EarlyCSE.cpp revision 0b8c9a80f20772c3793201ab5b251d3520b9cea3 
1//===- EarlyCSE.cpp - Simple and fast CSE pass ----------------------------===//
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 performs a simple dominator tree walk that eliminates trivially
11// redundant instructions.
12//
13//===----------------------------------------------------------------------===//
14
15#define DEBUG_TYPE "early-cse"
16#include "llvm/Transforms/Scalar.h"
17#include "llvm/ADT/Hashing.h"
18#include "llvm/ADT/ScopedHashTable.h"
19#include "llvm/ADT/Statistic.h"
20#include "llvm/Analysis/Dominators.h"
21#include "llvm/Analysis/InstructionSimplify.h"
22#include "llvm/IR/DataLayout.h"
23#include "llvm/IR/Instructions.h"
24#include "llvm/Pass.h"
25#include "llvm/Support/Debug.h"
26#include "llvm/Support/RecyclingAllocator.h"
27#include "llvm/Target/TargetLibraryInfo.h"
28#include "llvm/Transforms/Utils/Local.h"
29#include <deque>
30using namespace llvm;
31
32STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd");
33STATISTIC(NumCSE,      "Number of instructions CSE'd");
34STATISTIC(NumCSELoad,  "Number of load instructions CSE'd");
35STATISTIC(NumCSECall,  "Number of call instructions CSE'd");
36STATISTIC(NumDSE,      "Number of trivial dead stores removed");
37
38static unsigned getHash(const void *V) {
39  return DenseMapInfo<const void*>::getHashValue(V);
40}
41
42//===----------------------------------------------------------------------===//
43// SimpleValue
44//===----------------------------------------------------------------------===//
45
46namespace {
47  /// SimpleValue - Instances of this struct represent available values in the
48  /// scoped hash table.
49  struct SimpleValue {
50    Instruction *Inst;
51
52    SimpleValue(Instruction *I) : Inst(I) {
53      assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
54    }
55
56    bool isSentinel() const {
57      return Inst == DenseMapInfo<Instruction*>::getEmptyKey() ||
58             Inst == DenseMapInfo<Instruction*>::getTombstoneKey();
59    }
60
61    static bool canHandle(Instruction *Inst) {
62      // This can only handle non-void readnone functions.
63      if (CallInst *CI = dyn_cast<CallInst>(Inst))
64        return CI->doesNotAccessMemory() && !CI->getType()->isVoidTy();
65      return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) ||
66             isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) ||
67             isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) ||
68             isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) ||
69             isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst);
70    }
71  };
72}
73
74namespace llvm {
75// SimpleValue is POD.
76template<> struct isPodLike<SimpleValue> {
77  static const bool value = true;
78};
79
80template<> struct DenseMapInfo<SimpleValue> {
81  static inline SimpleValue getEmptyKey() {
82    return DenseMapInfo<Instruction*>::getEmptyKey();
83  }
84  static inline SimpleValue getTombstoneKey() {
85    return DenseMapInfo<Instruction*>::getTombstoneKey();
86  }
87  static unsigned getHashValue(SimpleValue Val);
88  static bool isEqual(SimpleValue LHS, SimpleValue RHS);
89};
90}
91
92unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) {
93  Instruction *Inst = Val.Inst;
94  // Hash in all of the operands as pointers.
