On Fri, 30 Apr 2021, Jonathan Wakely wrote:
> On 30/04/21 17:34 -0400, Patrick Palka via Libstdc++ wrote:
> > On Fri, 30 Apr 2021, Tim Song wrote:
> >
> > > On Fri, Apr 30, 2021 at 12:11 PM Patrick Palka via Libstdc++
> > > <libstd...@gcc.gnu.org> wrote:
> > > >
> > > > + template<typename _Iter>
> > > > + _Tp&
> > > > + _M_emplace_deref(const _Iter& __i)
> > > > + {
> > > > + this->reset();
> > > > + return this->emplace(*__i);
> > > > + }
> > >
> > > This incurs a move, avoiding of which is the sole reason for
> > > emplace-deref's existence.
> >
> > Ah thanks, I had missed that... IIUC, if we instead derive from
> > optional's internal base class _Optional_base, we can easily get at the
> > underlying storage for the object and directly initialize it with '*__i'
> > and avoid the move. How does the the following adjustment to the patch
> > look?
>
> Looks good to me, thanks.
Here's what I ended up committing, complete with a comment to
__non_propagating_cache::_M_emplace_deref and a test to verify we're
avoiding the extra move:
-- >8 --
Subject: [PATCH] libstdc++: Implement P2328 changes to join_view
This implements the wording changes of P2328R0 "join_view should join
all views of ranges".
libstdc++-v3/ChangeLog:
* include/std/ranges (__detail::__non_propating_cache): Define
as per P2328.
(join_view): Remove constraints on the value and reference types
of the wrapped iterator type as per P2328.
(join_view::_Iterator::_M_satisfy): Adjust as per P2328.
(join_view::_Iterator::operator++): Likewise.
(join_view::_M_inner): Use __non_propating_cache as per P2328.
Remove now-redundant use of __maybe_present_t.
* testsuite/std/ranges/adaptors/join.cc: Include <array>.
(test10): New test.
---
libstdc++-v3/include/std/ranges | 77 ++++++++++++++++---
.../testsuite/std/ranges/adaptors/join.cc | 24 ++++++
2 files changed, 90 insertions(+), 11 deletions(-)
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 4975d5c630b..4a7ca49b45d 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2241,10 +2241,68 @@ namespace views::__adaptor
inline constexpr _DropWhile drop_while;
} // namespace views
+ namespace __detail
+ {
+ template<typename _Tp>
+ struct __non_propagating_cache
+ {
+ // When _Tp is not an object type (e.g. is a reference type), we make
+ // __non_propagating_cache<_Tp> empty rather than ill-formed so that
+ // users can easily conditionally declare data members with this type
+ // (such as join_view::_M_inner).
+ };
+
+ template<typename _Tp>
+ requires is_object_v<_Tp>
+ struct __non_propagating_cache<_Tp> : private _Optional_base<_Tp>
+ {
+ __non_propagating_cache() = default;
+
+ constexpr
+ __non_propagating_cache(const __non_propagating_cache&) noexcept
+ { }
+
+ constexpr
+ __non_propagating_cache(__non_propagating_cache&& __other) noexcept
+ { __other._M_reset(); }
+
+ constexpr __non_propagating_cache&
+ operator=(const __non_propagating_cache& __other) noexcept
+ {
+ if (std::__addressof(__other) != this)
+ this->_M_reset();
+ return *this;
+ }
+
+ constexpr __non_propagating_cache&
+ operator=(__non_propagating_cache&& __other) noexcept
+ {
+ this->_M_reset();
+ __other._M_reset();
+ return *this;
+ }
+
+ constexpr _Tp&
+ operator*() noexcept
+ { return this->_M_get(); }
+
+ template<typename _Iter>
+ _Tp&
+ _M_emplace_deref(const _Iter& __i)
+ {
+ this->_M_reset();
+ // Using _Optional_base::_M_construct to initialize from '*__i'
+ // would incur an extra move due to the indirection, so we instead
+ // use placement new directly.
+ ::new ((void *) std::__addressof(this->_M_payload._M_payload))
_Tp(*__i);
+ this->_M_payload._M_engaged = true;
+ return this->_M_get();
+ }
+ };
+ }
+
template<input_range _Vp>
requires view<_Vp> && input_range<range_reference_t<_Vp>>
- && (is_reference_v<range_reference_t<_Vp>>
- || view<range_value_t<_Vp>>)
class join_view : public view_interface<join_view<_Vp>>
{
private:
@@ -2310,17 +2368,16 @@ namespace views::__adaptor
constexpr void
_M_satisfy()
{
- auto __update_inner = [this] (range_reference_t<_Base> __x) -> auto&
- {
+ auto __update_inner = [this] (const iterator_t<_Base>& __x) ->
auto&& {
if constexpr (_S_ref_is_glvalue)
- return __x;
+ return *__x;
else
- return (_M_parent->_M_inner = views::all(std::move(__x)));
+ return _M_parent->_M_inner._M_emplace_deref(__x);
};
for (; _M_outer != ranges::end(_M_parent->_M_base); ++_M_outer)
{
- auto& __inner = __update_inner(*_M_outer);
+ auto&& __inner = __update_inner(_M_outer);
_M_inner = ranges::begin(__inner);
if (_M_inner != ranges::end(__inner))
return;
@@ -2396,7 +2453,7 @@ namespace views::__adaptor
if constexpr (_S_ref_is_glvalue)
return *_M_outer;
else
- return _M_parent->_M_inner;
+ return *_M_parent->_M_inner;
}();
if (++_M_inner == ranges::end(__inner_range))
{
@@ -2507,10 +2564,8 @@ namespace views::__adaptor
friend _Sentinel<!_Const>;
};
- // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
[[no_unique_address]]
- __detail::__maybe_present_t<!is_reference_v<_InnerRange>,
- views::all_t<_InnerRange>> _M_inner;
+ __detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;
_Vp _M_base = _Vp();
public:
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
index e6c71d771de..d774e8d9385 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/join.cc
@@ -19,6 +19,7 @@
// { dg-do run { target c++2a } }
#include <algorithm>
+#include <array>
#include <ranges>
#include <string>
#include <string_view>
@@ -170,6 +171,28 @@ test10()
VERIFY( ranges::next(v.begin()) == v.end() );
}
+void
+test11()
+{
+ // Verify P2328 changes.
+ int r[] = {1, 2, 3};
+ auto v = r
+ | views::transform([] (int n) { return std::vector{{n, -n}}; })
+ | views::join;
+ VERIFY( ranges::equal(v, (int[]){1, -1, 2, -2, 3, -3}) );
+
+ struct S {
+ S() = default;
+ S(const S&) = delete;
+ S(S&&) = delete;
+ };
+ auto w = r
+ | views::transform([] (int) { return std::array<S, 2>{}; })
+ | views::join;
+ for (auto& i : w)
+ ;
+}
+
int
main()
{
@@ -183,4 +206,5 @@ main()
test08();
test09();
test10();
+ test11();
}
--
2.31.1.442.g7e39198978
