102 lines · c
1//===----------------------------------------------------------------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8 9#ifndef _LIBCPP___UTILITY_SMALL_BUFFER_H10#define _LIBCPP___UTILITY_SMALL_BUFFER_H11 12#include <__config>13#include <__cstddef/byte.h>14#include <__cstddef/size_t.h>15#include <__memory/construct_at.h>16#include <__new/allocate.h>17#include <__new/launder.h>18#include <__type_traits/decay.h>19#include <__type_traits/is_trivially_constructible.h>20#include <__type_traits/is_trivially_destructible.h>21#include <__utility/exception_guard.h>22#include <__utility/forward.h>23 24#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)25# pragma GCC system_header26#endif27 28#if _LIBCPP_STD_VER >= 2329 30// __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to31// allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an32// allocation.33//34// This small buffer class only allows storing trivially relocatable objects inside the local storage to allow35// __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user36// has to manage the object's lifetime, in particular the destruction of the object.37 38_LIBCPP_BEGIN_NAMESPACE_STD39 40template <size_t _BufferSize, size_t _BufferAlignment>41 requires(_BufferSize > 0 && _BufferAlignment > 0)42class __small_buffer {43public:44 template <class _Tp, class _Decayed = decay_t<_Tp>>45 static constexpr bool __fits_in_buffer =46 is_trivially_move_constructible_v<_Decayed> && is_trivially_destructible_v<_Decayed> &&47 sizeof(_Decayed) <= _BufferSize && alignof(_Decayed) <= _BufferAlignment;48 49 _LIBCPP_HIDE_FROM_ABI __small_buffer() = default;50 __small_buffer(const __small_buffer&) = delete;51 __small_buffer& operator=(const __small_buffer&) = delete;52 _LIBCPP_HIDE_FROM_ABI ~__small_buffer() = default;53 54 // Relocates the buffer - __delete() should never be called on a moved-from __small_buffer55 _LIBCPP_HIDE_FROM_ABI __small_buffer(__small_buffer&&) = default;56 _LIBCPP_HIDE_FROM_ABI __small_buffer& operator=(__small_buffer&&) = default;57 58 template <class _Stored>59 _LIBCPP_HIDE_FROM_ABI _Stored* __get() {60 if constexpr (__fits_in_buffer<_Stored>)61 return std::launder(reinterpret_cast<_Stored*>(__buffer_));62 else63 return *std::launder(reinterpret_cast<_Stored**>(__buffer_));64 }65 66 template <class _Stored>67 _LIBCPP_HIDE_FROM_ABI _Stored* __alloc() {68 if constexpr (__fits_in_buffer<_Stored>) {69 return std::launder(reinterpret_cast<_Stored*>(__buffer_));70 } else {71 byte* __allocation = reinterpret_cast<byte*>(std::__libcpp_allocate<_Stored>(__element_count(1)));72 std::construct_at(reinterpret_cast<byte**>(__buffer_), __allocation);73 return std::launder(reinterpret_cast<_Stored*>(__allocation));74 }75 }76 77 template <class _Stored>78 _LIBCPP_HIDE_FROM_ABI void __dealloc() noexcept {79 if constexpr (!__fits_in_buffer<_Stored>)80 std::__libcpp_deallocate<_Stored>(__get<_Stored>(), __element_count(1));81 }82 83 template <class _Stored, class... _Args>84 _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) {85 _Stored* __buffer = __alloc<_Stored>();86 auto __guard = std::__make_exception_guard([&] { __dealloc<_Stored>(); });87 std::construct_at(__buffer, std::forward<_Args>(__args)...);88 __guard.__complete();89 }90 91private:92 alignas(_BufferAlignment) byte __buffer_[_BufferSize];93};94 95# undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI96 97_LIBCPP_END_NAMESPACE_STD98 99#endif // _LIBCPP_STD_VER >= 23100 101#endif // _LIBCPP___UTILITY_SMALL_BUFFER_H102