lib/Transforms/Scalar/DeadStoreElimination.cpp

   1 //===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by Owen Anderson and is distributed under
   6 // the University of Illinois Open Source 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/Pass.h"
  24 #include "llvm/ADT/SetVector.h"
  25 #include "llvm/ADT/SmallPtrSet.h"
  26 #include "llvm/ADT/Statistic.h"
  27 #include "llvm/Analysis/AliasAnalysis.h"
  28 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
  29 #include "llvm/Target/TargetData.h"
  30 #include "llvm/Transforms/Utils/Local.h"
  31 #include "llvm/Support/Compiler.h"
  32 using namespace llvm;
  33
  34 STATISTIC(NumFastStores, "Number of stores deleted");
  35 STATISTIC(NumFastOther , "Number of other instrs removed");
  36
  37 namespace {
  38   struct VISIBILITY_HIDDEN DSE : public FunctionPass {
  39     static char ID; // Pass identification, replacement for typeid
  40     DSE() : FunctionPass((intptr_t)&ID) {}
  41
  42     virtual bool runOnFunction(Function &F) {
  43       bool Changed = false;
  44       for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  45         Changed |= runOnBasicBlock(*I);
  46       return Changed;
  47     }
  48
  49     bool runOnBasicBlock(BasicBlock &BB);
  50     bool handleFreeWithNonTrivialDependency(FreeInst* F,
  51                                             Instruction* dependency,
  52                                         SetVector<Instruction*>& possiblyDead);
  53     bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
  54     bool RemoveUndeadPointers(Value* pointer, unsigned pointerSize,
  55                               BasicBlock::iterator& BBI,
  56                               SmallPtrSet<AllocaInst*, 4>& deadPointers,
  57                               SetVector<Instruction*>& possiblyDead);
  58     void DeleteDeadInstructionChains(Instruction *I,
  59                                      SetVector<Instruction*> &DeadInsts);
  60
  61     // Find the base pointer that a pointer came from
  62     // Because this is used to find pointers that originate
  63     // from allocas, it is safe to ignore GEP indices, since
  64     // either the store will be in the alloca, and thus dead,
  65     // or beyond the end of the alloca, and thus undefined.
  66     void TranslatePointerBitCasts(Value*& v) {
  67       assert(isa<PointerType>(v->getType()) &&
  68              "Translating a non-pointer type?");
  69       while (true) {
  70         if (BitCastInst* C = dyn_cast<BitCastInst>(v))
  71           v = C->getOperand(0);
  72         else if (GetElementPtrInst* G = dyn_cast<GetElementPtrInst>(v))
  73           v = G->getOperand(0);
  74         else
  75           break;
  76       }
  77     }
  78
  79     // getAnalysisUsage - We require post dominance frontiers (aka Control
  80     // Dependence Graph)
  81     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  82       AU.setPreservesCFG();
  83       AU.addRequired<TargetData>();
  84       AU.addRequired<AliasAnalysis>();
  85       AU.addRequired<MemoryDependenceAnalysis>();
  86       AU.addPreserved<AliasAnalysis>();
  87       AU.addPreserved<MemoryDependenceAnalysis>();
  88     }
  89   };
  90   char DSE::ID = 0;
  91   RegisterPass<DSE> X("dse", "Dead Store Elimination");
  92 }
  93
  94 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
  95
  96 bool DSE::runOnBasicBlock(BasicBlock &BB) {
  97   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
  98
  99   // Record the last-seen store to this pointer
 100   DenseMap<Value*, StoreInst*> lastStore;
 101   // Record instructions possibly made dead by deleting a store
 102   SetVector<Instruction*> possiblyDead;
 103
 104   bool MadeChange = false;
 105
 106   // Do a top-down walk on the BB
 107   for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
 108        BBI != BBE; ++BBI) {
 109     // If we find a store or a free...
 110     if (!isa<StoreInst>(BBI) && !isa<FreeInst>(BBI))
 111       continue;
 112
 113     Value* pointer = 0;
 114     if (StoreInst* S = dyn_cast<StoreInst>(BBI))
 115       pointer = S->getPointerOperand();
 116     else if (FreeInst* F = dyn_cast<FreeInst>(BBI))
 117       pointer = F->getPointerOperand();
 118
 119     assert(pointer && "Not a free or a store?");
 120
 121     StoreInst*& last = lastStore[pointer];
 122     bool deletedStore = false;
 123
 124     // ... to a pointer that has been stored to before...