95  if (BinaryOperator* BinOp = dyn_cast<BinaryOperator>(Inst)) {
96    Value *LHS = BinOp->getOperand(0);
97    Value *RHS = BinOp->getOperand(1);
98    if (BinOp->isCommutative() && BinOp->getOperand(0) > BinOp->getOperand(1))
99      std::swap(LHS, RHS);
100
101    if (isa<OverflowingBinaryOperator>(BinOp)) {
102      // Hash the overflow behavior
103      unsigned Overflow =
104        BinOp->hasNoSignedWrap()   * OverflowingBinaryOperator::NoSignedWrap |
105        BinOp->hasNoUnsignedWrap() * OverflowingBinaryOperator::NoUnsignedWrap;
106      return hash_combine(BinOp->getOpcode(), Overflow, LHS, RHS);
107    }
108
109    return hash_combine(BinOp->getOpcode(), LHS, RHS);
110  }
111
112  if (CmpInst *CI = dyn_cast<CmpInst>(Inst)) {
113    Value *LHS = CI->getOperand(0);
114    Value *RHS = CI->getOperand(1);
115    CmpInst::Predicate Pred = CI->getPredicate();
116    if (Inst->getOperand(0) > Inst->getOperand(1)) {
117      std::swap(LHS, RHS);
118      Pred = CI->getSwappedPredicate();
119    }
120    return hash_combine(Inst->getOpcode(), Pred, LHS, RHS);
121  }
122
123  if (CastInst *CI = dyn_cast<CastInst>(Inst))
124    return hash_combine(CI->getOpcode(), CI->getType(), CI->getOperand(0));
125
126  if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst))
127    return hash_combine(EVI->getOpcode(), EVI->getOperand(0),
128                        hash_combine_range(EVI->idx_begin(), EVI->idx_end()));
129
130  if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst))
131    return hash_combine(IVI->getOpcode(), IVI->getOperand(0),
132                        IVI->getOperand(1),
133                        hash_combine_range(IVI->idx_begin(), IVI->idx_end()));
134
135  assert((isa<CallInst>(Inst) || isa<BinaryOperator>(Inst) ||
136          isa<GetElementPtrInst>(Inst) || isa<SelectInst>(Inst) ||
137          isa<ExtractElementInst>(Inst) || isa<InsertElementInst>(Inst) ||
138          isa<ShuffleVectorInst>(Inst)) && "Invalid/unknown instruction");
139
140  // Mix in the opcode.
141  return hash_combine(Inst->getOpcode(),
142                      hash_combine_range(Inst->value_op_begin(),
143                                         Inst->value_op_end()));
144}
145
146bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) {
147  Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
148
149  if (LHS.isSentinel() || RHS.isSentinel())
150    return LHSI == RHSI;
151
152  if (LHSI->getOpcode() != RHSI->getOpcode()) return false;
153  if (LHSI->isIdenticalTo(RHSI)) return true;
154
155  // If we're not strictly identical, we still might be a commutable instruction
156  if (BinaryOperator *LHSBinOp = dyn_cast<BinaryOperator>(LHSI)) {
157    if (!LHSBinOp->isCommutative())
158      return false;
159
160    assert(isa<BinaryOperator>(RHSI)
161           && "same opcode, but different instruction type?");
162    BinaryOperator *RHSBinOp = cast<BinaryOperator>(RHSI);
163
164    // Check overflow attributes
165    if (isa<OverflowingBinaryOperator>(LHSBinOp)) {
166      assert(isa<OverflowingBinaryOperator>(RHSBinOp)
167             && "same opcode, but different operator type?");
168      if (LHSBinOp->hasNoUnsignedWrap() != RHSBinOp->hasNoUnsignedWrap() ||
169          LHSBinOp->hasNoSignedWrap() != RHSBinOp->hasNoSignedWrap())
170        return false;
171    }
172
173    // Commuted equality
174    return LHSBinOp->getOperand(0) == RHSBinOp->getOperand(1) &&
175      LHSBinOp->getOperand(1) == RHSBinOp->getOperand(0);
176  }
177  if (CmpInst *LHSCmp = dyn_cast<CmpInst>(LHSI)) {
178    assert(isa<CmpInst>(RHSI)
179           && "same opcode, but different instruction type?");
180    CmpInst *RHSCmp = cast<CmpInst>(RHSI);
181    // Commuted equality
182    return LHSCmp->getOperand(0) == RHSCmp->getOperand(1) &&
183      LHSCmp->getOperand(1) == RHSCmp->getOperand(0) &&
184      LHSCmp->getSwappedPredicate() == RHSCmp->getPredicate();
185  }
186
187  return false;
188}
189
190//===----------------------------------------------------------------------===//
191// CallValue
192//===----------------------------------------------------------------------===//
193
194namespace {
195  /// CallValue - Instances of this struct represent available call values in
196  /// the scoped hash table.
