LLVM API Documentation
00001 //===-- CombineBranch.cpp -------------------------------------------------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // Combine multiple back-edges going to the same sink into a single 00011 // back-edge. This introduces a new basic block and back-edge branch for each 00012 // such sink. 00013 // 00014 //===----------------------------------------------------------------------===// 00015 00016 #include "llvm/Support/CFG.h" 00017 #include "llvm/Instructions.h" 00018 #include "llvm/Function.h" 00019 #include "llvm/Pass.h" 00020 00021 namespace llvm { 00022 00023 namespace { 00024 struct CombineBranches : public FunctionPass { 00025 private: 00026 /// Possible colors that a vertex can have during depth-first search for 00027 /// back-edges. 00028 /// 00029 enum Color { WHITE, GREY, BLACK }; 00030 00031 void getBackEdgesVisit(BasicBlock *u, 00032 std::map<BasicBlock *, Color > &color, 00033 std::map<BasicBlock *, int > &d, 00034 int &time, 00035 std::map<BasicBlock *, BasicBlock *> &be); 00036 void removeRedundant(std::map<BasicBlock *, BasicBlock *> &be); 00037 public: 00038 bool runOnFunction(Function &F); 00039 }; 00040 00041 RegisterOpt<CombineBranches> 00042 X("branch-combine", "Multiple backedges going to same target are merged"); 00043 } 00044 00045 /// getBackEdgesVisit - Get the back-edges of the control-flow graph for this 00046 /// function. We proceed recursively using depth-first search. We get 00047 /// back-edges by associating a time and a color with each vertex. The time of a 00048 /// vertex is the time when it was first visited. The color of a vertex is 00049 /// initially WHITE, changes to GREY when it is first visited, and changes to 00050 /// BLACK when ALL its neighbors have been visited. So we have a back edge when 00051 /// we meet a successor of a node with smaller time, and GREY color. 00052 /// 00053 void CombineBranches::getBackEdgesVisit(BasicBlock *u, 00054 std::map<BasicBlock *, Color > &color, 00055 std::map<BasicBlock *, int > &d, 00056 int &time, 00057 std::map<BasicBlock *, BasicBlock *> &be) { 00058 00059 color[u]=GREY; 00060 time++; 00061 d[u]=time; 00062 00063 for (succ_iterator vl = succ_begin(u), ve = succ_end(u); vl != ve; ++vl){ 00064 BasicBlock *BB = *vl; 00065 00066 if(color[BB]!=GREY && color[BB]!=BLACK) 00067 getBackEdgesVisit(BB, color, d, time, be); 00068 00069 //now checking for d and f vals 00070 else if(color[BB]==GREY){ 00071 //so v is ancestor of u if time of u > time of v 00072 if(d[u] >= d[BB]) // u->BB is a backedge 00073 be[u] = BB; 00074 } 00075 } 00076 color[u]=BLACK;//done with visiting the node and its neighbors 00077 } 00078 00079 /// removeRedundant - Remove all back-edges that are dominated by other 00080 /// back-edges in the set. 00081 /// 00082 void CombineBranches::removeRedundant(std::map<BasicBlock *, BasicBlock *> &be){ 00083 std::vector<BasicBlock *> toDelete; 00084 std::map<BasicBlock *, int> seenBB; 00085 00086 for(std::map<BasicBlock *, BasicBlock *>::iterator MI = be.begin(), 00087 ME = be.end(); MI != ME; ++MI){ 00088 00089 if(seenBB[MI->second]) 00090 continue; 00091 00092 seenBB[MI->second] = 1; 00093 00094 std::vector<BasicBlock *> sameTarget; 00095 sameTarget.clear(); 00096 00097 for(std::map<BasicBlock *, BasicBlock *>::iterator MMI = be.begin(), 00098 MME = be.end(); MMI != MME; ++MMI){ 00099 00100 if(MMI->first == MI->first) 00101 continue; 00102 00103 if(MMI->second == MI->second) 00104 sameTarget.push_back(MMI->first); 00105 00106 } 00107 00108 //so more than one branch to same target 00109 if(sameTarget.size()){ 00110 00111 sameTarget.push_back(MI->first); 00112 00113 BasicBlock *newBB = new BasicBlock("newCommon", MI->first->getParent()); 00114 BranchInst *newBranch = new BranchInst(MI->second, newBB); 00115 00116 std::map<PHINode *, std::vector<unsigned int> > phiMap; 00117 00118 for(std::vector<BasicBlock *>::iterator VBI = sameTarget.begin(), 00119 VBE = sameTarget.end(); VBI != VBE; ++VBI){ 00120 00121 BranchInst *ti = cast<BranchInst>((*VBI)->getTerminator()); 00122 unsigned char index = 1; 00123 if(ti->getSuccessor(0) == MI->second) 00124 index = 0; 00125 00126 ti->setSuccessor(index, newBB); 00127 00128 for(BasicBlock::iterator BB2Inst = MI->second->begin(), 00129 BBend = MI->second->end(); BB2Inst != BBend; ++BB2Inst){ 00130 00131 if (PHINode *phiInst = dyn_cast<PHINode>(BB2Inst)){ 00132 int bbIndex; 00133 bbIndex = phiInst->getBasicBlockIndex(*VBI); 00134 if(bbIndex>=0) 00135 phiMap[phiInst].push_back(bbIndex); 00136 } 00137 } 00138 } 00139 00140 for(std::map<PHINode *, std::vector<unsigned int> >::iterator 00141 PI = phiMap.begin(), PE = phiMap.end(); PI != PE; ++PI){ 00142 00143 PHINode *phiNode = new PHINode(PI->first->getType(), "phi", newBranch); 00144 for(std::vector<unsigned int>::iterator II = PI->second.begin(), 00145 IE = PI->second.end(); II != IE; ++II){ 00146 phiNode->addIncoming(PI->first->getIncomingValue(*II), 00147 PI->first->getIncomingBlock(*II)); 00148 } 00149 00150 std::vector<BasicBlock *> tempBB; 00151 for(std::vector<unsigned int>::iterator II = PI->second.begin(), 00152 IE = PI->second.end(); II != IE; ++II){ 00153 tempBB.push_back(PI->first->getIncomingBlock(*II)); 00154 } 00155 00156 for(std::vector<BasicBlock *>::iterator II = tempBB.begin(), 00157 IE = tempBB.end(); II != IE; ++II){ 00158 PI->first->removeIncomingValue(*II); 00159 } 00160 00161 PI->first->addIncoming(phiNode, newBB); 00162 } 00163 } 00164 } 00165 } 00166 00167 /// runOnFunction - Per function pass for combining branches. 00168 /// 00169 bool CombineBranches::runOnFunction(Function &F){ 00170 if (F.isExternal ()) 00171 return false; 00172 00173 // Find and remove "redundant" back-edges. 00174 std::map<BasicBlock *, Color> color; 00175 std::map<BasicBlock *, int> d; 00176 std::map<BasicBlock *, BasicBlock *> be; 00177 int time = 0; 00178 getBackEdgesVisit (F.begin (), color, d, time, be); 00179 removeRedundant (be); 00180 00181 return true; // FIXME: assumes a modification was always made. 00182 } 00183 00184 FunctionPass *createCombineBranchesPass () { 00185 return new CombineBranches(); 00186 } 00187 00188 } // End llvm namespace