Re: [PATCH 1/3] libstdc++: Separate construct/convertibility tests for std::tuple

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 23 Aug 2022 at 14:44, Patrick Palka <ppalka@redhat.com> wrote:
>
> On Tue, 23 Aug 2022, Jonathan Wakely wrote:
>
> > On Tue, 23 Aug 2022 at 02:35, Patrick Palka via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > P2321R2 adds new conditionally explicit constructors to std::tuple which
> > > we'll concisely implement in a subsequent patch using explicit(bool), like
> > > in our C++20 std::pair implementation.  But before we can do that, this
> > > patch first adds members to _TupleConstraints that test for constructibility
> > > and convertibility separately; we'll use the first in the new constructors'
> > > constraints, and the second in their explicit specifier.
> > >
> > > In passing, this patch also redefines the existing predicates
> > > __is_ex/implicitly_constructible in terms of these new members.  This
> > > seems to reduce compile time and memory usage by about 10% for large
> >
> > Nice.
> >
> > > tuples when using the relevant constructors constrained by
> > > _Explicit/_ImplicitCtor (since we no longer have to redundantly expand
> > > and process is_constructible<_Types, _UTypes>... twice for each pair of
> > > such constructors).  In order to retain maximal short circuiting, do
> > > this only when constexpr if is available.
> >
> > Would we get similar benefits for C++11 and C++14 by doing:
> >
> >        return __and_<__and_<is_constructible<_Types, _UTypes>...>,
> >                      __and_<is_convertible<_UTypes, _Types>...>
> >                      >::value;
> >
> > This is slightly more work in total, but if we have __and_<A,B> and
> > __and_<A,__not_<B>> then the A and B instantiations will be cached and
> > can be reused.
>
> Good idea, it seems we get pretty much the same 10% improvement for
> C++11/14 with this approach.  I reckon we might as well then define
> __convertible and __constructible as alias templates instead of as
> variable templates and use them unconditionally in
> __is_im/explicitly_constructible for benefit of all language versions.

I had a similar thought after hitting send.


> Using constexpr if instead of the outer __and_ seems to yield a marginal
> improvement at best and __and_v is currently just __and_::value, so it
> doesn't seem worth it to have different definitions for C++17 at least
> for now.
>
> What do you think about the following?

OK for trunk - thanks.