197  struct CallValue {
198    Instruction *Inst;
199
200    CallValue(Instruction *I) : Inst(I) {
201      assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
202    }
203
204    bool isSentinel() const {
205      return Inst == DenseMapInfo<Instruction*>::getEmptyKey() ||
206             Inst == DenseMapInfo<Instruction*>::getTombstoneKey();
207    }
208
209    static bool canHandle(Instruction *Inst) {
210      // Don't value number anything that returns void.
211      if (Inst->getType()->isVoidTy())
212        return false;
213
214      CallInst *CI = dyn_cast<CallInst>(Inst);
215      if (CI == 0 || !CI->onlyReadsMemory())
216        return false;
217      return true;
218    }
219  };
220}
221
222namespace llvm {
223  // CallValue is POD.
224  template<> struct isPodLike<CallValue> {
225    static const bool value = true;
226  };
227
228  template<> struct DenseMapInfo<CallValue> {
229    static inline CallValue getEmptyKey() {
230      return DenseMapInfo<Instruction*>::getEmptyKey();
231    }
232    static inline CallValue getTombstoneKey() {
233      return DenseMapInfo<Instruction*>::getTombstoneKey();
234    }
235    static unsigned getHashValue(CallValue Val);
236    static bool isEqual(CallValue LHS, CallValue RHS);
237  };
238}
239unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) {
240  Instruction *Inst = Val.Inst;
241  // Hash in all of the operands as pointers.
242  unsigned Res = 0;
243  for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) {
244    assert(!Inst->getOperand(i)->getType()->isMetadataTy() &&
245           "Cannot value number calls with metadata operands");
246    Res ^= getHash(Inst->getOperand(i)) << (i & 0xF);
247  }
248
249  // Mix in the opcode.
250  return (Res << 1) ^ Inst->getOpcode();
251}
252
253bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) {
254  Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
255  if (LHS.isSentinel() || RHS.isSentinel())
256    return LHSI == RHSI;
257  return LHSI->isIdenticalTo(RHSI);
258}
259
260
261//===----------------------------------------------------------------------===//
262// EarlyCSE pass.
263//===----------------------------------------------------------------------===//
264
265namespace {
266
267/// EarlyCSE - This pass does a simple depth-first walk over the dominator
268/// tree, eliminating trivially redundant instructions and using instsimplify
269/// to canonicalize things as it goes.  It is intended to be fast and catch
270/// obvious cases so that instcombine and other passes are more effective.  It
271/// is expected that a later pass of GVN will catch the interesting/hard
272/// cases.
273class EarlyCSE : public FunctionPass {
274public:
275  const DataLayout *TD;
276  const TargetLibraryInfo *TLI;
277  DominatorTree *DT;
278  typedef RecyclingAllocator<BumpPtrAllocator,
279                      ScopedHashTableVal<SimpleValue, Value*> > AllocatorTy;
280  typedef ScopedHashTable<SimpleValue, Value*, DenseMapInfo<SimpleValue>,
281                          AllocatorTy> ScopedHTType;
282
283  /// AvailableValues - This scoped hash table contains the current values of
284  /// all of our simple scalar expressions.  As we walk down the domtree, we
285  /// look to see if instructions are in this: if so, we replace them with what
286  /// we find, otherwise we insert them so that dominated values can succeed in
287  /// their lookup.
288  ScopedHTType *AvailableValues;
289
290  /// AvailableLoads - This scoped hash table contains the current values
291  /// of loads.  This allows us to get efficient access to dominating loads when
292  /// we have a fully redundant load.  In addition to the most recent load, we
293  /// keep track of a generation count of the read, which is compared against
294  /// the current generation count.  The current generation count is
295  /// incremented after every possibly writing memory operation, which ensures
296  /// that we only CSE loads with other loads that have no intervening store.
