From 42907ca9a495a4a535bbd995fa126afb76012023 Mon Sep 17 00:00:00 2001 From: Patrick Palka Date: Thu, 24 Sep 2020 12:58:39 -0400 Subject: [PATCH] libstdc++: Specialize ranges::__detail::__box for semiregular types The class template semiregular-box defined in [range.semi.wrap] is used by a number of views to accomodate non-semiregular subobjects while ensuring that the overall view remains semiregular. It provides a stand-in default constructor, copy assignment operator and move assignment operator whenever the underlying type lacks them. The wrapper derives from std::optional to support default construction when T is not default constructible. It would be nice for this wrapper to essentially be a no-op when the underlying type is already semiregular, but this is currently not the case due to its use of std::optional, which incurs space overhead compared to storing just T. To that end, this patch specializes the semiregular wrapper for semiregular T. Compared to the primary template, this specialization uses less space, and it allows [[no_unique_address]] to optimize away wrapped data members whose underlying type is empty and semiregular (e.g. a non-capturing lambda). This patch also applies [[no_unique_address]] to the five data members that use the wrapper. libstdc++-v3/ChangeLog: * include/std/ranges (__detail::__boxable): Split out the associated constraints of __box into here. (__detail::__box): Use the __boxable concept. Define a leaner partial specialization for semiregular types. (single_view::_M_value): Give it [[no_unique_address]]. (filter_view::_M_pred): Likewise. (transform_view::_M_fun): Likewise. (take_while_view::_M_pred): Likewise. (drop_while_view::_M_pred):: Likewise. * testsuite/std/ranges/adaptors/detail/semiregular_box.cc: New test. --- libstdc++-v3/include/std/ranges | 68 +++++++++++++++-- .../ranges/adaptors/detail/semiregular_box.cc | 73 +++++++++++++++++++ 2 files changed, 135 insertions(+), 6 deletions(-) create mode 100644 libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges index e7fa4493612..ed04fa0001d 100644 --- a/libstdc++-v3/include/std/ranges +++ b/libstdc++-v3/include/std/ranges @@ -86,7 +86,10 @@ namespace ranges namespace __detail { - template requires is_object_v<_Tp> + template + concept __boxable = copy_constructible<_Tp> && is_object_v<_Tp>; + + template<__boxable _Tp> struct __box : std::optional<_Tp> { using std::optional<_Tp>::optional; @@ -130,6 +133,59 @@ namespace ranges } }; + // For types which are already semiregular, this specialization of the + // semiregular wrapper stores the object directly without going through + // std::optional. It provides just the subset of the primary template's + // API that we currently use. + template<__boxable _Tp> requires semiregular<_Tp> + struct __box<_Tp> + { + private: + [[no_unique_address]] _Tp _M_value; + + public: + __box() = default; + + constexpr explicit + __box(const _Tp& __t) + noexcept(is_nothrow_copy_constructible_v<_Tp>) + : _M_value{__t} + { } + + constexpr explicit + __box(_Tp&& __t) + noexcept(is_nothrow_move_constructible_v<_Tp>) + : _M_value{std::move(__t)} + { } + + template + requires constructible_from<_Tp, _Args...> + constexpr + __box(in_place_t, _Args&&... __args) + noexcept(is_nothrow_constructible_v<_Tp, _Args...>) + : _M_value{std::forward<_Args>(__args)...} + { } + + constexpr bool + has_value() const noexcept + { return true; }; + + constexpr _Tp& + operator*() noexcept + { return _M_value; } + + constexpr const _Tp& + operator*() const noexcept + { return _M_value; } + + constexpr _Tp* + operator->() noexcept + { return &_M_value; } + + constexpr const _Tp* + operator->() const noexcept + { return &_M_value; } + }; } // namespace __detail /// A view that contains exactly one element. @@ -185,7 +241,7 @@ namespace ranges { return _M_value.operator->(); } private: - __detail::__box<_Tp> _M_value; + [[no_unique_address]] __detail::__box<_Tp> _M_value; }; namespace __detail @@ -1195,7 +1251,7 @@ namespace views }; _Vp _M_base = _Vp(); - __detail::__box<_Pred> _M_pred; + [[no_unique_address]] __detail::__box<_Pred> _M_pred; [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin; public: @@ -1533,7 +1589,7 @@ namespace views }; _Vp _M_base = _Vp(); - __detail::__box<_Fp> _M_fun; + [[no_unique_address]] __detail::__box<_Fp> _M_fun; public: transform_view() = default; @@ -1787,7 +1843,7 @@ namespace views }; _Vp _M_base = _Vp(); - __detail::__box<_Pred> _M_pred; + [[no_unique_address]] __detail::__box<_Pred> _M_pred; public: take_while_view() = default; @@ -1947,7 +2003,7 @@ namespace views { private: _Vp _M_base = _Vp(); - __detail::__box<_Pred> _M_pred; + [[no_unique_address]] __detail::__box<_Pred> _M_pred; [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin; public: diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc new file mode 100644 index 00000000000..392acff3eb6 --- /dev/null +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc @@ -0,0 +1,73 @@ +// Copyright (C) 2020 Free Software Foundation, Inc. +// +// This file is part of the GNU ISO C++ Library. This library is free +// software; you can redistribute it and/or modify it under the +// terms of the GNU General Public License as published by the +// Free Software Foundation; either version 3, or (at your option) +// any later version. + +// This library is distributed in the hope that it will be useful, +// but WITHOUT ANY WARRANTY; without even the implied warranty of +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +// GNU General Public License for more details. + +// You should have received a copy of the GNU General Public License along +// with this library; see the file COPYING3. If not see +// . + +// { dg-options "-std=gnu++2a" } +// { dg-do compile { target c++2a } } + +#include + +using std::ranges::__detail::__box; + +using T = decltype([] { return 0; }); +static_assert(std::is_empty_v<__box>); +static_assert(std::is_nothrow_copy_constructible_v<__box>); +static_assert(std::is_nothrow_move_constructible_v<__box>); +static_assert(std::is_nothrow_constructible_v<__box, std::in_place_t>); +static_assert(requires (__box a) { + a = a; + a = std::move(a); + a.operator*(); + a.operator->(); + a.has_value(); +}); + +struct S +{ + S(); + ~S(); + S(const S&); + S(S&&); + S& operator=(const S&); + S& operator=(S&&); +}; +static_assert(std::is_empty_v<__box>); +static_assert(!std::is_nothrow_copy_constructible_v<__box> + && std::is_copy_constructible_v<__box>); +static_assert(!std::is_nothrow_move_constructible_v<__box> + && std::is_move_constructible_v<__box>); +static_assert(!std::is_nothrow_constructible_v<__box, std::in_place_t> + && std::is_constructible_v<__box, std::in_place_t>); +static_assert(requires (__box a) { + a = a; + a = std::move(a); + a.operator*(); + a.operator->(); + a.has_value(); +}); + +using U = decltype([i=0] { return 0; }); +static_assert(!std::is_empty_v<__box>); +static_assert(std::is_nothrow_copy_constructible_v<__box>); +static_assert(std::is_nothrow_move_constructible_v<__box>); +static_assert(!std::is_nothrow_constructible_v<__box, std::in_place_t>); +static_assert(requires (__box a) { + a = a; + a = std::move(a); + a.operator*(); + a.operator->(); + a.has_value(); +}); -- 2.30.2