lib/CodeGen/StrongPHIElimination.cpp

   1 //===- StrongPhiElimination.cpp - Eliminate PHI nodes by inserting copies -===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass eliminates machine instruction PHI nodes by inserting copy
  11 // instructions, using an intelligent copy-folding technique based on
  12 // dominator information.  This is technique is derived from:
  13 //
  14 //    Budimlic, et al. Fast copy coalescing and live-range identification.
  15 //    In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
  16 //    Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
  17 //    PLDI '02. ACM, New York, NY, 25-32.
  18 //    DOI= http://doi.acm.org/10.1145/512529.512534
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #define DEBUG_TYPE "strongphielim"
  23 #include "llvm/CodeGen/Passes.h"
  24 #include "llvm/CodeGen/LiveVariables.h"
  25 #include "llvm/CodeGen/MachineDominators.h"
  26 #include "llvm/CodeGen/MachineFunctionPass.h"
  27 #include "llvm/CodeGen/MachineInstr.h"
  28 #include "llvm/CodeGen/SSARegMap.h"
  29 #include "llvm/Target/TargetInstrInfo.h"
  30 #include "llvm/Target/TargetMachine.h"
  31 #include "llvm/ADT/DepthFirstIterator.h"
  32 #include "llvm/ADT/Statistic.h"
  33 #include "llvm/Support/Compiler.h"
  34 using namespace llvm;
  35
  36
  37 namespace {
  38   struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
  39     static char ID; // Pass identification, replacement for typeid
  40     StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}
  41
  42     DenseMap<MachineBasicBlock*,
  43              std::map<unsigned, unsigned> > Waiting;
  44
  45     std::map<unsigned, std::vector<unsigned> > Stacks;
  46     std::set<unsigned> UsedByAnother;
  47
  48     bool runOnMachineFunction(MachineFunction &Fn);
  49
  50     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  51       AU.addPreserved<LiveVariables>();
  52       AU.addPreservedID(PHIEliminationID);
  53       AU.addRequired<MachineDominatorTree>();
  54       AU.addRequired<LiveVariables>();
  55       AU.setPreservesAll();
  56       MachineFunctionPass::getAnalysisUsage(AU);
  57     }
  58
  59     virtual void releaseMemory() {
  60       preorder.clear();
  61       maxpreorder.clear();
  62
  63       Waiting.clear();
  64     }
  65
  66   private:
  67     struct DomForestNode {
  68     private:
  69       std::vector<DomForestNode*> children;
  70       unsigned reg;
  71
  72       void addChild(DomForestNode* DFN) { children.push_back(DFN); }
  73
  74     public:
  75       typedef std::vector<DomForestNode*>::iterator iterator;
  76
  77       DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
  78         if (parent)
  79           parent->addChild(this);
  80       }
  81
  82       ~DomForestNode() {
  83         for (iterator I = begin(), E = end(); I != E; ++I)
  84           delete *I;
  85       }
  86
  87       inline unsigned getReg() { return reg; }
  88
  89       inline DomForestNode::iterator begin() { return children.begin(); }
  90       inline DomForestNode::iterator end() { return children.end(); }
  91     };
  92
  93     DenseMap<MachineBasicBlock*, unsigned> preorder;
  94     DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
  95
  96
  97     void computeDFS(MachineFunction& MF);
  98     void processBlock(MachineBasicBlock* MBB);
  99
 100     std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
 101     void processPHIUnion(MachineInstr* Inst,
 102                          std::set<unsigned>& PHIUnion,
 103                          std::vector<StrongPHIElimination::DomForestNode*>& DF,
 104                          std::vector<std::pair<unsigned, unsigned> >& locals);
 105     void ScheduleCopies(MachineBasicBlock* MBB, std::set<unsigned>& pushed);
 106     void InsertCopies(MachineBasicBlock* MBB);
 107   };
 108
 109   char StrongPHIElimination::ID = 0;
 110   RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
 111                   "Eliminate PHI nodes for register allocation, intelligently");
 112 }
 113
 114 const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();
 115
 116 /// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
 117 /// of the given MachineFunction.  These numbers are then used in other parts
 118 /// of the PHI elimination process.
