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
 117       pointer = cast<FreeInst>(BBI)->getPointerOperand();
 118
 119     StoreInst*& last = lastStore[pointer];
 120     bool deletedStore = false;
 121
 122     // ... to a pointer that has been stored to before...
 123     if (last) {
 124       Instruction* dep = MD.getDependency(BBI);
 125
 126       // ... and no other memory dependencies are between them....
 127       while (dep != MemoryDependenceAnalysis::None &&
 128              dep != MemoryDependenceAnalysis::NonLocal &&
 129              isa<StoreInst>(dep)) {
 130         if (dep != last) {
 131           dep = MD.getDependency(BBI, dep);
 132           continue;
 133         }
 134
 135         // Remove it!
 136         MD.removeInstruction(last);
 137
 138         // DCE instructions only used to calculate that store
 139         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
 140           possiblyDead.insert(D);
 141         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
 142           possiblyDead.insert(D);
 143
 144         last->eraseFromParent();
 145         NumFastStores++;
 146         deletedStore = true;
 147         MadeChange = true;
 148
 149         break;
 150       }
 151     }
 152
 153     // Handle frees whose dependencies are non-trivial.
 154     if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
 155       if (!deletedStore)
 156         MadeChange |= handleFreeWithNonTrivialDependency(F,
 157                                                          MD.getDependency(F),
 158                                                          possiblyDead);
 159       // No known stores after the free
 160       last = 0;
 161     } else {
 162       // Update our most-recent-store map.
 163       last = cast<StoreInst>(BBI);
 164     }
 165   }
 166
 167   // If this block ends in a return, unwind, unreachable, and eventually
 168   // tailcall, then all allocas are dead at its end.
 169   if (BB.getTerminator()->getNumSuccessors() == 0)
 170     MadeChange |= handleEndBlock(BB, possiblyDead);
 171
 172   // Do a trivial DCE
 173   while (!possiblyDead.empty()) {
 174     Instruction *I = possiblyDead.back();
 175     possiblyDead.pop_back();
 176     DeleteDeadInstructionChains(I, possiblyDead);
 177   }
 178
 179   return MadeChange;
 180 }
 181
 182 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 183 /// dependency is a store to a field of that structure
 184 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
 185                                        SetVector<Instruction*>& possiblyDead) {
 186   TargetData &TD = getAnalysis<TargetData>();
 187   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 188   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 189
 190   if (dep == MemoryDependenceAnalysis::None ||
 191       dep == MemoryDependenceAnalysis::NonLocal)
 192     return false;
 193
 194   StoreInst* dependency = dyn_cast<StoreInst>(dep);
 195   if (!dependency)
 196     return false;
 197
 198   Value* depPointer = dependency->getPointerOperand();
 199   const Type* depType = dependency->getOperand(0)->getType();
 200   unsigned depPointerSize = TD.getTypeSize(depType);
 201
 202   // Check for aliasing
 203   AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0UL,
 204                                           depPointer, depPointerSize);
 205
 206   if (A == AliasAnalysis::MustAlias) {
 207     // Remove it!
 208     MD.removeInstruction(dependency);
 209
 210     // DCE instructions only used to calculate that store
 211     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
 212       possiblyDead.insert(D);
 213     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
 214       possiblyDead.insert(D);
 215
 216     dependency->eraseFromParent();
 217     NumFastStores++;
 218     return true;
 219   }
 220
 221   return false;
 222 }
 223
 224 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
 225 /// function end block
 226 bool DSE::handleEndBlock(BasicBlock& BB,
 227                          SetVector<Instruction*>& possiblyDead) {
 228   TargetData &TD = getAnalysis<TargetData>();
 229   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 230   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 231
 232   bool MadeChange = false;
 233
 234   // Pointers alloca'd in this function are dead in the end block
 235   SmallPtrSet<AllocaInst*, 4> deadPointers;
 236
 237   // Find all of the alloca'd pointers in the entry block
 238   BasicBlock *Entry = BB.getParent()->begin();
 239   for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
 240     if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
 241       deadPointers.insert(AI);
 242
 243   // Scan the basic block backwards
 244   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
 245     --BBI;
 246
 247     if (deadPointers.empty())
 248       break;
 249
 250     Value* killPointer = 0;
 251     unsigned killPointerSize = 0;
 252
 253     // If we find a store whose pointer is dead...
 254     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
 255       Value* pointerOperand = S->getPointerOperand();
 256       // See through pointer-to-pointer bitcasts
 257       TranslatePointerBitCasts(pointerOperand);
 258
 259       if (deadPointers.count(pointerOperand)){
 260         // Remove it!
