llvm.org GIT mirror llvm / 59d3ae6 lib / IR / Value.cpp
59d3ae6

Tree @59d3ae6 (Download .tar.gz)

Value.cpp @59d3ae6raw · history · blame

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
//===-- Value.cpp - Implement the Value class -----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the Value, ValueHandle, and User classes.
//
//===----------------------------------------------------------------------===//

#include "llvm/IR/Value.h"
#include "LLVMContextImpl.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/ValueSymbolTable.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/LeakDetector.h"
#include "llvm/Support/ManagedStatic.h"
#include "llvm/Support/ValueHandle.h"
#include <algorithm>
using namespace llvm;

//===----------------------------------------------------------------------===//
//                                Value Class
//===----------------------------------------------------------------------===//

static inline Type *checkType(Type *Ty) {
  assert(Ty && "Value defined with a null type: Error!");
  return const_cast<Type*>(Ty);
}

Value::Value(Type *ty, unsigned scid)
  : SubclassID(scid), HasValueHandle(0),
    SubclassOptionalData(0), SubclassData(0), VTy((Type*)checkType(ty)),
    UseList(0), Name(0) {
  // FIXME: Why isn't this in the subclass gunk??
  // Note, we cannot call isa<CallInst> before the CallInst has been
  // constructed.
  if (SubclassID == Instruction::Call || SubclassID == Instruction::Invoke)
    assert((VTy->isFirstClassType() || VTy->isVoidTy() || VTy->isStructTy()) &&
           "invalid CallInst type!");
  else if (SubclassID != BasicBlockVal &&
           (SubclassID < ConstantFirstVal || SubclassID > ConstantLastVal))
    assert((VTy->isFirstClassType() || VTy->isVoidTy()) &&
           "Cannot create non-first-class values except for constants!");
}

Value::~Value() {
  // Notify all ValueHandles (if present) that this value is going away.
  if (HasValueHandle)
    ValueHandleBase::ValueIsDeleted(this);

#ifndef NDEBUG      // Only in -g mode...
  // Check to make sure that there are no uses of this value that are still
  // around when the value is destroyed.  If there are, then we have a dangling
  // reference and something is wrong.  This code is here to print out what is
  // still being referenced.  The value in question should be printed as
  // a <badref>
  //
  if (!use_empty()) {
    dbgs() << "While deleting: " << *VTy << " %" << getName() << "\n";
    for (use_iterator I = use_begin(), E = use_end(); I != E; ++I)
      dbgs() << "Use still stuck around after Def is destroyed:"
           << **I << "\n";
  }
#endif
  assert(use_empty() && "Uses remain when a value is destroyed!");

  // If this value is named, destroy the name.  This should not be in a symtab
  // at this point.
  if (Name && SubclassID != MDStringVal)
    Name->Destroy();

  // There should be no uses of this object anymore, remove it.
  LeakDetector::removeGarbageObject(this);
}

/// hasNUses - Return true if this Value has exactly N users.
///
bool Value::hasNUses(unsigned N) const {
  const_use_iterator UI = use_begin(), E = use_end();

  for (; N; --N, ++UI)
    if (UI == E) return false;  // Too few.
  return UI == E;
}

/// hasNUsesOrMore - Return true if this value has N users or more.  This is
/// logically equivalent to getNumUses() >= N.
///
bool Value::hasNUsesOrMore(unsigned N) const {
  const_use_iterator UI = use_begin(), E = use_end();

  for (; N; --N, ++UI)
    if (UI == E) return false;  // Too few.

  return true;
}

/// isUsedInBasicBlock - Return true if this value is used in the specified
/// basic block.
bool Value::isUsedInBasicBlock(const BasicBlock *BB) const {
  // This can be computed either by scanning the instructions in BB, or by
  // scanning the use list of this Value. Both lists can be very long, but
  // usually one is quite short.
  //
  // Scan both lists simultaneously until one is exhausted. This limits the
  // search to the shorter list.
  BasicBlock::const_iterator BI = BB->begin(), BE = BB->end();
  const_use_iterator UI = use_begin(), UE = use_end();
  for (; BI != BE && UI != UE; ++BI, ++UI) {
    // Scan basic block: Check if this Value is used by the instruction at BI.
    if (std::find(BI->op_begin(), BI->op_end(), this) != BI->op_end())
      return true;
    // Scan use list: Check if the use at UI is in BB.
    const Instruction *User = dyn_cast<Instruction>(*UI);
    if (User && User->getParent() == BB)
      return true;
  }
  return false;
}


