AA is imprecise, it's not
 423         // worth it to scan the rest of the deadPointers set.  Just
 424         // assume that the AA will return ModRef for everything, and
 425         // go ahead and bail.
 426         if (modRef >= 16 && other == 0) {
 427           deadPointers.clear();
 428           return MadeChange;
 429         }
 430
 431         // Get size information for the alloca
 432         unsigned pointerSize = ~0U;
 433         if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
 434           if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
 435             pointerSize = C->getZExtValue() * \
 436                           TD.getABITypeSize(A->getAllocatedType());
 437         } else {
 438           const PointerType* PT = cast<PointerType>(
 439                                                  cast<Argument>(*I)->getType());
 440           pointerSize = TD.getABITypeSize(PT->getElementType());
 441         }
 442
 443         // See if the call site touches it
 444         AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
 445
 446         if (A == AliasAnalysis::ModRef)
 447           modRef++;
 448         else
 449           other++;
 450
 451         if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
 452           dead.push_back(*I);
 453       }
 454
 455       for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
 456            I != E; ++I)
 457         deadPointers.erase(*I);
 458
 459       continue;
 460     } else {
 461       // For any non-memory-affecting non-terminators, DCE them as we reach them
 462       Instruction *CI = BBI;
 463       if (!CI->isTerminator() && CI->use_empty() && !isa<FreeInst>(CI)) {
 464
 465         // DCE instructions only used to calculate that load
 466         for (Instruction::op_iterator OI = CI->op_begin(), OE = CI->op_end();
 467              OI != OE; ++OI)
 468           if (Instruction* D = dyn_cast<Instruction>(OI))
 469             possiblyDead.insert(D);
 470
 471         BBI++;
 472         CI->eraseFromParent();
 473         NumFastOther++;
 474         MadeChange = true;
 475         possiblyDead.remove(CI);
 476
 477         continue;
 478       }
 479     }
 480
 481     if (!killPointer)
 482       continue;
 483
 484     TranslatePointerBitCasts(killPointer);
 485
 486     // Deal with undead pointers
 487     MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
 488                                        deadPointers, possiblyDead);
 489   }
 490
 491   return MadeChange;
 492 }
 493
 494 /// RemoveUndeadPointers - check for uses of a pointer that make it
 495 /// undead when scanning for dead stores to alloca's.
 496 bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
 497                                 BasicBlock::iterator& BBI,
 498                                 SmallPtrSet<Value*, 64>& deadPointers,
 499                                 SetVector<Instruction*>& possiblyDead) {
 500   TargetData &TD = getAnalysis<TargetData>();
 501   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 502   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 503
 504   // If the kill pointer can be easily reduced to an alloca,
 505   // don't bother doing extraneous AA queries
 506   if (deadPointers.count(killPointer)) {
 507     deadPointers.erase(killPointer);
 508     return false;
 509   } else if (isa<GlobalValue>(killPointer)) {
 510     // A global can't be in the dead pointer set
 511     return false;
 512   }
 513
 514   bool MadeChange = false;
 515
 516   std::vector<Value*> undead;
 517
 518   for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
 519       E = deadPointers.end(); I != E; ++I) {
 520     // Get size information for the alloca
 521     unsigned pointerSize = ~0U;
 522     if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
 523       if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
 524         pointerSize = C->getZExtValue() * \
 525                       TD.getABITypeSize(A->getAllocatedType());
 526     } else {
 527       const PointerType* PT = cast<PointerType>(
 528                                                 cast<Argument>(*I)->getType());
 529       pointerSize = TD.getABITypeSize(PT->getElementType());
 530     }
 531
 532     // See if this pointer could alias it
 533     AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
 534                                             killPointer, killPointerSize);
 535
 536     // If it must-alias and a store, we can delete it
 537     if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
 538       StoreInst* S = cast<StoreInst>(BBI);
 539
 540       // Remove it!
 541       MD.removeInstruction(S);
 542
 543       // DCE instructions only used to calculate that store
 544       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 545         possiblyDead.insert(D);
 546       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 547         possiblyDead.insert(D);
 548
 549       BBI++;
 550       S->eraseFromParent();
 551       NumFastStores++;
 552       MadeChange = true;
 553
 554       continue;
 555
 556       // Otherwise, it is undead
 557       } else if (A != AliasAnalysis::NoAlias)
 558         undead.push_back(*I);
 559   }
 560
 561   for (std::vector<Value*>::iterator I = undead.begin(), E = undead.end();
 562        I != E; ++I)
 563       deadPointers.erase(*I);
 564
 565   return MadeChange;
 566 }
 567
 568 /// DeleteDeadInstructionChains - takes an instruction and a setvector of
 569 /// dead instructions.  If I is dead, it is erased, and its operands are
 570 /// checked for deadness.  If they are dead, they are added to the dead
 571 /// setvector.
 572 void DSE::DeleteDeadInstructionChains(Instruction *I,
 573                                       SetVector<Instruction*> &DeadInsts) {
 574   // Instruction must be dead.
 575   if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
 576
 577   // Let the memory dependence know
 578   getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);
 579
 580   // See if this made any operands dead.  We do it this way in case the
 581   // instruction uses the same operand twice.  We don't want to delete a
 582   // value then reference it.
 583   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
 584     if (I->getOperand(i)->hasOneUse())
 585       if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
 586         DeadInsts.insert(Op);      // Attempt to nuke it later.
 587
 588     I->setOperand(i, 0);         // Drop from the operand list.
 589   }
 590
 591   I->eraseFromParent();
 592   ++NumFastOther;
 593 }