 125     if (last) {
 126       Instruction* dep = MD.getDependency(BBI);
 127
 128       // ... and no other memory dependencies are between them....
 129       while (dep != MemoryDependenceAnalysis::None &&
 130              dep != MemoryDependenceAnalysis::NonLocal &&
 131              isa<StoreInst>(dep)) {
 132         if (dep != last) {
 133           dep = MD.getDependency(BBI, dep);
 134           continue;
 135         }
 136
 137         // Remove it!
 138         MD.removeInstruction(last);
 139
 140         // DCE instructions only used to calculate that store
 141         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
 142           possiblyDead.insert(D);
 143         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
 144           possiblyDead.insert(D);
 145
 146         last->eraseFromParent();
 147         NumFastStores++;
 148         deletedStore = true;
 149         MadeChange = true;
 150
 151         break;
 152       }
 153     }
 154
 155     // Handle frees whose dependencies are non-trivial.
 156     if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
 157       if (!deletedStore)
 158         MadeChange |= handleFreeWithNonTrivialDependency(F,
 159                                                          MD.getDependency(F),
 160                                                          possiblyDead);
 161       // No known stores after the free
 162       last = 0;
 163     } else {
 164       // Update our most-recent-store map.
 165       last = cast<StoreInst>(BBI);
 166     }
 167   }
 168
 169   // If this block ends in a return, unwind, unreachable, and eventually
 170   // tailcall, then all allocas are dead at its end.
 171   if (BB.getTerminator()->getNumSuccessors() == 0)
 172     MadeChange |= handleEndBlock(BB, possiblyDead);
 173
 174   // Do a trivial DCE
 175   while (!possiblyDead.empty()) {
 176     Instruction *I = possiblyDead.back();
 177     possiblyDead.pop_back();
 178     DeleteDeadInstructionChains(I, possiblyDead);
 179   }
 180
 181   return MadeChange;
 182 }
 183
 184 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 185 /// dependency is a store to a field of that structure
 186 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
 187                                        SetVector<Instruction*>& possiblyDead) {
 188   TargetData &TD = getAnalysis<TargetData>();
 189   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 190   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 191
 192   if (dep == MemoryDependenceAnalysis::None ||
 193       dep == MemoryDependenceAnalysis::NonLocal)
 194     return false;
 195
 196   StoreInst* dependency = dyn_cast<StoreInst>(dep);
 197   if (!dependency)
 198     return false;
 199
 200   Value* depPointer = dependency->getPointerOperand();
 201   const Type* depType = dependency->getOperand(0)->getType();
 202   unsigned depPointerSize = TD.getTypeSize(depType);
 203
 204   // Check for aliasing
 205   AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0UL,
 206                                           depPointer, depPointerSize);
 207
 208   if (A == AliasAnalysis::MustAlias) {
 209     // Remove it!
 210     MD.removeInstruction(dependency);
 211
 212     // DCE instructions only used to calculate that store
 213     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
 214       possiblyDead.insert(D);
 215     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
 216       possiblyDead.insert(D);
 217
 218     dependency->eraseFromParent();
 219     NumFastStores++;
 220     return true;
 221   }
 222
 223   return false;
 224 }
 225
 226 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
 227 /// function end block
 228 bool DSE::handleEndBlock(BasicBlock& BB,
 229                          SetVector<Instruction*>& possiblyDead) {
 230   TargetData &TD = getAnalysis<TargetData>();
 231   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 232   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 233
 234   bool MadeChange = false;
 235
 236   // Pointers alloca'd in this function are dead in the end block
 237   SmallPtrSet<AllocaInst*, 4> deadPointers;
 238
 239   // Find all of the alloca'd pointers in the entry block
 240   BasicBlock *Entry = BB.getParent()->begin();
 241   for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
 242     if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
 243       deadPointers.insert(AI);
 244
 245   // Scan the basic block backwards
 246   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
 247     --BBI;
 248
 249     if (deadPointers.empty())
 250       break;
 251
 252     Value* killPointer = 0;
 253     unsigned killPointerSize = 0;
 254
 255     // If we find a store whose pointer is dead...
 256     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
 257       Value* pointerOperand = S->getPointerOperand();
 258       // See through pointer-to-pointer bitcasts
 259       TranslatePointerBitCasts(pointerOperand);
 260
 261       if (deadPointers.count(pointerOperand)){
 262         // Remove it!