 119 void StrongPHIElimination::computeDFS(MachineFunction& MF) {
 120   SmallPtrSet<MachineDomTreeNode*, 8> frontier;
 121   SmallPtrSet<MachineDomTreeNode*, 8> visited;
 122
 123   unsigned time = 0;
 124
 125   MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
 126
 127   MachineDomTreeNode* node = DT.getRootNode();
 128
 129   std::vector<MachineDomTreeNode*> worklist;
 130   worklist.push_back(node);
 131
 132   while (!worklist.empty()) {
 133     MachineDomTreeNode* currNode = worklist.back();
 134
 135     if (!frontier.count(currNode)) {
 136       frontier.insert(currNode);
 137       ++time;
 138       preorder.insert(std::make_pair(currNode->getBlock(), time));
 139     }
 140
 141     bool inserted = false;
 142     for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
 143          I != E; ++I)
 144       if (!frontier.count(*I) && !visited.count(*I)) {
 145         worklist.push_back(*I);
 146         inserted = true;
 147         break;
 148       }
 149
 150     if (!inserted) {
 151       frontier.erase(currNode);
 152       visited.insert(currNode);
 153       maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
 154
 155       worklist.pop_back();
 156     }
 157   }
 158 }
 159
 160 /// PreorderSorter - a helper class that is used to sort registers
 161 /// according to the preorder number of their defining blocks
 162 class PreorderSorter {
 163 private:
 164   DenseMap<MachineBasicBlock*, unsigned>& preorder;
 165   LiveVariables& LV;
 166
 167 public:
 168   PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
 169                 LiveVariables& L) : preorder(p), LV(L) { }
 170
 171   bool operator()(unsigned A, unsigned B) {
 172     if (A == B)
 173       return false;
 174
 175     MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
 176     MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();
 177
 178     if (preorder[ABlock] < preorder[BBlock])
 179       return true;
 180     else if (preorder[ABlock] > preorder[BBlock])
 181       return false;
 182
 183     assert(0 && "Error sorting by dominance!");
 184     return false;
 185   }
 186 };
 187
 188 /// computeDomForest - compute the subforest of the DomTree corresponding
 189 /// to the defining blocks of the registers in question
 190 std::vector<StrongPHIElimination::DomForestNode*>
 191 StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
 192   LiveVariables& LV = getAnalysis<LiveVariables>();
 193
 194   DomForestNode* VirtualRoot = new DomForestNode(0, 0);
 195   maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));
 196
 197   std::vector<unsigned> worklist;
 198   worklist.reserve(regs.size());
 199   for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
 200        I != E; ++I)
 201     worklist.push_back(*I);
 202
 203   PreorderSorter PS(preorder, LV);
 204   std::sort(worklist.begin(), worklist.end(), PS);
 205
 206   DomForestNode* CurrentParent = VirtualRoot;
 207   std::vector<DomForestNode*> stack;
 208   stack.push_back(VirtualRoot);
 209
 210   for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
 211        I != E; ++I) {
 212     unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
 213     MachineBasicBlock* parentBlock =
 214       LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
 215
 216     while (pre > maxpreorder[parentBlock]) {
 217       stack.pop_back();
 218       CurrentParent = stack.back();
 219
 220       parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
 221     }
 222
 223     DomForestNode* child = new DomForestNode(*I, CurrentParent);
 224     stack.push_back(child);
 225     CurrentParent = child;
 226   }
 227
 228   std::vector<DomForestNode*> ret;
 229   ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
 230   return ret;
 231 }
 232
 233 /// isLiveIn - helper method that determines, from a VarInfo, if a register
 234 /// is live into a block
 235 bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
 236   if (V.AliveBlocks.test(MBB->getNumber()))
 237     return true;
 238
 239   if (V.DefInst->getParent() != MBB &&
 240       V.UsedBlocks.test(MBB->getNumber()))
 241     return true;
 242
 243   return false;
 244 }
 245
 246 /// isLiveOut - help method that determines, from a VarInfo, if a register is
 247 /// live out of a block.
