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