/// getNumUses - This method computes the number of uses of this Value.  This
/// is a linear time operation.  Use hasOneUse or hasNUses to check for specific
/// values.
unsigned Value::getNumUses() const {
  return (unsigned)std::distance(use_begin(), use_end());
}

static bool getSymTab(Value *V, ValueSymbolTable *&ST) {
  ST = 0;
  if (Instruction *I = dyn_cast<Instruction>(V)) {
    if (BasicBlock *P = I->getParent())
      if (Function *PP = P->getParent())
        ST = &PP->getValueSymbolTable();
  } else if (BasicBlock *BB = dyn_cast<BasicBlock>(V)) {
    if (Function *P = BB->getParent())
      ST = &P->getValueSymbolTable();
  } else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
    if (Module *P = GV->getParent())
      ST = &P->getValueSymbolTable();
  } else if (Argument *A = dyn_cast<Argument>(V)) {
    if (Function *P = A->getParent())
      ST = &P->getValueSymbolTable();
  } else if (isa<MDString>(V))
    return true;
  else {
    assert(isa<Constant>(V) && "Unknown value type!");
    return true;  // no name is setable for this.
  }
  return false;
}

StringRef Value::getName() const {
  // Make sure the empty string is still a C string. For historical reasons,
  // some clients want to call .data() on the result and expect it to be null
  // terminated.
  if (!Name) return StringRef("", 0);
  return Name->getKey();
}

void Value::setName(const Twine &NewName) {
  assert(SubclassID != MDStringVal &&
         "Cannot set the name of MDString with this method!");

  // Fast path for common IRBuilder case of setName("") when there is no name.
  if (NewName.isTriviallyEmpty() && !hasName())
    return;

  SmallString<256> NameData;
  StringRef NameRef = NewName.toStringRef(NameData);

  // Name isn't changing?
  if (getName() == NameRef)
    return;

  assert(!getType()->isVoidTy() && "Cannot assign a name to void values!");

  // Get the symbol table to update for this object.
  ValueSymbolTable *ST;
  if (getSymTab(this, ST))
    return;  // Cannot set a name on this value (e.g. constant).

  if (Function *F = dyn_cast<Function>(this))
    getContext().pImpl->IntrinsicIDCache.erase(F);

  if (!ST) { // No symbol table to update?  Just do the change.
    if (NameRef.empty()) {
      // Free the name for this value.
      Name->Destroy();
      Name = 0;
      return;
    }

    if (Name)
      Name->Destroy();

    // NOTE: Could optimize for the case the name is shrinking to not deallocate
    // then reallocated.

    // Create the new name.
    Name = ValueName::Create(NameRef.begin(), NameRef.end());
    Name->setValue(this);
    return;
  }

  // NOTE: Could optimize for the case the name is shrinking to not deallocate
  // then reallocated.
  if (hasName()) {
    // Remove old name.
    ST->removeValueName(Name);
    Name->Destroy();
    Name = 0;

    if (NameRef.empty())
      return;
  }

  // Name is changing to something new.
  Name = ST->createValueName(NameRef, this);
}


/// takeName - transfer the name from V to this value, setting V's name to
/// empty.  It is an error to call V->takeName(V).
void Value::takeName(Value *V) {
  assert(SubclassID != MDStringVal && "Cannot take the name of an MDString!");

  ValueSymbolTable *ST = 0;
  // If this value has a name, drop it.
  if (hasName()) {
    // Get the symtab this is in.
    if (getSymTab(this, ST)) {
      // We can't set a name on this value, but we need to clear V's name if
      // it has one.
      if (V->hasName()) V->setName("");
      return;  // Cannot set a name on this value (e.g. constant).
    }

    // Remove old name.
    if (ST)
      ST->removeValueName(Name);
    Name->Destroy();
    Name = 0;
  }

  // Now we know that this has no name.

