Re: [PATCH 2/2] libstdc++: use new built-in trait __add_const

[Ken Matsui <kmatsui@cs.washington.edu>]

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Thank you for your information. Although it matches my intuition, I sent
this patch because I was unsure my intuition was correct. As Jonathan
pointed out, there appear to be several implementation errors. The
benchmark result for this trait is kind of trivial, so I will implement the
other traits I want to implement and then come back here.

Thank you all for your help.

On Tue, Mar 21, 2023 at 4:25 AM Jonathan Wakely <jwakely@redhat.com> wrote:

>
>
> On Tue, 21 Mar 2023 at 11:21, Marc Glisse via Libstdc++ <
> libstdc++@gcc.gnu.org> wrote:
>
>> On Tue, 21 Mar 2023, Ken Matsui via Libstdc++ wrote:
>>
>> >   /// add_const
>> > +#if __has_builtin(__add_const)
>> > +  template<typename _Tp>
>> > +    struct add_const
>> > +    { using type = __add_const(_Tp); };
>> > +#else
>> >   template<typename _Tp>
>> >     struct add_const
>> >     { using type = _Tp const; };
>> > +#endif
>>
>> Is that really better? You asked elsewhere if you should measure for each
>> patch, and I think that at least for such a trivial case, you need to
>> demonstrate that there is a point. The drawbacks are obvious: more code
>> in
>> libstdc++, non-standard, and more builtins in the compiler.
>>
>
> Right, this one isn't even getting rid of any partial specializations, but
> it is giving the preprocessor more work to do.
>
> Adding the extra built-ins to the compiler makes the compiler (very
> slightly) bigger and slower, so a real benchmark would require comparing an
> unpatched gcc (without the new built-in) to a patched gcc and patched
> libstdc++ sources.
>
>
>
>>
>> Using builtins makes more sense for complicated traits where you can save
>> several instantiations. Now that you have done a couple simple cases to
>> see how it works, I think you should concentrate on the more complicated
>> cases.
>>
>> --
>> Marc Glisse
>>
>>