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 :)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15101 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -0,0 +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;
+    }
+}