  // If V has no name either, we're done.
  if (!V->hasName()) return;

  // Get this's symtab if we didn't before.
  if (!ST) {
    if (getSymTab(this, ST)) {
      // Clear V's name.
      V->setName("");
      return;  // Cannot set a name on this value (e.g. constant).
    }
  }

  // Get V's ST, this should always succed, because V has a name.
  ValueSymbolTable *VST;
  bool Failure = getSymTab(V, VST);
  assert(!Failure && "V has a name, so it should have a ST!"); (void)Failure;

  // If these values are both in the same symtab, we can do this very fast.
  // This works even if both values have no symtab yet.
  if (ST == VST) {
    // Take the name!
    Name = V->Name;
    V->Name = 0;
    Name->setValue(this);
    return;
  }

  // Otherwise, things are slightly more complex.  Remove V's name from VST and
  // then reinsert it into ST.

  if (VST)
    VST->removeValueName(V->Name);
  Name = V->Name;
  V->Name = 0;
  Name->setValue(this);

  if (ST)
    ST->reinsertValue(this);
}


void Value::replaceAllUsesWith(Value *New) {
  assert(New && "Value::replaceAllUsesWith(<null>) is invalid!");
  assert(New != this && "this->replaceAllUsesWith(this) is NOT valid!");
  assert(New->getType() == getType() &&
         "replaceAllUses of value with new value of different type!");

  // Notify all ValueHandles (if present) that this value is going away.
  if (HasValueHandle)
    ValueHandleBase::ValueIsRAUWd(this, New);

  while (!use_empty()) {
    Use &U = *UseList;
    // Must handle Constants specially, we cannot call replaceUsesOfWith on a
    // constant because they are uniqued.
    if (Constant *C = dyn_cast<Constant>(U.getUser())) {
      if (!isa<GlobalValue>(C)) {
        C->replaceUsesOfWithOnConstant(this, New, &U);
        continue;
      }
    }

    U.set(New);
  }

  if (BasicBlock *BB = dyn_cast<BasicBlock>(this))
    BB->replaceSuccessorsPhiUsesWith(cast<BasicBlock>(New));
}

namespace {
// Various metrics for how much to strip off of pointers.
enum PointerStripKind {
  PSK_ZeroIndices,
  PSK_ZeroIndicesAndAliases,
  PSK_InBoundsConstantIndices,
  PSK_InBounds
};

template <PointerStripKind StripKind>
static Value *stripPointerCastsAndOffsets(Value *V) {
  if (!V->getType()->isPointerTy())
    return V;

  // Even though we don't look through PHI nodes, we could be called on an
  // instruction in an unreachable block, which may be on a cycle.
  SmallPtrSet<Value *, 4> Visited;

  Visited.insert(V);
  do {
    if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
      switch (StripKind) {
      case PSK_ZeroIndicesAndAliases:
      case PSK_ZeroIndices:
        if (!GEP->hasAllZeroIndices())
          return V;
        break;
      case PSK_InBoundsConstantIndices:
        if (!GEP->hasAllConstantIndices())
          return V;
        // fallthrough
      case PSK_InBounds:
        if (!GEP->isInBounds())
          return V;
        break;
      }
      V = GEP->getPointerOperand();
    } else if (Operator::getOpcode(V) == Instruction::BitCast ||
               Operator::getOpcode(V) == Instruction::AddrSpaceCast) {
      V = cast<Operator>(V)->getOperand(0);
    } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
      if (StripKind == PSK_ZeroIndices || GA->mayBeOverridden())
        return V;
      V = GA->getAliasee();
    } else {
      return V;
    }
    assert(V->getType()->isPointerTy() && "Unexpected operand type!");
  } while (Visited.insert(V));

  return V;
}
} // namespace

Value *Value::stripPointerCasts() {
  return stripPointerCastsAndOffsets<PSK_ZeroIndicesAndAliases>(this);
}

Value *Value::stripPointerCastsNoFollowAliases() {
  return stripPointerCastsAndOffsets<PSK_ZeroIndices>(this);
}

Value *Value::stripInBoundsConstantOffsets() {
  return stripPointerCastsAndOffsets<PSK_InBoundsConstantIndices>(this);
}

Value *Value::stripAndAccumulateInBoundsConstantOffsets(const DataLayout &DL,
                                                        APInt &Offset) {
  if (!getType()->isPointerTy())
    return this;

  assert(Offset.getBitWidth() == DL.getPointerSizeInBits(cast<PointerType>(
                                     getType())->getAddressSpace()) &&
         "The offset must have exactly as many bits as our pointer.");

