mirror of
https://github.com/mozilla/gecko-dev.git
synced 2024-12-29 03:44:37 +00:00
140 lines
3.0 KiB
C++
140 lines
3.0 KiB
C++
/* -*- 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 mChar;
|
|
T mT;
|
|
};
|
|
|
|
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 mBytes[Nbytes];
|
|
uint64_t mDummy;
|
|
} u;
|
|
|
|
const void* addr() const { return u.mBytes; }
|
|
void* addr() { return u.mBytes; }
|
|
};
|
|
|
|
template<typename T>
|
|
struct AlignedStorage2
|
|
{
|
|
union U
|
|
{
|
|
char mBytes[sizeof(T)];
|
|
uint64_t mDummy;
|
|
} u;
|
|
|
|
const T* addr() const { return reinterpret_cast<const T*>(u.mBytes); }
|
|
T* addr() { return static_cast<T*>(static_cast<void*>(u.mBytes)); }
|
|
};
|
|
|
|
} /* namespace mozilla */
|
|
|
|
#endif /* mozilla_Alignment_h */
|