[llvm-commits] CVS: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

[Chris Lattner]

Changes in directory llvm/lib/Transforms/Scalar:

DeadStoreElimination.cpp added (r1.1)

---
Log message:

This is a trivial dead store elimination pass.  It very very simple and
can be improved in many ways.  But: stop laughing, even with -basicaa it 
deletes 15% of the stores in 252.eon :)


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Diffs of the changes:  (+139 -0)

Index: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
diff -c /dev/null llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp:1.1
*** /dev/null	Thu Jul 22 03:00:38 2004
--- llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp	Thu Jul 22 03:00:28 2004
***************
*** 0 ****
--- 1,139 ----
+ //===- DeadStoreElimination.cpp - Dead Store Elimination ------------------===//
+ // 
+ //                     The LLVM Compiler Infrastructure
+ //
+ // This file was developed by the LLVM research group and is distributed under
+ // the University of Illinois Open Source License. See LICENSE.TXT for details.
+ // 
+ //===----------------------------------------------------------------------===//
+ //
+ // This file implements a trivial dead store elimination that only considers
+ // basic-block local redundant stores.
+ //
+ // FIXME: This should eventually be extended to be a post-dominator tree
+ // traversal.  Doing so would be pretty trivial.
+ //
+ //===----------------------------------------------------------------------===//
+ 
+ #include "llvm/Transforms/Scalar.h"
+ #include "llvm/Function.h"
+ #include "llvm/Instructions.h"
+ #include "llvm/Analysis/AliasAnalysis.h"
+ #include "llvm/Analysis/AliasSetTracker.h"
+ #include "llvm/Target/TargetData.h"
+ #include "llvm/Transforms/Utils/Local.h"
+ #include "Support/Statistic.h"
+ using namespace llvm;
+ 
+ namespace {
+   Statistic<> NumStores("dse", "Number of stores deleted");
+   Statistic<> NumOther ("dse", "Number of other instrs removed");
+ 
+   struct DSE : public FunctionPass {
+ 
+     virtual bool runOnFunction(Function &F) {
+       bool Changed = false;
+       for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
+         Changed |= runOnBasicBlock(*I);
+       return Changed;
+     }
+     
+     bool runOnBasicBlock(BasicBlock &BB);
+     
+     void DeleteDeadValueChains(Value *V, AliasSetTracker &AST);
+ 
+     // getAnalysisUsage - We require post dominance frontiers (aka Control
+     // Dependence Graph)
+     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+       AU.addRequired<TargetData>();
+       AU.addRequired<AliasAnalysis>();
+       AU.addPreserved<AliasAnalysis>();
+     }
+   };
+   RegisterOpt<DSE> X("dse", "Dead Store Elimination");
+ }
+ 
+ Pass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
+ 
+ bool DSE::runOnBasicBlock(BasicBlock &BB) {
+   TargetData &TD = getAnalysis<TargetData>();
+   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+   AliasSetTracker KillLocs(AA);
+ 
+   // If this block ends in a return, unwind, and eventually tailcall/barrier,
+   // then all allocas are dead at its end.
+   if (BB.getTerminator()->getNumSuccessors() == 0) {
+ 
+   }
+ 
+   bool MadeChange = false;
+   for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
+     Instruction *I = --BBI;   // Keep moving iterator backwards
+     
+ #if 0
+     // AST doesn't support malloc/free/alloca???
+     if (isa<FreeInst>(I)) {
+       // Free instructions make any stores to the free'd location dead.
+       KillLocs.insert(I);
+     }
+ #endif
+ 
+     if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
+       // If this is a non-store instruction, it makes everything referenced no
+       // longer killed.  Remove anything aliased from the alias set tracker.
+       KillLocs.remove(I);
+       continue;
+     }
+ 
+     // If this is a non-volatile store instruction, and if it is already in
+     // the stored location is already in the tracker, then this is a dead
+     // store.  We can just delete it here, but while we're at it, we also
+     // delete any trivially dead expression chains.
+     unsigned ValSize = TD.getTypeSize(I->getOperand(0)->getType());
+     Value *Ptr = I->getOperand(1);
+     if (AliasSet *AS = KillLocs.getAliasSetForPointerIfExists(Ptr, ValSize))
+       for (AliasSet::iterator ASI = AS->begin(), E = AS->end(); ASI != E; ++ASI)
+         if (AA.alias(ASI.getPointer(), ASI.getSize(), Ptr, ValSize)
+                == AliasAnalysis::MustAlias) {
+           // If we found a must alias in the killed set, then this store really
+           // is dead.  Delete it now.
+           ++BBI;                        // Don't invalidate iterator.
+           Value *Val = I->getOperand(0);
+           BB.getInstList().erase(I);    // Nuke the store!
+           ++NumStores;
+           DeleteDeadValueChains(Val, KillLocs);   // Delete any now-dead instrs
+           DeleteDeadValueChains(Ptr, KillLocs);   // Delete any now-dead instrs
+           MadeChange = true;
+           goto BigContinue;
+         }
+ 
+     // Otherwise, this is a non-dead store just add it to the set of dead
+     // locations.
+     KillLocs.add(cast<StoreInst>(I));
+   BigContinue:;
+   }
+   return MadeChange;
+ }
+ 
+ void DSE::DeleteDeadValueChains(Value *V, AliasSetTracker &AST) {
+   // Value must be dead.
+   if (!V->use_empty()) return;
+ 
+   if (Instruction *I = dyn_cast<Instruction>(V))
+     if (isInstructionTriviallyDead(I)) {
+       AST.deleteValue(I);
+       getAnalysis<AliasAnalysis>().deleteValue(I);
+ 
+       // See if this made any operands dead.  We do it this way in case the
+       // instruction uses the same operand twice.  We don't want to delete a
+       // value then reference it.
+       while (unsigned NumOps = I->getNumOperands()) {
+         Value *Op = I->getOperand(NumOps-1);
+         I->op_erase(I->op_end()-1);         // Drop from the operand list.
+         DeleteDeadValueChains(Op, AST);  // Attempt to nuke it.
+       }
+ 
+       I->getParent()->getInstList().erase(I);
+       ++NumOther;
+     }
+ }