  // Even though we don't look through PHI nodes, we could be called on an
  // instruction in an unreachable block, which may be on a cycle.
  SmallPtrSet<Value *, 4> Visited;
  Visited.insert(this);
  Value *V = this;
  do {
    if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
      if (!GEP->isInBounds())
        return V;
      APInt GEPOffset(Offset);
      if (!GEP->accumulateConstantOffset(DL, GEPOffset))
        return V;
      Offset = GEPOffset;
      V = GEP->getPointerOperand();
    } else if (Operator::getOpcode(V) == Instruction::BitCast) {
      V = cast<Operator>(V)->getOperand(0);
    } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
      V = GA->getAliasee();
    } else {
      return V;
    }
    assert(V->getType()->isPointerTy() && "Unexpected operand type!");
  } while (Visited.insert(V));

  return V;
}

Value *Value::stripInBoundsOffsets() {
  return stripPointerCastsAndOffsets<PSK_InBounds>(this);
}

/// isDereferenceablePointer - Test if this value is always a pointer to
/// allocated and suitably aligned memory for a simple load or store.
static bool isDereferenceablePointer(const Value *V,
                                     SmallPtrSet<const Value *, 32> &Visited) {
  // Note that it is not safe to speculate into a malloc'd region because
  // malloc may return null.
  // It's also not always safe to follow a bitcast, for example:
  //   bitcast i8* (alloca i8) to i32*
  // would result in a 4-byte load from a 1-byte alloca. Some cases could
  // be handled using DataLayout to check sizes and alignments though.

  // These are obviously ok.
  if (isa<AllocaInst>(V)) return true;

  // Global variables which can't collapse to null are ok.
  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
    return !GV->hasExternalWeakLinkage();

  // byval arguments are ok.
  if (const Argument *A = dyn_cast<Argument>(V))
    return A->hasByValAttr();

  // For GEPs, determine if the indexing lands within the allocated object.
  if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
    // Conservatively require that the base pointer be fully dereferenceable.
    if (!Visited.insert(GEP->getOperand(0)))
      return false;
    if (!isDereferenceablePointer(GEP->getOperand(0), Visited))
      return false;
    // Check the indices.
    gep_type_iterator GTI = gep_type_begin(GEP);
    for (User::const_op_iterator I = GEP->op_begin()+1,
         E = GEP->op_end(); I != E; ++I) {
      Value *Index = *I;
      Type *Ty = *GTI++;
      // Struct indices can't be out of bounds.
      if (isa<StructType>(Ty))
        continue;
      ConstantInt *CI = dyn_cast<ConstantInt>(Index);
      if (!CI)
        return false;
      // Zero is always ok.
      if (CI->isZero())
        continue;
      // Check to see that it's within the bounds of an array.
      ArrayType *ATy = dyn_cast<ArrayType>(Ty);
      if (!ATy)
        return false;
      if (CI->getValue().getActiveBits() > 64)
        return false;
      if (CI->getZExtValue() >= ATy->getNumElements())
        return false;
    }
    // Indices check out; this is dereferenceable.
    return true;
  }

  // If we don't know, assume the worst.
  return false;
}

/// isDereferenceablePointer - Test if this value is always a pointer to
/// allocated and suitably aligned memory for a simple load or store.
bool Value::isDereferenceablePointer() const {
  SmallPtrSet<const Value *, 32> Visited;
  return ::isDereferenceablePointer(this, Visited);
}

/// DoPHITranslation - If this value is a PHI node with CurBB as its parent,
/// return the value in the PHI node corresponding to PredBB.  If not, return
/// ourself.  This is useful if you want to know the value something has in a
/// predecessor block.
Value *Value::DoPHITranslation(const BasicBlock *CurBB,
                               const BasicBlock *PredBB) {
  PHINode *PN = dyn_cast<PHINode>(this);
  if (PN && PN->getParent() == CurBB)
    return PN->getIncomingValueForBlock(PredBB);
  return this;
}

LLVMContext &Value::getContext() const { return VTy->getContext(); }

//===----------------------------------------------------------------------===//
//                             ValueHandleBase Class
//===----------------------------------------------------------------------===//