 248 bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
 249   if (MBB == V.DefInst->getParent() ||
 250       V.UsedBlocks.test(MBB->getNumber())) {
 251     for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
 252          E = V.Kills.end(); I != E; ++I)
 253       if ((*I)->getParent() == MBB)
 254         return false;
 255
 256     return true;
 257   }
 258
 259   return false;
 260 }
 261
 262 /// isKillInst - helper method that determines, from a VarInfo, if an
 263 /// instruction kills a given register
 264 bool isKillInst(LiveVariables::VarInfo& V, MachineInstr* MI) {
 265   return std::find(V.Kills.begin(), V.Kills.end(), MI) != V.Kills.end();
 266 }
 267
 268 /// interferes - checks for local interferences by scanning a block.  The only
 269 /// trick parameter is 'mode' which tells it the relationship of the two
 270 /// registers. 0 - defined in the same block, 1 - first properly dominates
 271 /// second, 2 - second properly dominates first
 272 bool interferes(LiveVariables::VarInfo& First, LiveVariables::VarInfo& Second,
 273                 MachineBasicBlock* scan, unsigned mode) {
 274   MachineInstr* def = 0;
 275   MachineInstr* kill = 0;
 276
 277   bool interference = false;
 278
 279   // Wallk the block, checking for interferences
 280   for (MachineBasicBlock::iterator MBI = scan->begin(), MBE = scan->end();
 281        MBI != MBE; ++MBI) {
 282     MachineInstr* curr = MBI;
 283
 284     // Same defining block...
 285     if (mode == 0) {
 286       if (curr == First.DefInst) {
 287         // If we find our first DefInst, save it
 288         if (!def) {
 289           def = curr;
 290         // If there's already an unkilled DefInst, then
 291         // this is an interference
 292         } else if (!kill) {
 293           interference = true;
 294           break;
 295         // If there's a DefInst followed by a KillInst, then
 296         // they can't interfere
 297         } else {
 298           interference = false;
 299           break;
 300         }
 301       // Symmetric with the above
 302       } else if (curr == Second.DefInst ) {
 303         if (!def) {
 304           def = curr;
 305         } else if (!kill) {
 306           interference = true;
 307           break;
 308         } else {
 309           interference = false;
 310           break;
 311         }
 312       // Store KillInsts if they match up with the DefInst
 313       } else if (isKillInst(First, curr)) {
 314         if (def == First.DefInst) {
 315           kill = curr;
 316         } else if (isKillInst(Second, curr)) {
 317           if (def == Second.DefInst) {
 318             kill = curr;
 319           }
 320         }
 321       }
 322     // First properly dominates second...
 323     } else if (mode == 1) {
 324       if (curr == Second.DefInst) {
 325         // DefInst of second without kill of first is an interference
 326         if (!kill) {
 327           interference = true;
 328           break;
 329         // DefInst after a kill is a non-interference
 330         } else {
 331           interference = false;
 332           break;
 333         }
 334       // Save KillInsts of First
 335       } else if (isKillInst(First, curr)) {
 336         kill = curr;
 337       }
 338     // Symmetric with the above
 339     } else if (mode == 2) {
 340       if (curr == First.DefInst) {
 341         if (!kill) {
 342           interference = true;
 343           break;
 344         } else {
 345           interference = false;
 346           break;
 347         }
 348       } else if (isKillInst(Second, curr)) {
 349         kill = curr;
 350       }
 351     }
 352   }
 353
 354   return interference;
 355 }
 356
 357 /// processBlock - Eliminate PHIs in the given block
 358 void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
 359   LiveVariables& LV = getAnalysis<LiveVariables>();
 360
 361   // Holds names that have been added to a set in any PHI within this block
 362   // before the current one.
 363   std::set<unsigned> ProcessedNames;
 364
 365   MachineBasicBlock::iterator P = MBB->begin();
 366   while (P->getOpcode() == TargetInstrInfo::PHI) {
 367     LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());
 368
 369     unsigned DestReg = P->getOperand(0).getReg();
 370
 371     // Hold the names that are currently in the candidate set.
