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.
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?
-- >8 --
libstdc++-v3/ChangeLog:
* include/std/tuple (_TupleConstraints::__convertible): Define.
(_TupleConstraints::__constructible): Likewise.
(_TupleConstraints::__is_explicitly_constructible): Redefine this
in terms of __convertible and __constructible.
(_TupleConstraints::__is_implicitly_constructible): Likewise.
---
libstdc++-v3/include/std/tuple | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 6d0060a191c..f8f48ccc370 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -553,15 +553,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<bool, typename... _Types>
struct _TupleConstraints
{
+ template<typename... _UTypes>
+ using __constructible = __and_<is_constructible<_Types, _UTypes>...>;
+
+ template<typename... _UTypes>
+ using __convertible = __and_<is_convertible<_UTypes, _Types>...>;
+
// Constraint for a non-explicit constructor.
// True iff each Ti in _Types... can be constructed from Ui in _UTypes...
// and every Ui is implicitly convertible to Ti.
template<typename... _UTypes>
static constexpr bool __is_implicitly_constructible()
{
- return __and_<is_constructible<_Types, _UTypes>...,
- is_convertible<_UTypes, _Types>...
- >::value;
+ return __and_<__constructible<_UTypes...>,
+ __convertible<_UTypes...>>::value;
}
// Constraint for a non-explicit constructor.
@@ -570,9 +575,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename... _UTypes>
static constexpr bool __is_explicitly_constructible()
{
- return __and_<is_constructible<_Types, _UTypes>...,
- __not_<__and_<is_convertible<_UTypes, _Types>...>>
- >::value;
+ return __and_<__constructible<_UTypes...>,
+ __not_<__convertible<_UTypes...>>>::value;
}
static constexpr bool __is_implicitly_default_constructible()
--
2.37.2.382.g795ea8776b
