gecko-dev/xpcom/glue/nsTArray-inl.h

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim:set ts=2 sw=2 sts=2 et cindent: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef nsTArray_h__
#  error "Don't include this file directly"
#endif

template<class Alloc>
nsTArray_base<Alloc>::nsTArray_base()
  : mHdr(EmptyHdr()) {
  MOZ_COUNT_CTOR(nsTArray_base);
}

template<class Alloc>
nsTArray_base<Alloc>::~nsTArray_base() {
  if (mHdr != EmptyHdr() && !UsesAutoArrayBuffer()) {
    Alloc::Free(mHdr);
  }
  MOZ_COUNT_DTOR(nsTArray_base);
}

template<class Alloc>
const nsTArrayHeader* nsTArray_base<Alloc>::GetAutoArrayBufferUnsafe(size_t elemAlign) const {
  // Assuming |this| points to an nsAutoArray, we want to get a pointer to
  // mAutoBuf.  So just cast |this| to nsAutoArray* and read &mAutoBuf!

  const void* autoBuf = &reinterpret_cast<const nsAutoArrayBase<nsTArray<uint32_t>, 1>*>(this)->mAutoBuf;

  // If we're on a 32-bit system and elemAlign is 8, we need to adjust our
  // pointer to take into account the extra alignment in the auto array.

  MOZ_STATIC_ASSERT(sizeof(void*) != 4 ||
                    (MOZ_ALIGNOF(mozilla::AlignedElem<8>) == 8 &&
                     sizeof(nsAutoTArray<mozilla::AlignedElem<8>, 1>) ==
                       sizeof(void*) + sizeof(nsTArrayHeader) +
                       4 + sizeof(mozilla::AlignedElem<8>)),
                    "auto array padding wasn't what we expected");

  // We don't support alignments greater than 8 bytes.
  NS_ABORT_IF_FALSE(elemAlign <= 4 || elemAlign == 8, "unsupported alignment.");
  if (sizeof(void*) == 4 && elemAlign == 8) {
    autoBuf = reinterpret_cast<const char*>(autoBuf) + 4;
  }

  return reinterpret_cast<const Header*>(autoBuf);
}

template<class Alloc>
bool nsTArray_base<Alloc>::UsesAutoArrayBuffer() const {
  if (!mHdr->mIsAutoArray) {
    return false;
  }

  // This is nuts.  If we were sane, we'd pass elemAlign as a parameter to
  // this function.  Unfortunately this function is called in nsTArray_base's
  // destructor, at which point we don't know elem_type's alignment.
  //
  // We'll fall on our face and return true when we should say false if
  //
  //   * we're not using our auto buffer,
  //   * elemAlign == 4, and
  //   * mHdr == GetAutoArrayBuffer(8).
  //
  // This could happen if |*this| lives on the heap and malloc allocated our
  // buffer on the heap adjacent to |*this|.
  //
  // However, we can show that this can't happen.  If |this| is an auto array
  // (as we ensured at the beginning of the method), GetAutoArrayBuffer(8)
  // always points to memory owned by |*this|, because (as we assert below)
  //
  //   * GetAutoArrayBuffer(8) is at most 4 bytes past GetAutoArrayBuffer(4), and 
  //   * sizeof(nsTArrayHeader) > 4.
  //
  // Since nsAutoTArray always contains an nsTArrayHeader,
  // GetAutoArrayBuffer(8) will always point inside the auto array object,
  // even if it doesn't point at the beginning of the header.
  //
  // Note that this means that we can't store elements with alignment 16 in an
  // nsTArray, because GetAutoArrayBuffer(16) could lie outside the memory
  // owned by this nsAutoTArray.  We statically assert that elem_type's
  // alignment is 8 bytes or less in nsAutoArrayBase.

