michael@0: /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
michael@0: /* vim: set ts=8 sts=2 et sw=2 tw=80: */
michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: /* Functionality related to memory alignment. */
michael@0: 
michael@0: #ifndef mozilla_Alignment_h
michael@0: #define mozilla_Alignment_h
michael@0: 
michael@0: #include <stddef.h>
michael@0: #include <stdint.h>
michael@0: 
michael@0: namespace mozilla {
michael@0: 
michael@0: /*
michael@0:  * This class, and the corresponding macro MOZ_ALIGNOF, figures out how many
michael@0:  * bytes of alignment a given type needs.
michael@0:  */
michael@0: template<typename T>
michael@0: class AlignmentFinder
michael@0: {
michael@0:     struct Aligner
michael@0:     {
michael@0:         char c;
michael@0:         T t;
michael@0:     };
michael@0: 
michael@0:   public:
michael@0:     static const size_t alignment = sizeof(Aligner) - sizeof(T);
michael@0: };
michael@0: 
michael@0: #define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment
michael@0: 
michael@0: /*
michael@0:  * Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
michael@0:  *
michael@0:  * For instance,
michael@0:  *
michael@0:  *   MOZ_ALIGNED_DECL(char arr[2], 8);
michael@0:  *
michael@0:  * will declare a two-character array |arr| aligned to 8 bytes.
michael@0:  */
michael@0: 
michael@0: #if defined(__GNUC__)
michael@0: #  define MOZ_ALIGNED_DECL(_type, _align) \
michael@0:      _type __attribute__((aligned(_align)))
michael@0: #elif defined(_MSC_VER)
michael@0: #  define MOZ_ALIGNED_DECL(_type, _align) \
michael@0:      __declspec(align(_align)) _type
michael@0: #else
michael@0: #  warning "We don't know how to align variables on this compiler."
michael@0: #  define MOZ_ALIGNED_DECL(_type, _align) _type
michael@0: #endif
michael@0: 
michael@0: /*
michael@0:  * AlignedElem<N> is a structure whose alignment is guaranteed to be at least N
michael@0:  * bytes.
michael@0:  *
michael@0:  * We support 1, 2, 4, 8, and 16-bit alignment.
michael@0:  */
michael@0: template<size_t Align>
michael@0: struct AlignedElem;
michael@0: 
michael@0: /*
michael@0:  * We have to specialize this template because GCC doesn't like __attribute__((aligned(foo))) where
michael@0:  * foo is a template parameter.
michael@0:  */
michael@0: 
michael@0: template<>
michael@0: struct AlignedElem<1>
michael@0: {
michael@0:     MOZ_ALIGNED_DECL(uint8_t elem, 1);
michael@0: };
michael@0: 
michael@0: template<>
michael@0: struct AlignedElem<2>
michael@0: {
michael@0:     MOZ_ALIGNED_DECL(uint8_t elem, 2);
michael@0: };
michael@0: 
michael@0: template<>
michael@0: struct AlignedElem<4>
michael@0: {
michael@0:     MOZ_ALIGNED_DECL(uint8_t elem, 4);
michael@0: };
michael@0: 
michael@0: template<>
michael@0: struct AlignedElem<8>
michael@0: {
michael@0:     MOZ_ALIGNED_DECL(uint8_t elem, 8);
michael@0: };
michael@0: 
michael@0: template<>
michael@0: struct AlignedElem<16>
michael@0: {
michael@0:     MOZ_ALIGNED_DECL(uint8_t elem, 16);
michael@0: };
michael@0: 
michael@0: /*
michael@0:  * This utility pales in comparison to Boost's aligned_storage. The utility
michael@0:  * simply assumes that uint64_t is enough alignment for anyone. This may need
michael@0:  * to be extended one day...
michael@0:  *
michael@0:  * As an important side effect, pulling the storage into this template is
michael@0:  * enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
michael@0:  * false negatives when we cast from the char buffer to whatever type we've
michael@0:  * constructed using the bytes.
michael@0:  */
michael@0: template<size_t Nbytes>
michael@0: struct AlignedStorage
michael@0: {
michael@0:     union U {
michael@0:       char bytes[Nbytes];
michael@0:       uint64_t _;
michael@0:     } u;
michael@0: 
michael@0:     const void* addr() const { return u.bytes; }
michael@0:     void* addr() { return u.bytes; }
michael@0: };
michael@0: 
michael@0: template<typename T>
michael@0: struct AlignedStorage2
michael@0: {
michael@0:     union U {
michael@0:       char bytes[sizeof(T)];
michael@0:       uint64_t _;
michael@0:     } u;
michael@0: 
michael@0:     const T* addr() const { return reinterpret_cast<const T*>(u.bytes); }
michael@0:     T* addr() { return static_cast<T*>(static_cast<void*>(u.bytes)); }
michael@0: };
michael@0: 
michael@0: } /* namespace mozilla */
michael@0: 
michael@0: #endif /* mozilla_Alignment_h */