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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 -*- */
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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 <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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/*
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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-bit 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 __attribute__((aligned(foo))) where
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* 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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/*
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* This utility pales in comparison to Boost's aligned_storage. The utility
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* simply assumes that uint64_t is enough alignment for anyone. This may need
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* to be extended one day...
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*
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* As an important side effect, pulling the storage into this template is
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* enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
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* false negatives when we cast from the char buffer to whatever type we've
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* constructed using the bytes.
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*/
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template<size_t Nbytes>
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struct AlignedStorage
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{
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union U
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{
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char mBytes[Nbytes];
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uint64_t mDummy;
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} u;
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const void* addr() const { return u.mBytes; }
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void* addr() { return u.mBytes; }
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};
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template<typename T>
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struct 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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};
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} /* namespace mozilla */
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#endif /* mozilla_Alignment_h */
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