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46ffc41619
Sub-byte alignment isn't really a thing. DONTBUILD because documentation fixes are always safe, amirite?
145 lines
3.3 KiB
C++
145 lines
3.3 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Functionality related to memory alignment. */
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#ifndef mozilla_Alignment_h
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#define mozilla_Alignment_h
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#include "mozilla/Attributes.h"
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#include <stddef.h>
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#include <stdint.h>
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namespace mozilla {
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/*
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* This class, and the corresponding macro MOZ_ALIGNOF, figures out how many
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* bytes of alignment a given type needs.
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*/
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template<typename T>
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class AlignmentFinder
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{
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struct Aligner
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{
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char mChar;
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T mT;
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};
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public:
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static const size_t alignment = sizeof(Aligner) - sizeof(T);
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};
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#define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment
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namespace detail {
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template<typename T>
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struct AlignasHelper
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{
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T mT;
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};
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} // namespace detail
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/*
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* Use this instead of alignof to align struct field as if it is inside
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* a struct. On some platforms, there exist types which have different
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* alignment between when it is used on its own and when it is used on
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* a struct field.
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*
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* Known examples are 64bit types (uint64_t, double) on 32bit Linux,
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* where they have 8byte alignment on their own, and 4byte alignment
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* when in struct.
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*/
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#define MOZ_ALIGNAS_IN_STRUCT(T) alignas(mozilla::detail::AlignasHelper<T>)
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/*
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* Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
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*
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* For instance,
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*
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* MOZ_ALIGNED_DECL(char arr[2], 8);
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*
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* will declare a two-character array |arr| aligned to 8 bytes.
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*/
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#if defined(__GNUC__)
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# define MOZ_ALIGNED_DECL(_type, _align) \
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_type __attribute__((aligned(_align)))
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#elif defined(_MSC_VER)
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# define MOZ_ALIGNED_DECL(_type, _align) \
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__declspec(align(_align)) _type
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#else
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# warning "We don't know how to align variables on this compiler."
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# define MOZ_ALIGNED_DECL(_type, _align) _type
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#endif
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/*
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* AlignedElem<N> is a structure whose alignment is guaranteed to be at least N
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* bytes.
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*
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* We support 1, 2, 4, 8, and 16-byte alignment.
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*/
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template<size_t Align>
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struct AlignedElem;
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/*
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* We have to specialize this template because GCC doesn't like
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* __attribute__((aligned(foo))) where foo is a template parameter.
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*/
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template<>
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struct AlignedElem<1>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 1);
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};
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template<>
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struct AlignedElem<2>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 2);
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};
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template<>
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struct AlignedElem<4>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 4);
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};
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template<>
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struct AlignedElem<8>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 8);
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};
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template<>
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struct AlignedElem<16>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 16);
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};
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template<typename T>
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struct MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS AlignedStorage2
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{
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union U
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{
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char mBytes[sizeof(T)];
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uint64_t mDummy;
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} u;
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const T* addr() const { return reinterpret_cast<const T*>(u.mBytes); }
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T* addr() { return static_cast<T*>(static_cast<void*>(u.mBytes)); }
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AlignedStorage2() = default;
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// AlignedStorage2 is non-copyable: the default copy constructor violates
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// strict aliasing rules, per bug 1269319.
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AlignedStorage2(const AlignedStorage2&) = delete;
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void operator=(const AlignedStorage2&) = delete;
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};
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} /* namespace mozilla */
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#endif /* mozilla_Alignment_h */
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