From: Marek Polacek <polacek@redhat.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org,
Jason Merrill <jason@redhat.com>
Subject: Re: [PATCH] c++: Define built-in for std::tuple_element [PR100157]
Date: Thu, 7 Jul 2022 13:25:24 -0400 [thread overview]
Message-ID: <YscXBCXH7COO2vFi@redhat.com> (raw)
In-Reply-To: <20220707171436.1419387-1-jwakely@redhat.com>
On Thu, Jul 07, 2022 at 06:14:36PM +0100, Jonathan Wakely wrote:
> This adds a new built-in to replace the recursive class template
> instantiations done by traits such as std::tuple_element and
> std::variant_alternative. The purpose is to select the Nth type from a
> list of types, e.g. __builtin_type_pack_element(1, char, int, float) is
> int.
>
> For a pathological example tuple_element_t<1000, tuple<2000 types...>>
> the compilation time is reduced by more than 90% and the memory used by
> the compiler is reduced by 97%. In realistic examples the gains will be
> much smaller, but still relevant.
>
> Clang has a similar built-in, __type_pack_element<N, T...>, but that's a
> "magic template" built-in using <> syntax, which GCC doesn't support. So
> this provides an equivalent feature, but as a built-in function using
> parens instead of <>. I don't really like the name "type pack element"
> (it gives you an element from a pack of types) but the semi-consistency
> with Clang seems like a reasonable argument in favour of keeping the
> name. I'd be open to alternative names though, e.g. __builtin_nth_type
> or __builtin_type_at_index.
>
>
> The patch has some problems though ...
>
> FIXME 1: Marek pointed out that this this ICEs:
> template<class... T> using type = __builtin_type_pack_element(sizeof(T), T...);
> type<int, char> c;
>
> The sizeof(T) expression is invalid, because T is an unexpanded pack,
> but it's not rejected and instead crashes:
I think this could be fixed by
if (check_for_bare_parameter_packs (n))
return error_mark_node;
in finish_type_pack_element.
(I haven't looked at the rest of the patch yet.)
Marek
next prev parent reply other threads:[~2022-07-07 17:25 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-07 17:14 Jonathan Wakely
2022-07-07 17:25 ` Marek Polacek [this message]
2022-07-07 19:28 ` Jason Merrill
2022-07-07 20:46 ` Jonathan Wakely
2022-10-05 13:43 ` Patrick Palka
2023-01-09 15:53 ` Patrick Palka
2023-01-09 19:25 ` Patrick Palka
2023-01-10 10:17 ` Jonathan Wakely
2023-01-17 18:04 ` Jason Merrill
2023-01-25 20:35 ` Patrick Palka
2023-01-26 17:47 ` Jason Merrill
2023-04-11 14:21 ` Patrick Palka
2023-04-18 19:09 ` Jason Merrill
2023-04-18 19:23 ` Patrick Palka
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