[llvm-branch-commits] [libcxx] [libc++] Implement std::move_only_function (P0288R9) (PR #94670)

[Adrian Vogelsgesang via llvm-branch-commits]

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+//===----------------------------------------------------------------------===//
+//
+// 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___FUNCTIONAL_MOVE_ONLY_FUNCTION_H
+#define _LIBCPP___FUNCTIONAL_MOVE_ONLY_FUNCTION_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+// move_only_function design:
+//
+// move_only_function has a small buffer with a size of `3 * sizeof(void*)` bytes. This buffer can only be used when the
+// object that should be stored is trivially relocatable (currently only when it is trivially move constructible and
+// trivially destructible). There is also a bool in the lower bits of the vptr stored which is set when the contained
+// object is not trivially destructible.
+//
+// trivially relocatable: It would also be possible to store nothrow_move_constructible types, but that would mean
+// that move_only_function itself would not be trivially relocatable anymore. The decision to keep move_only_function
+// trivially relocatable was made because we expect move_only_function to be mostly used to store a functor. To only
+// forward functors there is std::function_ref (not voted in yet, expected in C++26).
+//
+// buffer size: We did a survey of six implementations from various vendors. Three of them had a buffer size of 24 bytes
+// on 64 bit systems. This also allows storing a std::string or std::vector inside the small buffer (once the compiler
+// has full support of trivially_relocatable annotations).
+//
+// trivially-destructible bit: This allows us to keep the overall binary size smaller because we don't have to store
+// a pointer to a noop function inside the vtable. It also avoids loading the vtable during destruction, potentially
+// resulting in fewer cache misses. The downside is that calling the function now also requires setting the lower bits
+// of the pointer to zero, but this is a very fast operation on modern CPUs.
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vogelsgesang wrote:

Does the same also apply for `function_ref`? Should it be possible to turn a `move_only_function` / `copyable_function` into a `function_ref` without double-wrapping?

https://github.com/llvm/llvm-project/pull/94670