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| 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-exception |
| 6 | // |
| 7 | //===----------------------------------------------------------------------===// |
| 8 | |
| 9 | #ifndef _LIBCPP___UTILITY_SMALL_BUFFER_H |
| 10 | #define _LIBCPP___UTILITY_SMALL_BUFFER_H |
| 11 | |
| 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_header |
| 26 | #endif |
| 27 | |
| 28 | #if _LIBCPP_STD_VER >= 23 |
| 29 | |
| 30 | // __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to |
| 31 | // allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an |
| 32 | // allocation. |
| 33 | // |
| 34 | // This small buffer class only allows storing trivially relocatable objects inside the local storage to allow |
| 35 | // __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user |
| 36 | // has to manage the object's lifetime, in particular the destruction of the object. |
| 37 | |
| 38 | _LIBCPP_BEGIN_NAMESPACE_STD |
| 39 | |
| 40 | template <size_t _BufferSize, size_t _BufferAlignment> |
| 41 | requires(_BufferSize > 0 && _BufferAlignment > 0) |
| 42 | class __small_buffer { |
| 43 | public: |
| 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_buffer |
| 55 | _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 | else |
| 63 | 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 | |
| 91 | private: |
| 92 | alignas(_BufferAlignment) byte __buffer_[_BufferSize]; |
| 93 | }; |
| 94 | |
| 95 | # undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI |
| 96 | |
| 97 | _LIBCPP_END_NAMESPACE_STD |
| 98 | |
| 99 | #endif // _LIBCPP_STD_VER >= 23 |
| 100 | |
| 101 | #endif // _LIBCPP___UTILITY_SMALL_BUFFER_H |
| 102 |
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