/// AddToExistingUseList - Add this ValueHandle to the use list for VP, where
/// List is known to point into the existing use list.
void ValueHandleBase::AddToExistingUseList(ValueHandleBase **List) {
  assert(List && "Handle list is null?");

  // Splice ourselves into the list.
  Next = *List;
  *List = this;
  setPrevPtr(List);
  if (Next) {
    Next->setPrevPtr(&Next);
    assert(VP.getPointer() == Next->VP.getPointer() && "Added to wrong list?");
  }
}

void ValueHandleBase::AddToExistingUseListAfter(ValueHandleBase *List) {
  assert(List && "Must insert after existing node");

  Next = List->Next;
  setPrevPtr(&List->Next);
  List->Next = this;
  if (Next)
    Next->setPrevPtr(&Next);
}

/// AddToUseList - Add this ValueHandle to the use list for VP.
void ValueHandleBase::AddToUseList() {
  assert(VP.getPointer() && "Null pointer doesn't have a use list!");

  LLVMContextImpl *pImpl = VP.getPointer()->getContext().pImpl;

  if (VP.getPointer()->HasValueHandle) {
    // If this value already has a ValueHandle, then it must be in the
    // ValueHandles map already.
    ValueHandleBase *&Entry = pImpl->ValueHandles[VP.getPointer()];
    assert(Entry != 0 && "Value doesn't have any handles?");
    AddToExistingUseList(&Entry);
    return;
  }

  // Ok, it doesn't have any handles yet, so we must insert it into the
  // DenseMap.  However, doing this insertion could cause the DenseMap to
  // reallocate itself, which would invalidate all of the PrevP pointers that
  // point into the old table.  Handle this by checking for reallocation and
  // updating the stale pointers only if needed.
  DenseMap<Value*, ValueHandleBase*> &Handles = pImpl->ValueHandles;
  const void *OldBucketPtr = Handles.getPointerIntoBucketsArray();

  ValueHandleBase *&Entry = Handles[VP.getPointer()];
  assert(Entry == 0 && "Value really did already have handles?");
  AddToExistingUseList(&Entry);
  VP.getPointer()->HasValueHandle = true;

  // If reallocation didn't happen or if this was the first insertion, don't
  // walk the table.
  if (Handles.isPointerIntoBucketsArray(OldBucketPtr) ||
      Handles.size() == 1) {
    return;
  }

  // Okay, reallocation did happen.  Fix the Prev Pointers.
  for (DenseMap<Value*, ValueHandleBase*>::iterator I = Handles.begin(),
       E = Handles.end(); I != E; ++I) {
    assert(I->second && I->first == I->second->VP.getPointer() &&
           "List invariant broken!");
    I->second->setPrevPtr(&I->second);
  }
}

/// RemoveFromUseList - Remove this ValueHandle from its current use list.
void ValueHandleBase::RemoveFromUseList() {
  assert(VP.getPointer() && VP.getPointer()->HasValueHandle &&
         "Pointer doesn't have a use list!");

  // Unlink this from its use list.
  ValueHandleBase **PrevPtr = getPrevPtr();
  assert(*PrevPtr == this && "List invariant broken");

  *PrevPtr = Next;
  if (Next) {
    assert(Next->getPrevPtr() == &Next && "List invariant broken");
    Next->setPrevPtr(PrevPtr);
    return;
  }

  // If the Next pointer was null, then it is possible that this was the last
  // ValueHandle watching VP.  If so, delete its entry from the ValueHandles
  // map.
  LLVMContextImpl *pImpl = VP.getPointer()->getContext().pImpl;
  DenseMap<Value*, ValueHandleBase*> &Handles = pImpl->ValueHandles;
  if (Handles.isPointerIntoBucketsArray(PrevPtr)) {
    Handles.erase(VP.getPointer());
    VP.getPointer()->HasValueHandle = false;
  }
}


void ValueHandleBase::ValueIsDeleted(Value *V) {
  assert(V->HasValueHandle && "Should only be called if ValueHandles present");

  // Get the linked list base, which is guaranteed to exist since the
  // HasValueHandle flag is set.
  LLVMContextImpl *pImpl = V->getContext().pImpl;
  ValueHandleBase *Entry = pImpl->ValueHandles[V];
  assert(Entry && "Value bit set but no entries exist");

