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From: Antony Polukhin <antoshkka@gmail.com>
Date: Tue, 17 Aug 2021 16:40:37 +0300
Message-ID: <CAKqmYPaK+DM+CUFCboWyV018LYN9jwzTsvEt1-7BVaw_hUS6sQ@mail.gmail.com>
Subject: Re: [PATCH] Optimize seed_seq construction
To: Jonathan Wakely <jwakely@redhat.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>,
 gcc-patches List <gcc-patches@gcc.gnu.org>
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=D0=B2=D1=82, 17 =D0=B0=D0=B2=D0=B3. 2021 =D0=B3. =D0=B2 16:37, Jonathan Wa=
kely <jwakely@redhat.com>:
<...>
> Thanks, this is a nice improvement. We can avoid tag dispatching to
> make it simpler though:
>
> @@ -3248,6 +3249,9 @@ namespace __detail
>   template<typename _InputIterator>
>     seed_seq::seed_seq(_InputIterator __begin, _InputIterator __end)
>     {
> +      if _GLIBCXX17_CONSTEXPR
> (__is_random_access_iter<_InputIterator>::value)
> +       _M_v.reserve(std::distance(__begin, __end));
> +
>       for (_InputIterator __iter =3D __begin; __iter !=3D __end; ++__iter=
)
>        _M_v.push_back(__detail::__mod<result_type,
>                       __detail::_Shift<result_type, 32>::__value>(*__iter=
));
>
> The call to std::distance is well-formed for input iterators, but we
> won't actually call it unless we have random access iterators.
>
> Unless you see a problem with this that I'm missing, I'll go with that ve=
rsion.

Looks much better. Thanks!

--=20
Best regards,
Antony Polukhin