Re: [GSoC] Question about Relatively Simple Library Traits

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Fri, 24 Mar 2023 at 07:10, Ken Matsui via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi,
>
> I am working on the GSoC project, "C++: Implement compiler built-in
> traits for the standard library traits". I found the following library
> traits that I am not sure if implementing built-in traits brings
> reasonable speed up.
>
> * std::is_fundamental
> * std::is_arithmetic
> * std::is_scalar
> * std::is_object
> * std::is_compound
> * std::is_scoped_enum
>
> For example, std::is_object has no template specializations, but its
> inheriting class looks complicated.
>
> __not_<__or_<is_function<_Tp>, is_reference<_Tp>, is_void<_Tp>>>::type
>
> If we define the built-in trait for this trait, we have: (as
> equivalence of the above code)
>
> __bool_constant<__is_object(_Tp)>
>
> And __is_object built-in trait should be like:
>
> !(type1 == FUNCTION_TYPE || type1 == ...)
>
> In this case, could someone tell me which one would be faster? Or, is
> there no other way to know which but to benchmark?

You should benchmark it anyway, I was always expecting that to be a
part of this GSoC project :-)

But is_object is NOT a "relatively simple" trait. What you show above
is very complex. One of the more complex traits we have. Partial
specializations are quite fast to match (in general) so eliminating
partial speclializations (e.g. like we have for is_const) should not
be the goal.

For is_object<const int> we instantiate:

is_function<const int>
is_const<const int>
is_reference<const int>
is_void<const int>
__bool_constant<false>
__or_<false_type, false_type, false_type>
__not_<false_type>
__bool_constant<true>

This is a ton of work! Instantiating class templates is the slowest
part of trait evaluation, not matching partial specializations.

A built-in for __is_object will mean we only instantiate
__bool_constant<true>. That will be MUCH faster. But I think it would
be a good idea for you to benchmark it to confirm that.