297  typedef RecyclingAllocator<BumpPtrAllocator,
298    ScopedHashTableVal<Value*, std::pair<Value*, unsigned> > > LoadMapAllocator;
299  typedef ScopedHashTable<Value*, std::pair<Value*, unsigned>,
300                          DenseMapInfo<Value*>, LoadMapAllocator> LoadHTType;
301  LoadHTType *AvailableLoads;
302
303  /// AvailableCalls - This scoped hash table contains the current values
304  /// of read-only call values.  It uses the same generation count as loads.
305  typedef ScopedHashTable<CallValue, std::pair<Value*, unsigned> > CallHTType;
306  CallHTType *AvailableCalls;
307
308  /// CurrentGeneration - This is the current generation of the memory value.
309  unsigned CurrentGeneration;
310
311  static char ID;
312  explicit EarlyCSE() : FunctionPass(ID) {
313    initializeEarlyCSEPass(*PassRegistry::getPassRegistry());
314  }
315
316  bool runOnFunction(Function &F);
317
318private:
319
320  // NodeScope - almost a POD, but needs to call the constructors for the
321  // scoped hash tables so that a new scope gets pushed on. These are RAII so
322  // that the scope gets popped when the NodeScope is destroyed.
323  class NodeScope {
324   public:
325    NodeScope(ScopedHTType *availableValues,
326              LoadHTType *availableLoads,
327              CallHTType *availableCalls) :
328        Scope(*availableValues),
329        LoadScope(*availableLoads),
330        CallScope(*availableCalls) {}
331
332   private:
333    NodeScope(const NodeScope&) LLVM_DELETED_FUNCTION;
334    void operator=(const NodeScope&) LLVM_DELETED_FUNCTION;
335
336    ScopedHTType::ScopeTy Scope;
337    LoadHTType::ScopeTy LoadScope;
338    CallHTType::ScopeTy CallScope;
339  };
340
341  // StackNode - contains all the needed information to create a stack for
342  // doing a depth first tranversal of the tree. This includes scopes for
343  // values, loads, and calls as well as the generation. There is a child
344  // iterator so that the children do not need to be store spearately.
345  class StackNode {
346   public:
347    StackNode(ScopedHTType *availableValues,
348              LoadHTType *availableLoads,
349              CallHTType *availableCalls,
350              unsigned cg, DomTreeNode *n,
351              DomTreeNode::iterator child, DomTreeNode::iterator end) :
352        CurrentGeneration(cg), ChildGeneration(cg), Node(n),
353        ChildIter(child), EndIter(end),
354        Scopes(availableValues, availableLoads, availableCalls),
355        Processed(false) {}
356
357    // Accessors.
358    unsigned currentGeneration() { return CurrentGeneration; }
359    unsigned childGeneration() { return ChildGeneration; }
360    void childGeneration(unsigned generation) { ChildGeneration = generation; }
361    DomTreeNode *node() { return Node; }
362    DomTreeNode::iterator childIter() { return ChildIter; }
363    DomTreeNode *nextChild() {
364      DomTreeNode *child = *ChildIter;
365      ++ChildIter;
366      return child;
367    }
368    DomTreeNode::iterator end() { return EndIter; }
369    bool isProcessed() { return Processed; }
370    void process() { Processed = true; }
371
372   private:
373    StackNode(const StackNode&) LLVM_DELETED_FUNCTION;
374    void operator=(const StackNode&) LLVM_DELETED_FUNCTION;
375
376    // Members.
377    unsigned CurrentGeneration;
378    unsigned ChildGeneration;
379    DomTreeNode *Node;
380    DomTreeNode::iterator ChildIter;
381    DomTreeNode::iterator EndIter;
382    NodeScope Scopes;
383    bool Processed;
384  };
385
386  bool processNode(DomTreeNode *Node);
387
388  // This transformation requires dominator postdominator info
389  virtual void getAnalysisUsage(AnalysisUsage &AU) const {
390    AU.addRequired<DominatorTree>();
391    AU.addRequired<TargetLibraryInfo>();
392    AU.setPreservesCFG();
393  }
394};
395}
396
397char EarlyCSE::ID = 0;
398
399// createEarlyCSEPass - The public interface to this file.
