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