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From: Patrick Palka <ppalka@redhat.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: Patrick Palka <ppalka@redhat.com>,
	gcc-patches@gcc.gnu.org,  libstdc++@gcc.gnu.org
Subject: Re: [PATCH 1/3] libstdc++: Separate construct/convertibility tests for std::tuple
Date: Tue, 23 Aug 2022 09:44:35 -0400 (EDT)	[thread overview]
Message-ID: <6f6c8fa3-26b0-cf5f-0926-7c2dab46b63a@idea> (raw)
In-Reply-To: <CACb0b4mrWhyBjT6j=Dk6ajCg1Au5o96pnd2fEvfArOojQT2Aog@mail.gmail.com>

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.
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


  reply	other threads:[~2022-08-23 13:44 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-23  1:34 Patrick Palka
2022-08-23  1:34 ` [PATCH 2/3] libstdc++: Implement std::pair/tuple/misc enhancements from P2321R2 Patrick Palka
2022-08-23 12:03   ` Jonathan Wakely
2022-08-23 15:14     ` Patrick Palka
2022-08-23  1:35 ` [PATCH 3/3] libstdc++: Implement ranges::zip_view " Patrick Palka
2022-08-24 12:15   ` Jonathan Wakely
2022-08-26 20:05   ` Jonathan Wakely
2022-08-31 10:12     ` Jonathan Wakely
2022-08-23  9:15 ` [PATCH 1/3] libstdc++: Separate construct/convertibility tests for std::tuple Jonathan Wakely
2022-08-23 13:44   ` Patrick Palka [this message]
2022-08-23 14:53     ` Jonathan Wakely

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