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 <__memory/construct_at.h> 14 #include <__type_traits/decay.h> 15 #include <__type_traits/is_trivially_destructible.h> 16 #include <__type_traits/is_trivially_move_constructible.h> 17 #include <__utility/exception_guard.h> 18 #include <__utility/forward.h> 19 #include <cstddef> 20 #include <new> 21 22 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 23 # pragma GCC system_header 24 #endif 25 26 #if _LIBCPP_STD_VER >= 23 27 28 // __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to 29 // allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an 30 // allocation. 31 // 32 // This small buffer class only allows storing trivially relocatable objects inside the local storage to allow 33 // __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user 34 // has to manage the object's lifetime, in particular the destruction of the object. 35 36 _LIBCPP_BEGIN_NAMESPACE_STD 37 38 template <size_t _BufferSize, size_t _BufferAlignment> 39 requires(_BufferSize > 0 && _BufferAlignment > 0) 40 class __small_buffer { 41 public: 42 template <class _Tp, class _Decayed = decay_t<_Tp>> 43 static constexpr bool __fits_in_buffer = 44 is_trivially_move_constructible_v<_Decayed> && is_trivially_destructible_v<_Decayed> && 45 sizeof(_Decayed) <= _BufferSize && alignof(_Decayed) <= _BufferAlignment; 46 47 _LIBCPP_HIDE_FROM_ABI __small_buffer() = default; 48 __small_buffer(const __small_buffer&) = delete; 49 __small_buffer& operator=(const __small_buffer&) = delete; 50 _LIBCPP_HIDE_FROM_ABI ~__small_buffer() = default; 51 52 // Relocates the buffer - __delete() should never be called on a moved-from __small_buffer 53 _LIBCPP_HIDE_FROM_ABI __small_buffer(__small_buffer&&) = default; 54 _LIBCPP_HIDE_FROM_ABI __small_buffer& operator=(__small_buffer&&) = default; 55 56 template <class _Stored> __get()57 _LIBCPP_HIDE_FROM_ABI _Stored* __get() { 58 if constexpr (__fits_in_buffer<_Stored>) 59 return std::launder(reinterpret_cast<_Stored*>(__buffer_)); 60 else 61 return *std::launder(reinterpret_cast<_Stored**>(__buffer_)); 62 } 63 64 template <class _Stored> __alloc()65 _LIBCPP_HIDE_FROM_ABI _Stored* __alloc() { 66 if constexpr (__fits_in_buffer<_Stored>) { 67 return std::launder(reinterpret_cast<_Stored*>(__buffer_)); 68 } else { 69 byte* __allocation = static_cast<byte*>(::operator new[](sizeof(_Stored), align_val_t{alignof(_Stored)})); 70 std::construct_at(reinterpret_cast<byte**>(__buffer_), __allocation); 71 return std::launder(reinterpret_cast<_Stored*>(__allocation)); 72 } 73 } 74 75 template <class _Stored> __dealloc()76 _LIBCPP_HIDE_FROM_ABI void __dealloc() noexcept { 77 if constexpr (!__fits_in_buffer<_Stored>) 78 ::operator delete[](*reinterpret_cast<void**>(__buffer_), sizeof(_Stored), align_val_t{alignof(_Stored)}); 79 } 80 81 template <class _Stored, class... _Args> __construct(_Args &&...__args)82 _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) { 83 _Stored* __buffer = __alloc<_Stored>(); 84 auto __guard = std::__make_exception_guard([&] { __dealloc<_Stored>(); }); 85 std::construct_at(__buffer, std::forward<_Args>(__args)...); 86 __guard.__complete(); 87 } 88 89 private: 90 alignas(_BufferAlignment) byte __buffer_[_BufferSize]; 91 }; 92 93 # undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI 94 95 _LIBCPP_END_NAMESPACE_STD 96 97 #endif // _LIBCPP_STD_VER >= 23 98 99 #endif // _LIBCPP___UTILITY_SMALL_BUFFER_H 100