third_party/WebKit/Source/platform/PurgeableVector.cpp

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#include "config.h"
#include "platform/PurgeableVector.h"

#include "public/platform/Platform.h"
#include "public/platform/WebDiscardableMemory.h"
#include "wtf/Assertions.h"
#include "wtf/OwnPtr.h"
#include "wtf/PassOwnPtr.h"

#include <cstring>

namespace blink {

// WebDiscardableMemory allocations are expensive and page-grained. We only use
// them when there's a reasonable amount of memory to be saved by the OS
// discarding the memory.
static const size_t minimumDiscardableAllocationSize = 4 * 4096;

PurgeableVector::PurgeableVector(PurgeableOption purgeable)
    : m_discardableCapacity(0),
      m_discardableSize(0),
      m_isPurgeable(purgeable == Purgeable),
      m_locksCount(1)  // The buffer is locked at creation.
{}

PurgeableVector::~PurgeableVector() {}

void PurgeableVector::reserveCapacity(size_t capacity) {
  ASSERT(isLocked());

  if (m_isPurgeable) {
    if (reservePurgeableCapacity(capacity, UseExactCapacity))
      return;
    // Fallback to non-purgeable buffer allocation in case discardable memory
    // allocation failed.
  }

  if (!m_vector.capacity()) {
    // Using reserveInitialCapacity() on the underlying vector ensures that the
    // vector uses the
    // exact specified capacity to avoid consuming too much memory for small
    // resources.
    m_vector.reserveInitialCapacity(capacity);
  } else {
    m_vector.reserveCapacity(capacity);
  }

  moveDataFromDiscardableToVector();
}

void PurgeableVector::moveDataFromDiscardableToVector() {
  if (m_discardable) {
    m_vector.append(static_cast<const char*>(m_discardable->data()),
                    m_discardableSize);
    clearDiscardable();
  }
}

void PurgeableVector::clearDiscardable() {
  m_discardable.clear();
  m_discardableCapacity = 0;
  m_discardableSize = 0;
}

void PurgeableVector::append(const char* data, size_t length) {
  ASSERT(isLocked());

  if (!m_isPurgeable) {
    m_vector.append(data, length);
    return;
  }

  const size_t currentSize =
      m_discardable ? m_discardableSize : m_vector.size();
  const size_t newBufferSize = currentSize + length;

  if (!reservePurgeableCapacity(newBufferSize, UseExponentialGrowth)) {
    moveDataFromDiscardableToVector();
    m_vector.append(data, length);
    return;
  }

  ASSERT(m_discardableSize + length <= m_discardableCapacity);
  memcpy(static_cast<char*>(m_discardable->data()) + m_discardableSize, data,
         length);
  m_discardableSize += length;
}

void PurgeableVector::grow(size_t newSize) {
  ASSERT(newSize >= size());

  if (m_isPurgeable) {
    if (reservePurgeableCapacity(newSize, UseExponentialGrowth)) {
      m_discardableSize = newSize;
      return;
    }
    moveDataFromDiscardableToVector();
  }

  m_vector.resize(newSize);
}

void PurgeableVector::clear() {
  clearDiscardable();
  m_vector.clear();
}

char* PurgeableVector::data() {
  ASSERT(isLocked());
  return m_discardable ? static_cast<char*>(m_discardable->data())
                       : m_vector.data();
}

size_t PurgeableVector::size() const {
  return m_discardable ? m_discardableSize : m_vector.size();
}

void PurgeableVector::adopt(Vector<char>& other) {
  if (size() > 0)
    clear();

  if (!m_isPurgeable) {
    m_vector.swap(other);
    return;
  }

  if (other.isEmpty())
    return;

  append(other.data(), other.size());
  other.clear();
}

bool PurgeableVector::lock() {
  ++m_locksCount;
  if (m_locksCount > 1)
    return true;

  ASSERT(m_locksCount == 1);
  if (!m_discardable)
    return true;

  return m_discardable->lock();
}

void PurgeableVector::unlock() {
  ASSERT(isLocked());
  --m_locksCount;
  if (m_locksCount > 0)
    return;

  if (!m_vector.isEmpty()) {
    ASSERT(!m_discardable);
    m_isPurgeable = true;
    if (!reservePurgeableCapacity(m_vector.size(), UseExactCapacity))
      return;
  }

  if (m_discardable)
    m_discardable->unlock();
}

bool PurgeableVector::isLocked() const {
  ASSERT(m_locksCount >= 0);
  return m_locksCount > 0;
}

bool PurgeableVector::reservePurgeableCapacity(
    size_t capacity,
    PurgeableAllocationStrategy allocationStrategy) {
  ASSERT(m_isPurgeable);

  if (m_discardable && m_discardableCapacity >= capacity) {
    ASSERT(!m_vector.capacity());
    return true;
  }

  if (capacity < minimumDiscardableAllocationSize)
    return false;

  if (allocationStrategy == UseExponentialGrowth)
    capacity = adjustPurgeableCapacity(capacity);

  OwnPtr<WebDiscardableMemory> discardable =
      adoptPtr(Platform::current()->allocateAndLockDiscardableMemory(capacity));
  if (!discardable) {
    // Discardable memory is not supported.
    m_isPurgeable = false;
    return false;
  }

  m_discardableCapacity = capacity;
  // Copy the data that was either in the previous purgeable buffer or in the
  // vector to the new
  // purgeable buffer.
  if (m_discardable) {
    memcpy(discardable->data(), m_discardable->data(), m_discardableSize);
  } else {
    memcpy(discardable->data(), m_vector.data(), m_vector.size());
    m_discardableSize = m_vector.size();
    m_vector.clear();
  }

  m_discardable.swap(discardable);
  ASSERT(!m_vector.capacity());
  return true;
}

size_t PurgeableVector::adjustPurgeableCapacity(size_t capacity) const {
  ASSERT(capacity >= minimumDiscardableAllocationSize);

  const float growthFactor = 1.5;
  size_t newCapacity = std::max(
      capacity, static_cast<size_t>(m_discardableCapacity * growthFactor));

  // Discardable memory has page-granularity so align to the next page here to
  // minimize
  // fragmentation.
  // Since the page size is only used below to minimize fragmentation it's still
  // safe to use it
  // even if it gets out of sync (e.g. due to the use of huge pages).
  const size_t kPageSize = 4096;
  newCapacity = (newCapacity + kPageSize - 1) & ~(kPageSize - 1);

  return std::max(capacity, newCapacity);  // Overflow check.
}

}  // namespace blink