  MOZ_STATIC_ASSERT(sizeof(nsTArrayHeader) > 4,
                    "see comment above");

#ifdef DEBUG
  ptrdiff_t diff = reinterpret_cast<const char*>(GetAutoArrayBuffer(8)) -
                   reinterpret_cast<const char*>(GetAutoArrayBuffer(4));
  NS_ABORT_IF_FALSE(diff >= 0 && diff <= 4, "GetAutoArrayBuffer doesn't do what we expect.");
#endif

  return mHdr == GetAutoArrayBuffer(4) || mHdr == GetAutoArrayBuffer(8);
}


template<class Alloc>
bool
nsTArray_base<Alloc>::EnsureCapacity(size_type capacity, size_type elemSize) {
  // This should be the most common case so test this first
  if (capacity <= mHdr->mCapacity)
    return true;

  // If the requested memory allocation exceeds size_type(-1)/2, then
  // our doubling algorithm may not be able to allocate it.
  // Additionally we couldn't fit in the Header::mCapacity
  // member. Just bail out in cases like that.  We don't want to be
  // allocating 2 GB+ arrays anyway.
  if ((uint64_t)capacity * elemSize > size_type(-1)/2) {
    Alloc::SizeTooBig();
    return false;
  }

  if (mHdr == EmptyHdr()) {
    // Malloc() new data
    Header *header = static_cast<Header*>
                     (Alloc::Malloc(sizeof(Header) + capacity * elemSize));
    if (!header)
      return false;
    header->mLength = 0;
    header->mCapacity = capacity;
    header->mIsAutoArray = 0;
    mHdr = header;

    return true;
  }

  // We increase our capacity so |capacity * elemSize + sizeof(Header)| is the
  // next power of two, if this value is less than pageSize bytes, or otherwise
  // so it's the next multiple of pageSize.
  const uint32_t pageSizeBytes = 12;
  const uint32_t pageSize = 1 << pageSizeBytes;

  uint32_t minBytes = capacity * elemSize + sizeof(Header);
  uint32_t bytesToAlloc;
  if (minBytes >= pageSize) {
    // Round up to the next multiple of pageSize.
    bytesToAlloc = pageSize * ((minBytes + pageSize - 1) / pageSize);
  }
  else {
    // Round up to the next power of two.  See
    // http://graphics.stanford.edu/~seander/bithacks.html
    bytesToAlloc = minBytes - 1;
    bytesToAlloc |= bytesToAlloc >> 1;
    bytesToAlloc |= bytesToAlloc >> 2;
    bytesToAlloc |= bytesToAlloc >> 4;
    bytesToAlloc |= bytesToAlloc >> 8;
    bytesToAlloc |= bytesToAlloc >> 16;
    bytesToAlloc++;

    MOZ_ASSERT((bytesToAlloc & (bytesToAlloc - 1)) == 0,
               "nsTArray's allocation size should be a power of two!");
  }

  Header *header;
  if (UsesAutoArrayBuffer()) {
    // Malloc() and copy
    header = static_cast<Header*>(Alloc::Malloc(bytesToAlloc));
    if (!header)
      return false;

    memcpy(header, mHdr, sizeof(Header) + Length() * elemSize);
  } else {
    // Realloc() existing data
    header = static_cast<Header*>(Alloc::Realloc(mHdr, bytesToAlloc));
    if (!header)
      return false;
  }

  // How many elements can we fit in bytesToAlloc?
  uint32_t newCapacity = (bytesToAlloc - sizeof(Header)) / elemSize;
  MOZ_ASSERT(newCapacity >= capacity, "Didn't enlarge the array enough!");
  header->mCapacity = newCapacity;

  mHdr = header;

  return true;
}

template<class Alloc>
void
nsTArray_base<Alloc>::ShrinkCapacity(size_type elemSize, size_t elemAlign) {
  if (mHdr == EmptyHdr() || UsesAutoArrayBuffer())
    return;

  if (mHdr->mLength >= mHdr->mCapacity)  // should never be greater than...
    return;

  size_type length = Length();

  if (IsAutoArray() && GetAutoArrayBuffer(elemAlign)->mCapacity >= length) {
    Header* header = GetAutoArrayBuffer(elemAlign);

    // Copy data, but don't copy the header to avoid overwriting mCapacity
    header->mLength = length;
    memcpy(header + 1, mHdr + 1, length * elemSize);