400FunctionPass *llvm::createEarlyCSEPass() {
401  return new EarlyCSE();
402}
403
404INITIALIZE_PASS_BEGIN(EarlyCSE, "early-cse", "Early CSE", false, false)
405INITIALIZE_PASS_DEPENDENCY(DominatorTree)
406INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
407INITIALIZE_PASS_END(EarlyCSE, "early-cse", "Early CSE", false, false)
408
409bool EarlyCSE::processNode(DomTreeNode *Node) {
410  BasicBlock *BB = Node->getBlock();
411
412  // If this block has a single predecessor, then the predecessor is the parent
413  // of the domtree node and all of the live out memory values are still current
414  // in this block.  If this block has multiple predecessors, then they could
415  // have invalidated the live-out memory values of our parent value.  For now,
416  // just be conservative and invalidate memory if this block has multiple
417  // predecessors.
418  if (BB->getSinglePredecessor() == 0)
419    ++CurrentGeneration;
420
421  /// LastStore - Keep track of the last non-volatile store that we saw... for
422  /// as long as there in no instruction that reads memory.  If we see a store
423  /// to the same location, we delete the dead store.  This zaps trivial dead
424  /// stores which can occur in bitfield code among other things.
425  StoreInst *LastStore = 0;
426
427  bool Changed = false;
428
429  // See if any instructions in the block can be eliminated.  If so, do it.  If
430  // not, add them to AvailableValues.
431  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
432    Instruction *Inst = I++;
433
434    // Dead instructions should just be removed.
435    if (isInstructionTriviallyDead(Inst, TLI)) {
436      DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n');
437      Inst->eraseFromParent();
438      Changed = true;
439      ++NumSimplify;
440      continue;
441    }
442
443    // If the instruction can be simplified (e.g. X+0 = X) then replace it with
444    // its simpler value.
445    if (Value *V = SimplifyInstruction(Inst, TD, TLI, DT)) {
446      DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << "  to: " << *V << '\n');
447      Inst->replaceAllUsesWith(V);
448      Inst->eraseFromParent();
449      Changed = true;
450      ++NumSimplify;
451      continue;
452    }
453
454    // If this is a simple instruction that we can value number, process it.
455    if (SimpleValue::canHandle(Inst)) {
456      // See if the instruction has an available value.  If so, use it.
457      if (Value *V = AvailableValues->lookup(Inst)) {
458        DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << "  to: " << *V << '\n');
459        Inst->replaceAllUsesWith(V);
460        Inst->eraseFromParent();
461        Changed = true;
462        ++NumCSE;
463        continue;
464      }
465
466      // Otherwise, just remember that this value is available.
467      AvailableValues->insert(Inst, Inst);
468      continue;
469    }
470
471    // If this is a non-volatile load, process it.
472    if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
473      // Ignore volatile loads.
474      if (!LI->isSimple()) {
475        LastStore = 0;
476        continue;
477      }
478
479      // If we have an available version of this load, and if it is the right
480      // generation, replace this instruction.
481      std::pair<Value*, unsigned> InVal =
482        AvailableLoads->lookup(Inst->getOperand(0));
483      if (InVal.first != 0 && InVal.second == CurrentGeneration) {
484        DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst << "  to: "
485              << *InVal.first << '\n');
486        if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first);
487        Inst->eraseFromParent();
488        Changed = true;
489        ++NumCSELoad;
490        continue;
491      }
492
493      // Otherwise, remember that we have this instruction.
494      AvailableLoads->insert(Inst->getOperand(0),
495                          std::pair<Value*, unsigned>(Inst, CurrentGeneration));
496      LastStore = 0;
497      continue;
498    }
499
500    // If this instruction may read from memory, forget LastStore.
501    if (Inst->mayReadFromMemory())
502      LastStore = 0;
503
504    // If this is a read-only call, process it.
505    if (CallValue::canHandle(Inst)) {
506      // If we have an available version of this call, and if it is the right
507      // generation, replace this instruction.
508      std::pair<Value*, unsigned> InVal = AvailableCalls->lookup(Inst);
509      if (InVal.first != 0 && InVal.second == CurrentGeneration) {
510        DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst << "  to: "
511                     << *InVal.first << '\n');
512        if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first);
513        Inst->eraseFromParent();
514        Changed = true;
515        ++NumCSECall;
516        continue;
517      }
518
519      // Otherwise, remember that we have this instruction.
