[libcxx-commits] [libcxx] a349092 - [libc++] Add __small_buffer
Nikolas Klauser via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Oct 26 08:25:58 PDT 2023
Author: Nikolas Klauser
Date: 2023-10-26T17:25:53+02:00
New Revision: a3490920615e963732d70df2ef77eafa35b8ee0f
URL: https://github.com/llvm/llvm-project/commit/a3490920615e963732d70df2ef77eafa35b8ee0f
DIFF: https://github.com/llvm/llvm-project/commit/a3490920615e963732d70df2ef77eafa35b8ee0f.diff
LOG: [libc++] Add __small_buffer
This is an implementation detail for `move_only_function` (and potentially other type-erasing classes).
Reviewed By: #libc, ldionne
Spies: Mordante, ldionne, EricWF, libcxx-commits
Differential Revision: https://reviews.llvm.org/D140259
Added:
libcxx/include/__utility/small_buffer.h
libcxx/test/libcxx/utilities/utility/small_buffer.pass.cpp
Modified:
libcxx/include/CMakeLists.txt
libcxx/include/module.modulemap.in
Removed:
################################################################################
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 1f36ff6b3896e46..b7b14200498a298 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -854,6 +854,7 @@ set(files
__utility/piecewise_construct.h
__utility/priority_tag.h
__utility/rel_ops.h
+ __utility/small_buffer.h
__utility/swap.h
__utility/to_underlying.h
__utility/unreachable.h
diff --git a/libcxx/include/__utility/small_buffer.h b/libcxx/include/__utility/small_buffer.h
new file mode 100644
index 000000000000000..0c6d4986a58514b
--- /dev/null
+++ b/libcxx/include/__utility/small_buffer.h
@@ -0,0 +1,99 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___UTILITY_SMALL_BUFFER_H
+#define _LIBCPP___UTILITY_SMALL_BUFFER_H
+
+#include <__config>
+#include <__memory/construct_at.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/is_trivially_destructible.h>
+#include <__type_traits/is_trivially_move_constructible.h>
+#include <__utility/exception_guard.h>
+#include <__utility/forward.h>
+#include <cstddef>
+#include <new>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+# pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+// __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to
+// allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an
+// allocation.
+//
+// This small buffer class only allows storing trivially relocatable objects inside the local storage to allow
+// __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user
+// has to manage the object's lifetime, in particular the destruction of the object.
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <size_t _BufferSize, size_t _BufferAlignment>
+ requires(_BufferSize > 0 && _BufferAlignment > 0)
+class __small_buffer {
+public:
+ template <class _Tp, class _Decayed = decay_t<_Tp>>
+ static constexpr bool __fits_in_buffer =
+ is_trivially_move_constructible_v<_Decayed> && is_trivially_destructible_v<_Decayed> &&
+ sizeof(_Decayed) <= _BufferSize && alignof(_Decayed) <= _BufferAlignment;
+
+ _LIBCPP_HIDE_FROM_ABI __small_buffer() = default;
+ __small_buffer(const __small_buffer&) = delete;
+ __small_buffer& operator=(const __small_buffer&) = delete;
+ _LIBCPP_HIDE_FROM_ABI ~__small_buffer() = default;
+
+ // Relocates the buffer - __delete() should never be called on a moved-from __small_buffer
+ _LIBCPP_HIDE_FROM_ABI __small_buffer(__small_buffer&&) = default;
+ _LIBCPP_HIDE_FROM_ABI __small_buffer& operator=(__small_buffer&&) = default;
+
+ template <class _Stored>
+ _LIBCPP_HIDE_FROM_ABI _Stored* __get() {
+ if constexpr (__fits_in_buffer<_Stored>)
+ return std::launder(reinterpret_cast<_Stored*>(__buffer_));
+ else
+ return *std::launder(reinterpret_cast<_Stored**>(__buffer_));
+ }
+
+ template <class _Stored>
+ _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE _LIBCPP_HIDE_FROM_ABI _Stored* __alloc() {
+ if constexpr (__fits_in_buffer<_Stored>) {
+ return std::launder(reinterpret_cast<_Stored*>(__buffer_));
+ } else {
+ byte* __allocation = static_cast<byte*>(::operator new[](sizeof(_Stored), align_val_t{alignof(_Stored)}));
+ std::construct_at(reinterpret_cast<byte**>(__buffer_), __allocation);
+ return std::launder(reinterpret_cast<_Stored*>(__allocation));
+ }
+ }
+
+ template <class _Stored>
+ _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE _LIBCPP_HIDE_FROM_ABI void __dealloc() noexcept {
+ if constexpr (!__fits_in_buffer<_Stored>)
+ ::operator delete[](*reinterpret_cast<void**>(__buffer_), sizeof(_Stored), align_val_t{alignof(_Stored)});
+ }
+
+ template <class _Stored, class... _Args>
+ _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) {
+ _Stored* __buffer = __alloc<_Stored>();
+ auto __guard = std::__make_exception_guard([&] { __dealloc<_Stored>(); });
+ std::construct_at(__buffer, std::forward<_Args>(__args)...);
+ __guard.__complete();
+ }
+
+private:
+ alignas(_BufferAlignment) byte __buffer_[_BufferSize];
+};
+
+# undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+#endif // _LIBCPP___UTILITY_SMALL_BUFFER_H
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 41158212c9a86cf..f447b2fa0a0ce94 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -2079,6 +2079,7 @@ module std_private_utility_pair_fwd [system] { header "__fwd/pair.
