[PATCH] libstdc++: Specialize ranges::__detail::__box for semiregular types

[Patrick Palka]

The class template semiregular-box<T> 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<T> 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<T>, 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 currently use the
wrapper.

Tested on x86_64-pc-linux-gnu, does this look OK to commit?

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): Mark 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..8a302a7918f 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -86,7 +86,10 @@ namespace ranges
 
   namespace __detail
   {
-    template<copy_constructible _Tp> requires is_object_v<_Tp>
+    template<typename _Tp>
+      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 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
+	__box(const _Tp& __t)
+	noexcept(is_nothrow_copy_constructible_v<_Tp>)
+	: _M_value{__t}
+	{ }
+
+	constexpr
+	__box(_Tp&& __t)
+	noexcept(is_nothrow_move_constructible_v<_Tp>)
+	: _M_value{std::move(__t)}
+	{ }
+
+	template<typename... _Args>
+	  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..41de3189fec
--- /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
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <ranges>
+
+using std::ranges::__detail::__box;
+
+using T = decltype([] { return 0; });
+static_assert(std::is_empty_v<__box<T>>);
+static_assert(std::is_nothrow_copy_constructible_v<__box<T>>);
+static_assert(std::is_nothrow_move_constructible_v<__box<T>>);
+static_assert(std::is_nothrow_constructible_v<__box<T>, std::in_place_t>);
+static_assert(requires (__box<T> 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<S>>);
+static_assert(!std::is_nothrow_copy_constructible_v<__box<S>>
+	      && std::is_copy_constructible_v<__box<S>>);
+static_assert(!std::is_nothrow_move_constructible_v<__box<S>>
+	      && std::is_move_constructible_v<__box<S>>);
+static_assert(!std::is_nothrow_constructible_v<__box<S>, std::in_place_t>
+	      && std::is_constructible_v<__box<S>, std::in_place_t>);
+static_assert(requires (__box<S> a) {
+  a = a;
+  a = std::move(a);
+  a.operator*();
+  a.operator->();
+  a.has_value();
+});
+
+using U = decltype([a=0] { return 0; });
+static_assert(!std::is_empty_v<__box<U>>);
+static_assert(std::is_nothrow_copy_constructible_v<__box<U>>);
+static_assert(std::is_nothrow_move_constructible_v<__box<U>>);
+static_assert(!std::is_nothrow_constructible_v<__box<U>, std::in_place_t>);
+static_assert(requires (__box<U> a) {
+  a = a;
+  a = std::move(a);
+  a.operator*();
+  a.operator->();
+  a.has_value();
+});
-- 
2.28.0.585.ge1cfff6765