520      AvailableCalls->insert(Inst,
521                         std::pair<Value*, unsigned>(Inst, CurrentGeneration));
522      continue;
523    }
524
525    // Okay, this isn't something we can CSE at all.  Check to see if it is
526    // something that could modify memory.  If so, our available memory values
527    // cannot be used so bump the generation count.
528    if (Inst->mayWriteToMemory()) {
529      ++CurrentGeneration;
530
531      if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
532        // We do a trivial form of DSE if there are two stores to the same
533        // location with no intervening loads.  Delete the earlier store.
534        if (LastStore &&
535            LastStore->getPointerOperand() == SI->getPointerOperand()) {
536          DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << "  due to: "
537                       << *Inst << '\n');
538          LastStore->eraseFromParent();
539          Changed = true;
540          ++NumDSE;
541          LastStore = 0;
542          continue;
543        }
544
545        // Okay, we just invalidated anything we knew about loaded values.  Try
546        // to salvage *something* by remembering that the stored value is a live
547        // version of the pointer.  It is safe to forward from volatile stores
548        // to non-volatile loads, so we don't have to check for volatility of
549        // the store.
550        AvailableLoads->insert(SI->getPointerOperand(),
551         std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration));
552
553        // Remember that this was the last store we saw for DSE.
554        if (SI->isSimple())
555          LastStore = SI;
556      }
557    }
558  }
559
560  return Changed;
561}
562
563
564bool EarlyCSE::runOnFunction(Function &F) {
565  std::deque<StackNode *> nodesToProcess;
566
567  TD = getAnalysisIfAvailable<DataLayout>();
568  TLI = &getAnalysis<TargetLibraryInfo>();
569  DT = &getAnalysis<DominatorTree>();
570
571  // Tables that the pass uses when walking the domtree.
572  ScopedHTType AVTable;
573  AvailableValues = &AVTable;
574  LoadHTType LoadTable;
575  AvailableLoads = &LoadTable;
576  CallHTType CallTable;
577  AvailableCalls = &CallTable;
578
579  CurrentGeneration = 0;
580  bool Changed = false;
581
582  // Process the root node.
583  nodesToProcess.push_front(
584      new StackNode(AvailableValues, AvailableLoads, AvailableCalls,
585                    CurrentGeneration, DT->getRootNode(),
586                    DT->getRootNode()->begin(),
587                    DT->getRootNode()->end()));
588
589  // Save the current generation.
590  unsigned LiveOutGeneration = CurrentGeneration;
591
592  // Process the stack.
593  while (!nodesToProcess.empty()) {
594    // Grab the first item off the stack. Set the current generation, remove
595    // the node from the stack, and process it.
596    StackNode *NodeToProcess = nodesToProcess.front();
597
598    // Initialize class members.
599    CurrentGeneration = NodeToProcess->currentGeneration();
600
601    // Check if the node needs to be processed.
602    if (!NodeToProcess->isProcessed()) {
603      // Process the node.
604      Changed |= processNode(NodeToProcess->node());
605      NodeToProcess->childGeneration(CurrentGeneration);
606      NodeToProcess->process();
607    } else if (NodeToProcess->childIter() != NodeToProcess->end()) {
608      // Push the next child onto the stack.
609      DomTreeNode *child = NodeToProcess->nextChild();
610      nodesToProcess.push_front(
611          new StackNode(AvailableValues,
612                        AvailableLoads,
613                        AvailableCalls,
614                        NodeToProcess->childGeneration(), child,
615                        child->begin(), child->end()));
616    } else {
617      // It has been processed, and there are no more children to process,
618      // so delete it and pop it off the stack.
619      delete NodeToProcess;
620      nodesToProcess.pop_front();
621    }
622  } // while (!nodes...)
623
624  // Reset the current generation.
625  CurrentGeneration = LiveOutGeneration;
626
627  return Changed;
628}
629