llvm-project/llvm/lib/Transforms/Instrumentation/ProfilePaths/CombineBranch.cpp

//===-- 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