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->getNameStr() << " to " << *New->getType() << " %"
          << New->getNameStr() << "\n";
        llvm_unreachable("A tracking or weak value handle still pointed to the"
                         " old value!\n");
      default:
        break;
      }
#endif
}

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.
  ValueHandleBase *Entry = (*ValueHandles)[Old];
  assert(Entry && "Value bit set but no entries exist");
  
  while (Entry) {
    // Advance pointer to avoid invalidation.
    ValueHandleBase *ThisNode = Entry;
    Entry = Entry->Next;
    
    switch (ThisNode->getKind()) {
    case Assert:
      // Asserting handle does not follow RAUW implicitly.
      break;
    case Weak:
      // Weak goes to the new value, which will unlink it from Old's list.
      ThisNode->operator=(New);
      break;
    case Callback:
      // Forward to the subclass's implementation.
      static_cast<CallbackVH*>(ThisNode)->allUsesReplacedWith(New);
      break;
    }
  }
}

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=(nullptr);
      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!");
  assert(Old->getType() == New->getType() &&
         "replaceAllUses of value with new value of different type!");

  // 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:
    case Weak:
      // Asserting and Weak handles do not follow RAUW implicitly.
      break;
    case WeakTracking:
      // 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 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 WeakTracking:
        dbgs() << "After RAUW from " << *Old->getType() << " %"
               << Old->getName() << " to " << *New->getType() << " %"
               << New->getName() << "\n";
        llvm_unreachable(
            "A weak tracking value handle still pointed to the  old value!\n");
      default:
        break;
      }
#endif
}

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 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:
      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->getNameStr()
           << "\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?");
  }
}