  // We use a local ValueHandleBase as an iterator so that ValueHandles can add
  // and remove themselves from the list without breaking our iteration.  This
  // is not really an AssertingVH; we just have to give ValueHandleBase a kind.
  // Note that we deliberately do not the support the case when dropping a value
  // handle results in a new value handle being permanently added to the list
  // (as might occur in theory for CallbackVH's): the new value handle will not
  // be processed and the checking code will mete out righteous punishment if
  // the handle is still present once we have finished processing all the other
  // value handles (it is fine to momentarily add then remove a value handle).
  for (ValueHandleBase Iterator(Assert, *Entry); Entry; Entry = Iterator.Next) {
    Iterator.RemoveFromUseList();
    Iterator.AddToExistingUseListAfter(Entry);
    assert(Entry->Next == &Iterator && "Loop invariant broken.");

    switch (Entry->getKind()) {
    case Assert:
      break;
    case Tracking:
      // Mark that this value has been deleted by setting it to an invalid Value
      // pointer.
      Entry->operator=(DenseMapInfo<Value *>::getTombstoneKey());
      break;
    case Weak:
      // Weak just goes to null, which will unlink it from the list.
      Entry->operator=(0);
      break;
    case Callback:
      // Forward to the subclass's implementation.
      static_cast<CallbackVH*>(Entry)->deleted();
      break;
    }
  }

  // All callbacks, weak references, and assertingVHs should be dropped by now.
  if (V->HasValueHandle) {
#ifndef NDEBUG      // Only in +Asserts mode...
    dbgs() << "While deleting: " << *V->getType() << " %" << V->getName()
           << "\n";
    if (pImpl->ValueHandles[V]->getKind() == Assert)
      llvm_unreachable("An asserting value handle still pointed to this"
                       " value!");

#endif
    llvm_unreachable("All references to V were not removed?");
  }
}


void ValueHandleBase::ValueIsRAUWd(Value *Old, Value *New) {
  assert(Old->HasValueHandle &&"Should only be called if ValueHandles present");
  assert(Old != New && "Changing value into itself!");

  // Get the linked list base, which is guaranteed to exist since the
  // HasValueHandle flag is set.
  LLVMContextImpl *pImpl = Old->getContext().pImpl;
  ValueHandleBase *Entry = pImpl->ValueHandles[Old];

  assert(Entry && "Value bit set but no entries exist");

  // We use a local ValueHandleBase as an iterator so that
  // ValueHandles can add and remove themselves from the list without
  // breaking our iteration.  This is not really an AssertingVH; we
  // just have to give ValueHandleBase some kind.
  for (ValueHandleBase Iterator(Assert, *Entry); Entry; Entry = Iterator.Next) {
    Iterator.RemoveFromUseList();
    Iterator.AddToExistingUseListAfter(Entry);
    assert(Entry->Next == &Iterator && "Loop invariant broken.");

    switch (Entry->getKind()) {
    case Assert:
      // Asserting handle does not follow RAUW implicitly.
      break;
    case Tracking:
      // Tracking goes to new value like a WeakVH. Note that this may make it
      // something incompatible with its templated type. We don't want to have a
      // virtual (or inline) interface to handle this though, so instead we make
      // the TrackingVH accessors guarantee that a client never sees this value.

      // FALLTHROUGH
    case Weak:
      // Weak goes to the new value, which will unlink it from Old's list.
      Entry->operator=(New);
      break;
    case Callback:
      // Forward to the subclass's implementation.
      static_cast<CallbackVH*>(Entry)->allUsesReplacedWith(New);
      break;
    }
  }

#ifndef NDEBUG
  // If any new tracking or weak value handles were added while processing the
  // list, then complain about it now.
  if (Old->HasValueHandle)
    for (Entry = pImpl->ValueHandles[Old]; Entry; Entry = Entry->Next)
      switch (Entry->getKind()) {
      case Tracking:
      case Weak:
        dbgs() << "After RAUW from " << *Old->getType() << " %"
               << Old->getName() << " to " << *New->getType() << " %"
               << New->getName() << "\n";
        llvm_unreachable("A tracking or weak value handle still pointed to the"
                         " old value!\n");
      default:
        break;
      }
#endif
}

// Default implementation for CallbackVH.
void CallbackVH::allUsesReplacedWith(Value *) {}

void CallbackVH::deleted() {
  setValPtr(NULL);
}