llvm/lib/Transforms/Scalar/ADCE.cpp

//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
//
// This file implements "aggressive" dead code elimination.  ADCE is DCe where
// values are assumed to be dead until proven otherwise.  This is similar to 
// SCCP, except applied to the liveness of values.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Type.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/Writer.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Constant.h"
#include "llvm/Support/CFG.h"
#include "Support/STLExtras.h"
#include "Support/DepthFirstIterator.h"
#include "Support/StatisticReporter.h"
#include <algorithm>
#include <iostream>
using std::cerr;

static Statistic<> NumBlockRemoved("adce\t\t- Number of basic blocks removed");
static Statistic<> NumInstRemoved ("adce\t\t- Number of instructions removed");

namespace {

//===----------------------------------------------------------------------===//
// ADCE Class
//
// This class does all of the work of Aggressive Dead Code Elimination.
// It's public interface consists of a constructor and a doADCE() method.
//
class ADCE : public FunctionPass {
  Function *Func;                       // The function that we are working on
  std::vector<Instruction*> WorkList;   // Instructions that just became live
  std::set<Instruction*>    LiveSet;    // The set of live instructions

  //===--------------------------------------------------------------------===//
  // The public interface for this class
  //
public:
  const char *getPassName() const { return "Aggressive Dead Code Elimination"; }
  
  // Execute the Aggressive Dead Code Elimination Algorithm
  //
  virtual bool runOnFunction(Function &F) {
    Func = &F;
    bool Changed = doADCE();
    assert(WorkList.empty());
    LiveSet.clear();
    return Changed;
  }
  // getAnalysisUsage - We require post dominance frontiers (aka Control
  // Dependence Graph)
  virtual void getAnalysisUsage(AnalysisUsage &AU) const {
    AU.addRequired(DominatorTree::PostDomID);
    AU.addRequired(DominanceFrontier::PostDomID);
  }


  //===--------------------------------------------------------------------===//
  // The implementation of this class
  //
private:
  // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
  // true if the function was modified.
  //
  bool doADCE();

  void markBlockAlive(BasicBlock *BB);

  inline void markInstructionLive(Instruction *I) {
    if (LiveSet.count(I)) return;
    DEBUG(cerr << "Insn Live: " << I);
    LiveSet.insert(I);
    WorkList.push_back(I);
  }

  inline void markTerminatorLive(const BasicBlock *BB) {
    DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
    markInstructionLive((Instruction*)BB->getTerminator());
  }
};

} // End of anonymous namespace

Pass *createAggressiveDCEPass() { return new ADCE(); }


void ADCE::markBlockAlive(BasicBlock *BB) {
  // Mark the basic block as being newly ALIVE... and mark all branches that
  // this block is control dependant on as being alive also...
  //
  DominanceFrontier &CDG =
    getAnalysis<DominanceFrontier>(DominanceFrontier::PostDomID);

  DominanceFrontier::const_iterator It = CDG.find(BB);
  if (It != CDG.end()) {
    // Get the blocks that this node is control dependant on...
    const DominanceFrontier::DomSetType &CDB = It->second;
    for_each(CDB.begin(), CDB.end(),   // Mark all their terminators as live
             bind_obj(this, &ADCE::markTerminatorLive));
  }
  
  // If this basic block is live, then the terminator must be as well!
  markTerminatorLive(BB);
}


// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
// true if the function was modified.
//
bool ADCE::doADCE() {
  bool MadeChanges = false;

  // Iterate over all of the instructions in the function, eliminating trivially
  // dead instructions, and marking instructions live that are known to be 
  // needed.  Perform the walk in depth first order so that we avoid marking any
  // instructions live in basic blocks that are unreachable.  These blocks will
  // be eliminated later, along with the instructions inside.
  //
  for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
       BBI != BBE; ++BBI) {
    BasicBlock *BB = *BBI;
    for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
      if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
	markInstructionLive(II);
        ++II;  // Increment the inst iterator if the inst wasn't deleted
      } else if (isInstructionTriviallyDead(II)) {
        // Remove the instruction from it's basic block...
        II = BB->getInstList().erase(II);
        ++NumInstRemoved;
        MadeChanges = true;
      } else {
        ++II;  // Increment the inst iterator if the inst wasn't deleted
      }
    }
  }

  DEBUG(cerr << "Processing work list\n");

  // AliveBlocks - Set of basic blocks that we know have instructions that are
  // alive in them...
  //
  std::set<BasicBlock*> AliveBlocks;

  // Process the work list of instructions that just became live... if they
  // became live, then that means that all of their operands are neccesary as
  // well... make them live as well.
  //
  while (!WorkList.empty()) {
    Instruction *I = WorkList.back(); // Get an instruction that became live...
    WorkList.pop_back();

    BasicBlock *BB = I->getParent();
    if (!AliveBlocks.count(BB)) {     // Basic block not alive yet...
      AliveBlocks.insert(BB);         // Block is now ALIVE!
      markBlockAlive(BB);             // Make it so now!
    }