    Alloc::Free(mHdr);
    mHdr = header;
    return;
  }

  if (length == 0) {
    MOZ_ASSERT(!IsAutoArray(), "autoarray should have fit 0 elements");
    Alloc::Free(mHdr);
    mHdr = EmptyHdr();
    return;
  }

  size_type size = sizeof(Header) + length * elemSize;
  void *ptr = Alloc::Realloc(mHdr, size);
  if (!ptr)
    return;
  mHdr = static_cast<Header*>(ptr);
  mHdr->mCapacity = length;
}

template<class Alloc>
void
nsTArray_base<Alloc>::ShiftData(index_type start,
                                size_type oldLen, size_type newLen,
                                size_type elemSize, size_t elemAlign) {
  if (oldLen == newLen)
    return;

  // Determine how many elements need to be shifted
  size_type num = mHdr->mLength - (start + oldLen);

  // Compute the resulting length of the array
  mHdr->mLength += newLen - oldLen;
  if (mHdr->mLength == 0) {
    ShrinkCapacity(elemSize, elemAlign);
  } else {
    // Maybe nothing needs to be shifted
    if (num == 0)
      return;
    // Perform shift (change units to bytes first)
    start *= elemSize;
    newLen *= elemSize;
    oldLen *= elemSize;
    num *= elemSize;
    char *base = reinterpret_cast<char*>(mHdr + 1) + start;
    memmove(base + newLen, base + oldLen, num);
  }
}

template<class Alloc>
bool
nsTArray_base<Alloc>::InsertSlotsAt(index_type index, size_type count,
                                    size_type elementSize, size_t elemAlign)  {
  MOZ_ASSERT(index <= Length(), "Bogus insertion index");
  size_type newLen = Length() + count;

  EnsureCapacity(newLen, elementSize);

  // Check for out of memory conditions
  if (Capacity() < newLen)
    return false;

  // Move the existing elements as needed.  Note that this will
  // change our mLength, so no need to call IncrementLength.
  ShiftData(index, 0, count, elementSize, elemAlign);
      
  return true;
}

// nsTArray_base::IsAutoArrayRestorer is an RAII class which takes
// |nsTArray_base &array| in its constructor.  When it's destructed, it ensures
// that
//
//   * array.mIsAutoArray has the same value as it did when we started, and
//   * if array has an auto buffer and mHdr would otherwise point to sEmptyHdr,
//     array.mHdr points to array's auto buffer.

template<class Alloc>
nsTArray_base<Alloc>::IsAutoArrayRestorer::IsAutoArrayRestorer(
  nsTArray_base<Alloc> &array,
  size_t elemAlign) 
  : mArray(array),
    mElemAlign(elemAlign),
    mIsAuto(array.IsAutoArray())
{
}

template<class Alloc>
nsTArray_base<Alloc>::IsAutoArrayRestorer::~IsAutoArrayRestorer() {
  // Careful: We don't want to set mIsAutoArray = 1 on sEmptyHdr.
  if (mIsAuto && mArray.mHdr == mArray.EmptyHdr()) {
    // Call GetAutoArrayBufferUnsafe() because GetAutoArrayBuffer() asserts
    // that mHdr->mIsAutoArray is true, which surely isn't the case here.
    mArray.mHdr = mArray.GetAutoArrayBufferUnsafe(mElemAlign);
    mArray.mHdr->mLength = 0;
  }
  else {
    mArray.mHdr->mIsAutoArray = mIsAuto;
  }
}

template<class Alloc>
template<class Allocator>
bool
nsTArray_base<Alloc>::SwapArrayElements(nsTArray_base<Allocator>& other,
                                        size_type elemSize,
                                        size_t elemAlign) {

  // EnsureNotUsingAutoArrayBuffer will set mHdr = sEmptyHdr even if we have an
  // auto buffer.  We need to point mHdr back to our auto buffer before we
  // return, otherwise we'll forget that we have an auto buffer at all!
  // IsAutoArrayRestorer takes care of this for us.

