lib/Transforms/Scalar/DeadStoreElimination.cpp

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