 261         MD.removeInstruction(S);
 262
 263         // DCE instructions only used to calculate that store
 264         if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 265           possiblyDead.insert(D);
 266         if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 267           possiblyDead.insert(D);
 268
 269         BBI++;
 270         S->eraseFromParent();
 271         NumFastStores++;
 272         MadeChange = true;
 273       }
 274
 275     // If we encounter a use of the pointer, it is no longer considered dead
 276     } else if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
 277       killPointer = L->getPointerOperand();
 278       killPointerSize = TD.getTypeSize(L->getType());
 279     } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
 280       killPointer = V->getOperand(0);
 281       killPointerSize = TD.getTypeSize(V->getType());
 282     } else if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
 283       killPointer = F->getPointerOperand();
 284       killPointerSize = ~0UL;
 285     } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
 286       deadPointers.erase(A);
 287       continue;
 288     } else if (CallSite::get(BBI).getInstruction() != 0) {
 289       // Remove any pointers made undead by the call from the dead set
 290       std::vector<Instruction*> dead;
 291       for (SmallPtrSet<AllocaInst*, 4>::iterator I = deadPointers.begin(),
 292            E = deadPointers.end(); I != E; ++I) {
 293         // Get size information for the alloca
 294         unsigned pointerSize = ~0UL;
 295         if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
 296           pointerSize = C->getZExtValue() * \
 297                         TD.getTypeSize((*I)->getAllocatedType());
 298
 299         // See if the call site touches it
 300         AliasAnalysis::ModRefResult A = AA.getModRefInfo(CallSite::get(BBI),
 301                                                          *I, pointerSize);
 302         if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
 303           dead.push_back(*I);
 304       }
 305
 306       for (std::vector<Instruction*>::iterator I = dead.begin(), E = dead.end();
 307            I != E; ++I)
 308         deadPointers.erase(*I);
 309
 310       continue;
 311     }
 312
 313     if (!killPointer)
 314       continue;
 315
 316     // Deal with undead pointers
 317     MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
 318                                        deadPointers, possiblyDead);
 319   }
 320
 321   return MadeChange;
 322 }
 323
 324 bool DSE::RemoveUndeadPointers(Value* killPointer, unsigned killPointerSize,
 325                                 BasicBlock::iterator& BBI,
 326                                 SmallPtrSet<AllocaInst*, 4>& deadPointers,
 327                                 SetVector<Instruction*>& possiblyDead) {
 328   TargetData &TD = getAnalysis<TargetData>();
 329   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
 330   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
 331
 332   bool MadeChange = false;
 333
 334   std::vector<Instruction*> undead;
 335
 336   for (SmallPtrSet<AllocaInst*, 4>::iterator I = deadPointers.begin(),
 337       E = deadPointers.end(); I != E; ++I) {
 338     // Get size information for the alloca
 339     unsigned pointerSize = ~0UL;
 340     if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
 341       pointerSize = C->getZExtValue() * \
 342                     TD.getTypeSize((*I)->getAllocatedType());
 343
 344     // See if this pointer could alias it
 345     AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
 346                                             killPointer, killPointerSize);
 347
 348     // If it must-alias and a store, we can delete it
 349     if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
 350       StoreInst* S = cast<StoreInst>(BBI);
 351
 352       // Remove it!
 353       MD.removeInstruction(S);
 354
 355       // DCE instructions only used to calculate that store
 356       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
 357         possiblyDead.insert(D);
 358       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
 359         possiblyDead.insert(D);
 360
 361       BBI++;
 362       S->eraseFromParent();
 363       NumFastStores++;
 364       MadeChange = true;
 365
 366       continue;
 367
 368       // Otherwise, it is undead
 369       } else if (A != AliasAnalysis::NoAlias)
 370         undead.push_back(*I);
 371   }
 372
 373   for (std::vector<Instruction*>::iterator I = undead.begin(), E = undead.end();
 374        I != E; ++I)
 375     deadPointers.erase(*I);
 376
 377   return MadeChange;
 378 }
 379
 380 void DSE::DeleteDeadInstructionChains(Instruction *I,
 381                                       SetVector<Instruction*> &DeadInsts) {
 382   // Instruction must be dead.
 383   if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
 384
 385   // Let the memory dependence know
 386   getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);
 387
 388   // See if this made any operands dead.  We do it this way in case the
 389   // instruction uses the same operand twice.  We don't want to delete a
 390   // value then reference it.
 391   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
 392     if (I->getOperand(i)->hasOneUse())
 393       if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
 394         DeadInsts.insert(Op);      // Attempt to nuke it later.
 395
 396     I->setOperand(i, 0);         // Drop from the operand list.
 397   }
 398
 399   I->eraseFromParent();
 400   ++NumFastOther;
 401 }