  IsAutoArrayRestorer ourAutoRestorer(*this, elemAlign);
  typename nsTArray_base<Allocator>::IsAutoArrayRestorer otherAutoRestorer(other, elemAlign);

  // If neither array uses an auto buffer which is big enough to store the
  // other array's elements, then ensure that both arrays use malloc'ed storage
  // and swap their mHdr pointers.
  if ((!UsesAutoArrayBuffer() || Capacity() < other.Length()) &&
      (!other.UsesAutoArrayBuffer() || other.Capacity() < Length())) {

    if (!EnsureNotUsingAutoArrayBuffer(elemSize) ||
        !other.EnsureNotUsingAutoArrayBuffer(elemSize)) {
      return false;
    }

    Header *temp = mHdr;
    mHdr = other.mHdr;
    other.mHdr = temp;

    return true;
  }

  // Swap the two arrays using memcpy, since at least one is using an auto
  // buffer which is large enough to hold all of the other's elements.  We'll
  // copy the shorter array into temporary storage.
  //
  // (We could do better than this in some circumstances.  Suppose we're
  // swapping arrays X and Y.  X has space for 2 elements in its auto buffer,
  // but currently has length 4, so it's using malloc'ed storage.  Y has length
  // 2.  When we swap X and Y, we don't need to use a temporary buffer; we can
  // write Y straight into X's auto buffer, write X's malloc'ed buffer on top
  // of Y, and then switch X to using its auto buffer.)

  if (!EnsureCapacity(other.Length(), elemSize) ||
      !other.EnsureCapacity(Length(), elemSize)) {
    return false;
  }

  // The EnsureCapacity calls above shouldn't have caused *both* arrays to
  // switch from their auto buffers to malloc'ed space.
  NS_ABORT_IF_FALSE(UsesAutoArrayBuffer() ||
                    other.UsesAutoArrayBuffer(),
                    "One of the arrays should be using its auto buffer.");

  size_type smallerLength = NS_MIN(Length(), other.Length());
  size_type largerLength = NS_MAX(Length(), other.Length());
  void *smallerElements, *largerElements;
  if (Length() <= other.Length()) {
    smallerElements = Hdr() + 1;
    largerElements = other.Hdr() + 1;
  }
  else {
    smallerElements = other.Hdr() + 1;
    largerElements = Hdr() + 1;
  }

  // Allocate temporary storage for the smaller of the two arrays.  We want to
  // allocate this space on the stack, if it's not too large.  Sounds like a
  // job for AutoTArray!  (One of the two arrays we're swapping is using an
  // auto buffer, so we're likely not allocating a lot of space here.  But one
  // could, in theory, allocate a huge AutoTArray on the heap.)
  nsAutoTArray<uint8_t, 64, Alloc> temp;
  if (!temp.SetCapacity(smallerLength * elemSize)) {
    return false;
  }

  memcpy(temp.Elements(), smallerElements, smallerLength * elemSize);
  memcpy(smallerElements, largerElements, largerLength * elemSize);
  memcpy(largerElements, temp.Elements(), smallerLength * elemSize);

  // Swap the arrays' lengths.
  NS_ABORT_IF_FALSE((other.Length() == 0 || mHdr != EmptyHdr()) &&
                    (Length() == 0 || other.mHdr != EmptyHdr()),
                    "Don't set sEmptyHdr's length.");
  size_type tempLength = Length();
  mHdr->mLength = other.Length();
  other.mHdr->mLength = tempLength;

  return true;
}

template<class Alloc>
bool
nsTArray_base<Alloc>::EnsureNotUsingAutoArrayBuffer(size_type elemSize) {
  if (UsesAutoArrayBuffer()) {

    // If you call this on a 0-length array, we'll set that array's mHdr to
    // sEmptyHdr, in flagrant violation of the nsAutoTArray invariants.  It's
    // up to you to set it back!  (If you don't, the nsAutoTArray will forget
    // that it has an auto buffer.)
    if (Length() == 0) {
      mHdr = EmptyHdr();
      return true;
    }

    size_type size = sizeof(Header) + Length() * elemSize;

    Header* header = static_cast<Header*>(Alloc::Malloc(size));
    if (!header)
      return false;

    memcpy(header, mHdr, size);
    header->mCapacity = Length();
    mHdr = header;
  }
  
  return true;
}