 372     std::set<unsigned> PHIUnion;
 373     std::set<MachineBasicBlock*> UnionedBlocks;
 374
 375     for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
 376       unsigned SrcReg = P->getOperand(i-1).getReg();
 377       LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
 378
 379       // Check for trivial interferences
 380       if (isLiveIn(SrcInfo, P->getParent()) ||
 381           isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
 382           ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
 383             isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
 384           ProcessedNames.count(SrcReg) ||
 385           UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
 386
 387         // add a copy from a_i to p in Waiting[From[a_i]]
 388         MachineBasicBlock* From = P->getOperand(i).getMBB();
 389         Waiting[From].insert(std::make_pair(SrcReg, DestReg));
 390         UsedByAnother.insert(SrcReg);
 391       } else {
 392         PHIUnion.insert(SrcReg);
 393         UnionedBlocks.insert(SrcInfo.DefInst->getParent());
 394       }
 395     }
 396
 397     std::vector<StrongPHIElimination::DomForestNode*> DF =
 398                                                      computeDomForest(PHIUnion);
 399
 400     // Walk DomForest to resolve interferences
 401     std::vector<std::pair<unsigned, unsigned> > localInterferences;
 402     processPHIUnion(P, PHIUnion, DF, localInterferences);
 403
 404     // Check for local interferences
 405     for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
 406         localInterferences.begin(), E = localInterferences.end(); I != E; ++I) {
 407       std::pair<unsigned, unsigned> p = *I;
 408
 409       LiveVariables::VarInfo& FirstInfo = LV.getVarInfo(p.first);
 410       LiveVariables::VarInfo& SecondInfo = LV.getVarInfo(p.second);
 411
 412       MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
 413
 414       // Determine the block we need to scan and the relationship between
 415       // the two registers
 416       MachineBasicBlock* scan = 0;
 417       unsigned mode = 0;
 418       if (FirstInfo.DefInst->getParent() == SecondInfo.DefInst->getParent()) {
 419         scan = FirstInfo.DefInst->getParent();
 420         mode = 0; // Same block
 421       } else if (MDT.dominates(FirstInfo.DefInst->getParent(),
 422                              SecondInfo.DefInst->getParent())) {
 423         scan = SecondInfo.DefInst->getParent();
 424         mode = 1; // First dominates second
 425       } else {
 426         scan = FirstInfo.DefInst->getParent();
 427         mode = 2; // Second dominates first
 428       }
 429
 430       // If there's an interference, we need to insert  copies
 431       if (interferes(FirstInfo, SecondInfo, scan, mode)) {
 432         // Insert copies for First
 433         for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
 434           if (P->getOperand(i-1).getReg() == p.first) {
 435             unsigned SrcReg = p.first;
 436             MachineBasicBlock* From = P->getOperand(i).getMBB();
 437
 438             Waiting[From].insert(std::make_pair(SrcReg,
 439                                                 P->getOperand(0).getReg()));
 440             UsedByAnother.insert(SrcReg);
 441
 442             PHIUnion.erase(SrcReg);
 443           }
 444         }
 445       }
 446     }
 447
 448     // FIXME: Cache renaming information
 449
 450     ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
 451     ++P;
 452   }
 453 }
 454
 455 /// processPHIUnion - Take a set of candidate registers to be coallesced when
 456 /// decomposing the PHI instruction.  Use the DominanceForest to remove the ones
 457 /// that are known to interfere, and flag others that need to be checked for
 458 /// local interferences.
 459 void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
 460                                            std::set<unsigned>& PHIUnion,
 461                         std::vector<StrongPHIElimination::DomForestNode*>& DF,
 462                         std::vector<std::pair<unsigned, unsigned> >& locals) {
 463
 464   std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
 465   SmallPtrSet<DomForestNode*, 4> visited;
 466
 467   LiveVariables& LV = getAnalysis<LiveVariables>();
 468   unsigned DestReg = Inst->getOperand(0).getReg();
 469
 470   // DF walk on the DomForest
 471   while (!worklist.empty()) {
 472     DomForestNode* DFNode = worklist.back();
 473
 474     LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
 475     visited.insert(DFNode);
 476
 477     bool inserted = false;
 478     for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
 479          CI != CE; ++CI) {
 480       DomForestNode* child = *CI;
 481       LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());
 482
 483       if (isLiveOut(Info, CInfo.DefInst->getParent())) {
 484         // Insert copies for parent
 485         for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
 486           if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
 487             unsigned SrcReg = DFNode->getReg();
 488             MachineBasicBlock* From = Inst->getOperand(i).getMBB();
 489
 490             Waiting[From].insert(std::make_pair(SrcReg, DestReg));
 491             UsedByAnother.insert(SrcReg);
 492
 493             PHIUnion.erase(SrcReg);
 494           }
 495         }
 496       } else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
 497                  Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
 498         // Add (p, c) to possible local interferences
 499         locals.push_back(std::make_pair(DFNode->getReg(), child->getReg()));
 500       }
 501
 502       if (!visited.count(child)) {
 503         worklist.push_back(child);
 504         inserted = true;
 505       }
 506     }
 507
 508     if (!inserted) worklist.pop_back();
 509   }
 510 }
 511
 512 /// ScheduleCopies - Insert copies into predecessor blocks, scheduling
 513 /// them properly so as to avoid the 'lost copy' and the 'virtual swap'
 514 /// problems.
