Re: [PATCH] Implement "P0631R4 Math Constants" for C++20

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Jul 31, 2019 at 07:37:03PM +0100, Jonathan Wakely wrote:
> On 31/07/19 19:07 +0200, Marc Glisse wrote:
> > On Wed, 31 Jul 2019, Jonathan Wakely wrote:
> > 
> > > The values of the constants are taken from Glibc where the equivalent
> > > constant exists, or by rounding the actual constant to the same number
> > > of digits as the Glibc constants have.
> > 
> > How does it behave with __float128? I think that with -std=gnu++2a, it
> > counts as a floating point type, but the constant first becomes a long
> > double and thus loses precision.
> 
> Indeed it does:
> 
> #if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
>  template<>
>    struct __is_floating_point_helper<__float128>
>    : public true_type { };
> #endif
> 
> We could add explicit specializations for __float128 and use the Q
> suffix. That's not valid when __STRICT_ANSI__ is defined (unless
> -fext-numeric-literals is also used) but as the snippet above shows,
> __float128 isn't a floating-point type when __STRICT_ANSI__ is
> defined.

Perhaps add __extension__ before the literals too?

> I don't know if we need more accuracy for IBM double double.

I'd think the *L constants should be good enough.

	Jakub