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