On Tue, 23 Aug 2022 at 14:44, Patrick Palka <ppa...@redhat.com> wrote: > > On Tue, 23 Aug 2022, Jonathan Wakely wrote: > > > On Tue, 23 Aug 2022 at 02:35, Patrick Palka via Libstdc++ > > <libstd...@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.