 263         MD.removeInstruction(S);
 264
 265         // DCE instructions only used to calculate that store
 266         if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 267           possiblyDead.insert(D);
 268         if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 269           possiblyDead.insert(D);
 270
 271         BBI++;
 272         S->eraseFromParent();
 273         NumFastStores++;
 274         MadeChange = true;
 275       }
 276
 277     // If we encounter a use of the pointer, it is no longer considered dead
 278     } else if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
 279       killPointer = L->getPointerOperand();
 280       killPointerSize = TD.getTypeSize(L->getType());
 281     } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
 282       killPointer = V->getOperand(0);
 283       killPointerSize = TD.getTypeSize(V->getType());
 284     } else if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
 285       killPointer = F->getPointerOperand();
 286       killPointerSize = ~0UL;
 287     } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
 288       deadPointers.erase(A);
 289       continue;
 290     } else if (CallSite::get(BBI).getInstruction() != 0) {
 291       // Remove any pointers made undead by the call from the dead set
 292       std::vector<Instruction*> dead;
 293       for (SmallPtrSet<AllocaInst*, 4>::iterator I = deadPointers.begin(),
 294            E = deadPointers.end(); I != E; ++I) {
 295         // Get size information for the alloca
 296         unsigned pointerSize = ~0UL;
 297         if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
 298           pointerSize = C->getZExtValue() * \
 299                         TD.getTypeSize((*I)->getAllocatedType());
 300
 301         // See if the call site touches it
 302         AliasAnalysis::ModRefResult A = AA.getModRefInfo(CallSite::get(BBI),
 303                                                          *I, pointerSize);
 304         if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
 305           dead.push_back(*I);
 306       }
 307
 308       for (std::vector<Instruction*>::iterator I = dead.begin(), E = dead.end();
 309            I != E; ++I)
 310         deadPointers.erase(*I);
 311
 312       continue;
 313     }
 314
 315     if (!killPointer)
 316       continue;
 317
 318     // Deal with undead pointers
 319     MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
 320                                        deadPointers, possiblyDead);
 321   }
 322
 323   return MadeChange;
 324 }
 325
 326 bool DSE::RemoveUndeadPointers(Value* killPointer, unsigned killPointerSize,
 327                                 BasicBlock::iterator& BBI,
 328                                 SmallPtrSet<AllocaInst*, 4>& deadPointers,
 329                                 SetVector<Instruction*>& possiblyDead) {
 330   TargetData &TD = getAnalysis<TargetData>();
 331   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 332   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 333
 334   bool MadeChange = false;
 335
 336   std::vector<Instruction*> undead;
 337
 338   for (SmallPtrSet<AllocaInst*, 4>::iterator I = deadPointers.begin(),
 339       E = deadPointers.end(); I != E; ++I) {
 340     // Get size information for the alloca
 341     unsigned pointerSize = ~0UL;
 342     if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
 343       pointerSize = C->getZExtValue() * \
 344                     TD.getTypeSize((*I)->getAllocatedType());
 345
 346     // See if this pointer could alias it
 347     AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
 348                                             killPointer, killPointerSize);
 349
 350     // If it must-alias and a store, we can delete it
 351     if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
 352       StoreInst* S = cast<StoreInst>(BBI);
 353
 354       // Remove it!
 355       MD.removeInstruction(S);
 356
 357       // DCE instructions only used to calculate that store
 358       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 359         possiblyDead.insert(D);
 360       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 361         possiblyDead.insert(D);
 362
 363       BBI++;
 364       S->eraseFromParent();
 365       NumFastStores++;
 366       MadeChange = true;
 367
 368       continue;
 369
 370       // Otherwise, it is undead
 371       } else if (A != AliasAnalysis::NoAlias)
 372         undead.push_back(*I);
 373   }
 374
 375   for (std::vector<Instruction*>::iterator I = undead.begin(), E = undead.end();
 376        I != E; ++I)
 377     deadPointers.erase(*I);
 378
 379   return MadeChange;
 380 }
 381
 382 void DSE::DeleteDeadInstructionChains(Instruction *I,
 383                                       SetVector<Instruction*> &DeadInsts) {
 384   // Instruction must be dead.
 385   if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
 386
 387   // Let the memory dependence know
 388   getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);
 389
 390   // See if this made any operands dead.  We do it this way in case the
 391   // instruction uses the same operand twice.  We don't want to delete a
 392   // value then reference it.
 393   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
 394     if (I->getOperand(i)->hasOneUse())
 395       if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
 396         DeadInsts.insert(Op);      // Attempt to nuke it later.
 397
 398     I->setOperand(i, 0);         // Drop from the operand list.
 399   }
 400
 401   I->eraseFromParent();
 402   ++NumFastOther;
 403 }