module std_private_utility_piecewise_construct [system] { header "__utility/piecewise_construct.h" }
module std_private_utility_priority_tag [system] { header "__utility/priority_tag.h" }
module std_private_utility_rel_ops [system] { header "__utility/rel_ops.h" }
+module std_private_utility_small_buffer [system] { header "__utility/small_buffer.h" }
module std_private_utility_swap [system] {
header "__utility/swap.h"
export std_private_type_traits_is_swappable
diff --git a/libcxx/test/libcxx/utilities/utility/small_buffer.pass.cpp b/libcxx/test/libcxx/utilities/utility/small_buffer.pass.cpp
new file mode 100644
index 000000000000000..2214efa486870d9
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/utility/small_buffer.pass.cpp
@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// XFAIL: availability-aligned_allocation-missing
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+#include "test_macros.h"
+
+TEST_DIAGNOSTIC_PUSH
+TEST_CLANG_DIAGNOSTIC_IGNORED("-Wprivate-header")
+#include <__utility/small_buffer.h>
+TEST_DIAGNOSTIC_POP
+
+#include <cassert>
+#include <memory>
+#include <utility>
+
+struct NotTriviallyRelocatable {
+ char c_;
+
+ NotTriviallyRelocatable(char c) : c_(c) {}
+ ~NotTriviallyRelocatable() {}
+};
+
+struct alignas(16) Overaligned {
+ int i;
+};
+
+int main(int, char**) {
+ using BufferT = std::__small_buffer<8, 8>;
+ static_assert(sizeof(BufferT) == 8);
+ static_assert(alignof(BufferT) == 8);
+ static_assert(BufferT::__fits_in_buffer<int>);
+ static_assert(!BufferT::__fits_in_buffer<Overaligned>);
+ static_assert(!BufferT::__fits_in_buffer<NotTriviallyRelocatable>);
+
+ BufferT buf;
+
+ { // construct/destroy in the same place
+ buf.__construct<int>(3);
+ assert(*buf.__get<int>() == 3);
+ std::destroy_at(buf.__get<int>());
+ buf.__dealloc<int>();
+
+ buf.__construct<NotTriviallyRelocatable>(3);
+ assert(buf.__get<NotTriviallyRelocatable>()->c_ == 3);
+ std::destroy_at(buf.__get<NotTriviallyRelocatable>());
+ buf.__dealloc<NotTriviallyRelocatable>();
+ }
+
+ { // Move the buffer around
+ buf.__construct<int>(3);
+ assert(*buf.__get<int>() == 3);
+ auto buf2 = std::move(buf);
+ assert(*buf2.__get<int>() == 3);
+ std::destroy_at(buf2.__get<int>());
+ buf2.__dealloc<int>();
+
+ buf.__construct<NotTriviallyRelocatable>(3);
+ assert(buf.__get<NotTriviallyRelocatable>()->c_ == 3);
+ auto buf3 = std::move(buf);
+ assert(buf3.__get<NotTriviallyRelocatable>()->c_ == 3);
+ std::destroy_at(buf3.__get<NotTriviallyRelocatable>());
+ buf3.__dealloc<NotTriviallyRelocatable>();
+ }
+
+ return 0;
+}
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