 515 ///
 516 /// Based on "Practical Improvements to the Construction and Destruction
 517 /// of Static Single Assignment Form" by Briggs, et al.
 518 void StrongPHIElimination::ScheduleCopies(MachineBasicBlock* MBB,
 519                                           std::set<unsigned>& pushed) {
 520   std::map<unsigned, unsigned>& copy_set= Waiting[MBB];
 521
 522   std::map<unsigned, unsigned> worklist;
 523   std::map<unsigned, unsigned> map;
 524
 525   // Setup worklist of initial copies
 526   for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
 527        E = copy_set.end(); I != E; ) {
 528     map.insert(std::make_pair(I->first, I->first));
 529     map.insert(std::make_pair(I->second, I->second));
 530
 531     if (!UsedByAnother.count(I->first)) {
 532       worklist.insert(*I);
 533
 534       // Avoid iterator invalidation
 535       unsigned first = I->first;
 536       ++I;
 537       copy_set.erase(first);
 538     } else {
 539       ++I;
 540     }
 541   }
 542
 543   LiveVariables& LV = getAnalysis<LiveVariables>();
 544
 545   // Iterate over the worklist, inserting copies
 546   while (!worklist.empty() || !copy_set.empty()) {
 547     while (!worklist.empty()) {
 548       std::pair<unsigned, unsigned> curr = *worklist.begin();
 549       worklist.erase(curr.first);
 550
 551       if (isLiveOut(LV.getVarInfo(curr.second), MBB)) {
 552         // Insert copy from curr.second to a temporary
 553         // Push temporary on Stacks
 554         // Insert temporary in pushed
 555       }
 556
 557       // Insert copy from map[curr.first] to curr.second
 558       map[curr.first] = curr.second;
 559
 560       // If curr.first is a destination in copy_set...
 561       for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
 562            E = copy_set.end(); I != E; )
 563         if (curr.first == I->second) {
 564           std::pair<unsigned, unsigned> temp = *I;
 565
 566           // Avoid iterator invalidation
 567           ++I;
 568           copy_set.erase(temp.first);
 569           worklist.insert(temp);
 570
 571           break;
 572         } else {
 573           ++I;
 574         }
 575     }
 576
 577     if (!copy_set.empty()) {
 578       std::pair<unsigned, unsigned> curr = *copy_set.begin();
 579       copy_set.erase(curr.first);
 580
 581       // Insert a copy from dest to a new temporary t at the end of b
 582       // map[curr.second] = t;
 583
 584       worklist.insert(curr);
 585     }
 586   }
 587 }
 588
 589 /// InsertCopies - insert copies into MBB and all of its successors
 590 void StrongPHIElimination::InsertCopies(MachineBasicBlock* MBB) {
 591   std::set<unsigned> pushed;
 592
 593   // Rewrite register uses from Stacks
 594   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
 595       I != E; ++I)
 596     for (unsigned i = 0; i < I->getNumOperands(); ++i)
 597       if (I->getOperand(i).isRegister() &&
 598           Stacks[I->getOperand(i).getReg()].size()) {
 599         I->getOperand(i).setReg(Stacks[I->getOperand(i).getReg()].back());
 600       }
 601
 602   // Schedule the copies for this block
 603   ScheduleCopies(MBB, pushed);
 604
 605   // Recur to our successors
 606   for (GraphTraits<MachineBasicBlock*>::ChildIteratorType I =
 607        GraphTraits<MachineBasicBlock*>::child_begin(MBB), E =
 608        GraphTraits<MachineBasicBlock*>::child_end(MBB); I != E; ++I)
 609     InsertCopies(*I);
 610
 611   // As we exit this block, pop the names we pushed while processing it
 612   for (std::set<unsigned>::iterator I = pushed.begin(),
 613        E = pushed.end(); I != E; ++I)
 614     Stacks[*I].pop_back();
 615 }
 616
 617 bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
 618   // Compute DFS numbers of each block
 619   computeDFS(Fn);
 620
 621   // Determine which phi node operands need copies
 622   for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
 623     if (!I->empty() &&
 624         I->begin()->getOpcode() == TargetInstrInfo::PHI)
 625       processBlock(I);
 626
 627   // Insert copies
 628   // FIXME: This process should probably preserve LiveVariables
 629   InsertCopies(Fn.begin());
 630
 631   // FIXME: Perform renaming
 632
 633   return false;
 634 }