//===-- CombineBranch.cpp -------------------------------------------------===//
//
//                     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.
//
//===----------------------------------------------------------------------===//
//
// Combine multiple back-edges going to the same sink into a single
// back-edge. This introduces a new basic block and back-edge branch for each
// such sink.
//
//===----------------------------------------------------------------------===//

#include "llvm/Support/CFG.h"
#include "llvm/Instructions.h"
#include "llvm/Function.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"

namespace llvm {

namespace {
  struct CombineBranches : public FunctionPass {
  private:
    /// Possible colors that a vertex can have during depth-first search for
    /// back-edges.
    ///
    enum Color { WHITE, GREY, BLACK };

    void getBackEdgesVisit(BasicBlock *u,
                           std::map<BasicBlock *, Color > &color,
                           std::map<BasicBlock *, int > &d,
                           int &time,
                           std::map<BasicBlock *, BasicBlock *> &be);
    void removeRedundant(std::map<BasicBlock *, BasicBlock *> &be);
  public:
    bool runOnFunction(Function &F);
  };

  RegisterOpt<CombineBranches>
  X("branch-combine", "Multiple backedges going to same target are merged");
}

/// getBackEdgesVisit - Get the back-edges of the control-flow graph for this
/// function.  We proceed recursively using depth-first search.  We get
/// back-edges by associating a time and a color with each vertex.  The time of a
/// vertex is the time when it was first visited.  The color of a vertex is
/// initially WHITE, changes to GREY when it is first visited, and changes to
/// BLACK when ALL its neighbors have been visited.  So we have a back edge when
/// we meet a successor of a node with smaller time, and GREY color.
///
void CombineBranches::getBackEdgesVisit(BasicBlock *u,
                       std::map<BasicBlock *, Color > &color,
                       std::map<BasicBlock *, int > &d,
                       int &time,
                       std::map<BasicBlock *, BasicBlock *> &be) {

  color[u]=GREY;
  time++;
  d[u]=time;

  for (succ_iterator vl = succ_begin(u), ve = succ_end(u); vl != ve; ++vl){
    BasicBlock *BB = *vl;

    if(color[BB]!=GREY && color[BB]!=BLACK)
      getBackEdgesVisit(BB, color, d, time, be);

    //now checking for d and f vals
    else if(color[BB]==GREY){
      //so v is ancestor of u if time of u > time of v
      if(d[u] >= d[BB]) // u->BB is a backedge
        be[u] = BB;
    }
  }
  color[u]=BLACK;//done with visiting the node and its neighbors
}

/// removeRedundant - Remove all back-edges that are dominated by other
/// back-edges in the set.
///
void CombineBranches::removeRedundant(std::map<BasicBlock *, BasicBlock *> &be){
  std::vector<BasicBlock *> toDelete;
  std::map<BasicBlock *, int> seenBB;

  for(std::map<BasicBlock *, BasicBlock *>::iterator MI = be.begin(),
        ME = be.end(); MI != ME; ++MI){

    if(seenBB[MI->second])
      continue;

    seenBB[MI->second] = 1;

    std::vector<BasicBlock *> sameTarget;
    sameTarget.clear();

    for(std::map<BasicBlock *, BasicBlock *>::iterator MMI = be.begin(),
          MME = be.end(); MMI != MME; ++MMI){

      if(MMI->first == MI->first)
        continue;

      if(MMI->second == MI->second)
        sameTarget.push_back(MMI->first);

    }

    //so more than one branch to same target
    if(sameTarget.size()){

      sameTarget.push_back(MI->first);

      BasicBlock *newBB = new BasicBlock("newCommon", MI->first->getParent());
      BranchInst *newBranch = new BranchInst(MI->second, newBB);

      std::map<PHINode *, std::vector<unsigned int> > phiMap;

      for(std::vector<BasicBlock *>::iterator VBI = sameTarget.begin(),
            VBE = sameTarget.end(); VBI != VBE; ++VBI){

        BranchInst *ti = cast<BranchInst>((*VBI)->getTerminator());
        unsigned char index = 1;
        if(ti->getSuccessor(0) == MI->second)
          index = 0;

        ti->setSuccessor(index, newBB);

        for(BasicBlock::iterator BB2Inst = MI->second->begin(),
              BBend = MI->second->end(); BB2Inst != BBend; ++BB2Inst){

          if (PHINode *phiInst = dyn_cast<PHINode>(BB2Inst)){
            int bbIndex;
            bbIndex = phiInst->getBasicBlockIndex(*VBI);
            if(bbIndex>=0)
              phiMap[phiInst].push_back(bbIndex);
          }
        }
      }

      for(std::map<PHINode *, std::vector<unsigned int> >::iterator
            PI = phiMap.begin(), PE = phiMap.end(); PI != PE; ++PI){

        PHINode *phiNode = new PHINode(PI->first->getType(), "phi", newBranch);
        for(std::vector<unsigned int>::iterator II = PI->second.begin(),
              IE = PI->second.end(); II != IE; ++II){
          phiNode->addIncoming(PI->first->getIncomingValue(*II),
                               PI->first->getIncomingBlock(*II));
        }

        std::vector<BasicBlock *> tempBB;
        for(std::vector<unsigned int>::iterator II = PI->second.begin(),
              IE = PI->second.end(); II != IE; ++II){
          tempBB.push_back(PI->first->getIncomingBlock(*II));
        }

        for(std::vector<BasicBlock *>::iterator II = tempBB.begin(),
              IE = tempBB.end(); II != IE; ++II){
          PI->first->removeIncomingValue(*II);
        }

        PI->first->addIncoming(phiNode, newBB);
      }
    }
  }
}

/// runOnFunction - Per function pass for combining branches.
///
bool CombineBranches::runOnFunction(Function &F){
  if (F.isExternal ())
    return false;

  // Find and remove "redundant" back-edges.
  std::map<BasicBlock *, Color> color;
  std::map<BasicBlock *, int> d;
  std::map<BasicBlock *, BasicBlock *> be;
  int time = 0;
  getBackEdgesVisit (F.begin (), color, d, time, be);
  removeRedundant (be);

  return true; // FIXME: assumes a modification was always made.
}

FunctionPass *createCombineBranchesPass () {
  return new CombineBranches();
}

} // End llvm namespace