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//===-- 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
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