    // PHI nodes are a special case, because the incoming values are actually
    // defined in the predecessor nodes of this block, meaning that the PHI
    // makes the predecessors alive.
    //
    if (PHINode *PN = dyn_cast<PHINode>(I))
      for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
        if (!AliveBlocks.count(*PI)) {
          AliveBlocks.insert(BB);         // Block is now ALIVE!
          markBlockAlive(*PI);
        }

    // Loop over all of the operands of the live instruction, making sure that
    // they are known to be alive as well...
    //
    for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
      if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
	markInstructionLive(Operand);
  }

  if (DebugFlag) {
    cerr << "Current Function: X = Live\n";
    for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
      for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
        if (LiveSet.count(BI)) cerr << "X ";
        cerr << *BI;
      }
  }

  // Find the first postdominator of the entry node that is alive.  Make it the
  // new entry node...
  //
  DominatorTree &DT = getAnalysis<DominatorTree>(DominatorTree::PostDomID);

  // If there are some blocks dead...
  if (AliveBlocks.size() != Func->size()) {
    // Insert a new entry node to eliminate the entry node as a special case.
    BasicBlock *NewEntry = new BasicBlock();
    NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
    Func->getBasicBlockList().push_front(NewEntry);
    AliveBlocks.insert(NewEntry);    // This block is always alive!
    
    // Loop over all of the alive blocks in the function.  If any successor
    // blocks are not alive, we adjust the outgoing branches to branch to the
    // first live postdominator of the live block, adjusting any PHI nodes in
    // the block to reflect this.
    //
    for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
      if (AliveBlocks.count(I)) {
        BasicBlock *BB = I;
        TerminatorInst *TI = BB->getTerminator();
      
        // Loop over all of the successors, looking for ones that are not alive
        for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
          if (!AliveBlocks.count(TI->getSuccessor(i))) {
            // Scan up the postdominator tree, looking for the first
            // postdominator that is alive, and the last postdominator that is
            // dead...
            //
            DominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
            DominatorTree::Node *NextNode = LastNode->getIDom();
            while (!AliveBlocks.count(NextNode->getNode())) {
              LastNode = NextNode;
              NextNode = NextNode->getIDom();
            }
            
            // Get the basic blocks that we need...
            BasicBlock *LastDead = LastNode->getNode();
            BasicBlock *NextAlive = NextNode->getNode();
            
            // Make the conditional branch now go to the next alive block...
            TI->getSuccessor(i)->removePredecessor(BB);
            TI->setSuccessor(i, NextAlive);
            
            // If there are PHI nodes in NextAlive, we need to add entries to
            // the PHI nodes for the new incoming edge.  The incoming values
            // should be identical to the incoming values for LastDead.
            //
            for (BasicBlock::iterator II = NextAlive->begin();
                 PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
              // Get the incoming value for LastDead...
              int OldIdx = PN->getBasicBlockIndex(LastDead);
              assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
              Value *InVal = PN->getIncomingValue(OldIdx);
              
              // Add an incoming value for BB now...
              PN->addIncoming(InVal, BB);
            }
          }

        // Now loop over all of the instructions in the basic block, telling
        // dead instructions to drop their references.  This is so that the next
        // sweep over the program can safely delete dead instructions without
        // other dead instructions still refering to them.
        //
        for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ++I)
          if (!LiveSet.count(I))                // Is this instruction alive?
            I->dropAllReferences();             // Nope, drop references... 
      }
  }

  // Loop over all of the basic blocks in the function, dropping references of
  // the dead basic blocks
  //
  for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
    if (!AliveBlocks.count(BB)) {
      // Remove all outgoing edges from this basic block and convert the
      // terminator into a return instruction.
      vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
      
      if (!Succs.empty()) {
        // Loop over all of the successors, removing this block from PHI node
        // entries that might be in the block...
        while (!Succs.empty()) {
          Succs.back()->removePredecessor(BB);
          Succs.pop_back();
        }
        
        // Delete the old terminator instruction...
        BB->getInstList().pop_back();
        const Type *RetTy = Func->getReturnType();
        Instruction *New = new ReturnInst(RetTy != Type::VoidTy ?
                                          Constant::getNullValue(RetTy) : 0);
        BB->getInstList().push_back(New);
      }

      BB->dropAllReferences();
      ++NumBlockRemoved;
      MadeChanges = true;
    }
  }

  // Now loop through all of the blocks and delete the dead ones.  We can safely
  // do this now because we know that there are no references to dead blocks
  // (because they have dropped all of their references...  we also remove dead
  // instructions from alive blocks.
  //
  for (Function::iterator BI = Func->begin(); BI != Func->end(); )
    if (!AliveBlocks.count(BI))
      BI = Func->getBasicBlockList().erase(BI);
    else {
      for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
        if (!LiveSet.count(II)) {             // Is this instruction alive?
          // Nope... remove the instruction from it's basic block...
          II = BI->getInstList().erase(II);
          ++NumInstRemoved;
          MadeChanges = true;
        } else {
          ++II;
        }

      ++BI;                                           // Increment iterator...
    }

  return MadeChanges;
}