On Fri, 4 Apr 2025, Patrick Palka wrote:
> On Fri, 4 Apr 2025, Tomasz Kaminski wrote:
>
> >
> >
> > On Fri, Apr 4, 2025 at 6:07 PM Tomasz Kaminski <[email protected]> wrote:
> >
> >
> > On Fri, Apr 4, 2025 at 5:50 PM Patrick Palka <[email protected]> wrote:
> > flat_set::emplace (and flat_multiset) currently unconditionally
> > constructs an object outside of the container, but if we're passed
> > a value_type object we can and should avoid that.
> >
> > PR libstdc++/119620
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/std/flat_set (_Flat_set_impl::_M_try_emplace): Split
> > out into two overloads, one taking at least one argument and
> > one
> > taking zero arguments. Turn __k into an auto&& reference
> > bound
> > to __arg if it's already a value_type otherwise bound to a
> > lifetime-extended value_type temporary.
> > * testsuite/23_containers/flat_multiset/1.cc (test08): New
> > test.
> > * testsuite/23_containers/flat_set/1.cc (test08): New test.
> > ---
> > libstdc++-v3/include/std/flat_set | 20 +++++++++++++----
> > .../23_containers/flat_multiset/1.cc | 22
> > +++++++++++++++++++
> > .../testsuite/23_containers/flat_set/1.cc | 20 +++++++++++++++++
> > 3 files changed, 58 insertions(+), 4 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/std/flat_set
> > b/libstdc++-v3/include/std/flat_set
> > index a7b0b8aef15..3e15d1af416 100644
> > --- a/libstdc++-v3/include/std/flat_set
> > +++ b/libstdc++-v3/include/std/flat_set
> > @@ -350,12 +350,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > { return _M_cont.max_size(); }
> >
> > // modifiers
> > - template<typename... _Args>
> > + template<typename _Arg, typename... _Args>
> > pair<iterator, bool>
> > - _M_try_emplace(optional<const_iterator> __hint, _Args&&...
> > __args)
> > + _M_try_emplace(optional<const_iterator> __hint, _Arg&& __arg,
> > _Args&&... __args)
> >
> > map.try_emplace(key) is valid construct, and commonly used to value
> > construct value type. For example operator[] is defined in terms of it.
> > {
> > // TODO: Simplify and audit the hint handling.
> > - value_type __k(std::forward<_Args>(__args)...);
> > + auto&& __k = [&] -> decltype(auto) {
> > + if constexpr (sizeof...(_Args) == 0
> > + && same_as<remove_cvref_t<_Arg>,
> > value_type>)
> > + return std::forward<_Arg>(__arg);
> >
> > I think you can do sizeof..(_Args) == 1 and return
> > (std::forward<_Args>(__args), ...);
I tried something like that, but I think it'd mean we'd have to
then change the same_as check to
same_as<remove_cvref_t<_Args...>, value_type>
or
same_as<remove_cvref_t<_Args>..., value_type>
both of which are ill-formed because we can't pass a pack expansion
to a non-variadic template parameter of an alias template or concept.
> > + else
> > + return value_type(std::forward<_Arg>(__arg),
> > + std::forward<_Args>(__args)...);
> > + }();
> > typename container_type::iterator __it;
> > int __r = -1, __s = -1;
> > if (__hint.has_value()
> > @@ -397,11 +404,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > return {__it, false};
> >
> > auto __guard = _M_make_clear_guard();
> > - __it = _M_cont.insert(__it, std::move(__k));
> > + __it = _M_cont.insert(__it,
> > std::forward<decltype(__k)>(__k));
> > __guard._M_disable();
> > return {__it, true};
> > }
> >
> > + template<typename... _Args>
> > + pair<iterator, bool>
> > + _M_try_emplace(optional<const_iterator> __hint)
> > + { return _M_try_emplace(__hint, value_type()); }
> >
> > Ah, I haven't noticed this overload that restores _M_try_empalce. But this
> > causes additional move, so I think I prefer my suggestion.
>
> I'd expect there would be just one move in that case, when __k is moved
> from in the _M_cont.insert call? There should be no move when we e.g.
> bind __k to the default-constructed value_type.
>
> > +
> > template<typename... _Args>
> > requires is_constructible_v<value_type, _Args...>
> > __emplace_result_t
> > diff --git
> > a/libstdc++-v3/testsuite/23_containers/flat_multiset/1.cc
> > b/libstdc++-v3/testsuite/23_containers/flat_multiset/1.cc
> > index dc3cecd9720..7d9a33c6b00 100644
> > --- a/libstdc++-v3/testsuite/23_containers/flat_multiset/1.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/flat_multiset/1.cc
> > @@ -214,6 +214,27 @@ void test07()
> > #endif
> > }
> >
> > +void
> > +test08()
> > +{
> > + // PR libstdc++/119620 -- flat_set::emplace always
> > constructs element on the stack
> > + static int copy_counter;
> > + struct A {
> > + A() { }
> > + A(const A&) { ++copy_counter; }
> > + A& operator=(const A&) { ++copy_counter; return *this; }
> > + auto operator<=>(const A&) const = default;
> > + };
> > + std::vector<A> v;
> > + v.reserve(2);
> > + std::flat_multiset<A> s(std::move(v));
> > + A a;
> > + s.emplace(a);
> > + VERIFY( copy_counter == 1 );
> > + s.emplace(a);
> > + VERIFY( copy_counter == 2 );
> > +}
> > +
> > int
> > main()
> > {
> > @@ -225,4 +246,5 @@ main()
> > test05();
> > test06();
> > test07();
> > + test08();
> > }
> > diff --git a/libstdc++-v3/testsuite/23_containers/flat_set/1.cc
> > b/libstdc++-v3/testsuite/23_containers/flat_set/1.cc
> > index 90f5855859f..ed24fab785b 100644
> > --- a/libstdc++-v3/testsuite/23_containers/flat_set/1.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/flat_set/1.cc
> > @@ -229,6 +229,25 @@ void test07()
> > #endif
> > }
> >
> > +void
> > +test08()
> > +{
> > + // PR libstdc++/119620 -- flat_set::emplace always
> > constructs element on the stack
> > + static int copy_counter;
> > + struct A {
> > + A() { }
> > + A(const A&) { ++copy_counter; }
> > + A& operator=(const A&) { ++copy_counter; return *this; }
> > + auto operator<=>(const A&) const = default;
> > + };
> > + std::flat_set<A> s;
> > + A a;
> > + s.emplace(a);
> > + VERIFY( copy_counter == 1 );
> > + s.emplace(a);
> > + VERIFY( copy_counter == 1 );
> > +}
> > +
> > int
> > main()
> > {
> > @@ -240,4 +259,5 @@ main()
> > test05();
> > test06();
> > test07();
> > + test08();
> > }
> > --
> > 2.49.0.111.g5b97a56fa0
> >
> >
> >
