gecko/mfbt/Alignment.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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/. */

/* Functionality related to memory alignment. */

#ifndef mozilla_Alignment_h
#define mozilla_Alignment_h

#include <stddef.h>
#include <stdint.h>

namespace mozilla {

/*
 * This class, and the corresponding macro MOZ_ALIGNOF, figures out how many
 * bytes of alignment a given type needs.
 */
template<typename T>
class AlignmentFinder
{
    struct Aligner
    {
        char c;
        T t;
    };

  public:
    static const size_t alignment = sizeof(Aligner) - sizeof(T);
};

#define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment

/*
 * Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
 *
 * For instance,
 *
 *   MOZ_ALIGNED_DECL(char arr[2], 8);
 *
 * will declare a two-character array |arr| aligned to 8 bytes.
 */

#if defined(__GNUC__)
#  define MOZ_ALIGNED_DECL(_type, _align) \
     _type __attribute__((aligned(_align)))
#elif defined(_MSC_VER)
#  define MOZ_ALIGNED_DECL(_type, _align) \
     __declspec(align(_align)) _type
#else
#  warning "We don't know how to align variables on this compiler."
#  define MOZ_ALIGNED_DECL(_type, _align) _type
#endif

/*
 * AlignedElem<N> is a structure whose alignment is guaranteed to be at least N
 * bytes.
 *
 * We support 1, 2, 4, 8, and 16-bit alignment.
 */
template<size_t Align>
struct AlignedElem;

/*
 * We have to specialize this template because GCC doesn't like __attribute__((aligned(foo))) where
 * foo is a template parameter.
 */

template<>
struct AlignedElem<1>
{
    MOZ_ALIGNED_DECL(uint8_t elem, 1);
};

template<>
struct AlignedElem<2>
{
    MOZ_ALIGNED_DECL(uint8_t elem, 2);
};

template<>
struct AlignedElem<4>
{
    MOZ_ALIGNED_DECL(uint8_t elem, 4);
};

template<>
struct AlignedElem<8>
{
    MOZ_ALIGNED_DECL(uint8_t elem, 8);
};

template<>
struct AlignedElem<16>
{
    MOZ_ALIGNED_DECL(uint8_t elem, 16);
};

/*
 * This utility pales in comparison to Boost's aligned_storage. The utility
 * simply assumes that uint64_t is enough alignment for anyone. This may need
 * to be extended one day...
 *
 * As an important side effect, pulling the storage into this template is
 * enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
 * false negatives when we cast from the char buffer to whatever type we've
 * constructed using the bytes.
 */
template<size_t Nbytes>
struct AlignedStorage
{
    union U {
      char bytes[Nbytes];
      uint64_t _;
    } u;

    const void* addr() const { return u.bytes; }
    void* addr() { return u.bytes; }
};

template<typename T>
struct AlignedStorage2
{
    union U {
      char bytes[sizeof(T)];
      uint64_t _;
    } u;

    const T* addr() const { return reinterpret_cast<const T*>(u.bytes); }
    T* addr() { return static_cast<T*>(static_cast<void*>(u.bytes)); }
};

} /* namespace mozilla */

#endif /* mozilla_Alignment_h */