lib/Transforms/Scalar/DeadStoreElimination.cpp

   1 //===- DeadStoreElimination.cpp - Dead Store Elimination ------------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by the LLVM research group 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 #include "llvm/Transforms/Scalar.h"
  19 #include "llvm/DerivedTypes.h"
  20 #include "llvm/Function.h"
  21 #include "llvm/Instructions.h"
  22 #include "llvm/Analysis/AliasAnalysis.h"
  23 #include "llvm/Analysis/AliasSetTracker.h"
  24 #include "llvm/Target/TargetData.h"
  25 #include "llvm/Transforms/Utils/Local.h"
  26 #include "llvm/ADT/SetVector.h"
  27 #include "llvm/ADT/Statistic.h"
  28 using namespace llvm;
  29
  30 namespace {
  31   Statistic<> NumStores("dse", "Number of stores deleted");
  32   Statistic<> NumOther ("dse", "Number of other instrs removed");
  33
  34   struct DSE : public FunctionPass {
  35
  36     virtual bool runOnFunction(Function &F) {
  37       bool Changed = false;
  38       for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  39         Changed |= runOnBasicBlock(*I);
  40       return Changed;
  41     }
  42
  43     bool runOnBasicBlock(BasicBlock &BB);
  44
  45     void DeleteDeadInstructionChains(Instruction *I,
  46                                      SetVector<Instruction*> &DeadInsts);
  47
  48     // getAnalysisUsage - We require post dominance frontiers (aka Control
  49     // Dependence Graph)
  50     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  51       AU.setPreservesCFG();
  52       AU.addRequired<TargetData>();
  53       AU.addRequired<AliasAnalysis>();
  54       AU.addPreserved<AliasAnalysis>();
  55     }
  56   };
  57   RegisterOpt<DSE> X("dse", "Dead Store Elimination");
  58 }
  59
  60 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
  61
  62 bool DSE::runOnBasicBlock(BasicBlock &BB) {
  63   TargetData &TD = getAnalysis<TargetData>();
  64   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  65   AliasSetTracker KillLocs(AA);
  66
  67   // If this block ends in a return, unwind, unreachable, and eventually
  68   // tailcall, then all allocas are dead at its end.
  69   if (BB.getTerminator()->getNumSuccessors() == 0) {
  70     BasicBlock *Entry = BB.getParent()->begin();
  71     for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
  72       if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
  73         unsigned Size = ~0U;
  74         if (!AI->isArrayAllocation() &&
  75             AI->getType()->getElementType()->isSized())
  76           Size = (unsigned)TD.getTypeSize(AI->getType()->getElementType());
  77         KillLocs.add(AI, Size);
  78       }
  79   }
  80
  81   // PotentiallyDeadInsts - Deleting dead stores from the program can make other
  82   // instructions die if they were only used as operands to stores.  Keep track
  83   // of the operands to stores so that we can try deleting them at the end of
  84   // the traversal.
  85   SetVector<Instruction*> PotentiallyDeadInsts;
  86
  87   bool MadeChange = false;
  88   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
  89     Instruction *I = --BBI;   // Keep moving iterator backwards
  90
  91     // If this is a free instruction, it makes the free'd location dead!
  92     if (FreeInst *FI = dyn_cast<FreeInst>(I)) {
  93       // Free instructions make any stores to the free'd location dead.
  94       KillLocs.add(FI);
  95       continue;
  96     }
  97
  98     if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
  99       // If this is a vaarg instruction, it reads its operand.  We don't model
 100       // it correctly, so just conservatively remove all entries.
 101       if (isa<VAArgInst>(I)) {
 102         KillLocs.clear();
 103         continue;
 104       }
 105
 106       // If this is a non-store instruction, it makes everything referenced no
 107       // longer killed.  Remove anything aliased from the alias set tracker.
 108       KillLocs.remove(I);
 109       continue;
 110     }
 111
 112     // If this is a non-volatile store instruction, and if it is already in
 113     // the stored location is already in the tracker, then this is a dead
 114     // store.  We can just delete it here, but while we're at it, we also
 115     // delete any trivially dead expression chains.
 116     unsigned ValSize = (unsigned)TD.getTypeSize(I->getOperand(0)->getType());
 117     Value *Ptr = I->getOperand(1);
 118
 119     if (AliasSet *AS = KillLocs.getAliasSetForPointerIfExists(Ptr, ValSize))
 120       for (AliasSet::iterator ASI = AS->begin(), E = AS->end(); ASI != E; ++ASI)
 121         if (ASI.getSize() >= ValSize &&  // Overwriting all of this store.
 122             AA.alias(ASI.getPointer(), ASI.getSize(), Ptr, ValSize)
 123                == AliasAnalysis::MustAlias) {
 124           // If we found a must alias in the killed set, then this store really
 125           // is dead.  Remember that the various operands of the store now have
 126           // fewer users.  At the end we will see if we can delete any values
 127           // that are dead as part of the store becoming dead.
 128           if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(0)))
 129             PotentiallyDeadInsts.insert(Op);
 130           if (Instruction *Op = dyn_cast<Instruction>(Ptr))
 131             PotentiallyDeadInsts.insert(Op);
 132
 133           // Delete it now.
 134           ++BBI;                        // Don't invalidate iterator.
 135           BB.getInstList().erase(I);    // Nuke the store!
 136           ++NumStores;
 137           MadeChange = true;
 138           goto BigContinue;
 139         }
 140
 141     // Otherwise, this is a non-dead store just add it to the set of dead
 142     // locations.
 143     KillLocs.add(cast<StoreInst>(I));
 144   BigContinue:;
 145   }
 146
 147   while (!PotentiallyDeadInsts.empty()) {
 148     Instruction *I = PotentiallyDeadInsts.back();
 149     PotentiallyDeadInsts.pop_back();
 150     DeleteDeadInstructionChains(I, PotentiallyDeadInsts);
 151   }
 152   return MadeChange;
 153 }
 154
 155 void DSE::DeleteDeadInstructionChains(Instruction *I,
 156                                       SetVector<Instruction*> &DeadInsts) {
 157   // Instruction must be dead.
 158   if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
 159
 160   // Let the alias analysis know that we have nuked a value.
 161   getAnalysis<AliasAnalysis>().deleteValue(I);
 162
 163   // See if this made any operands dead.  We do it this way in case the
 164   // instruction uses the same operand twice.  We don't want to delete a
 165   // value then reference it.
 166   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
 167     if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(i)))
 168       DeadInsts.insert(Op);      // Attempt to nuke it later.
 169     I->setOperand(i, 0);         // Drop from the operand list.
 170   }
 171
 172   I->eraseFromParent();
 173   ++NumOther;
 174 }