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

[Segher Boessenkool]

On Wed, Jul 31, 2019 at 08:43:50PM +0200, Marc Glisse wrote:
> On Wed, 31 Jul 2019, Jonathan Wakely wrote:
> 
> >So something like the attached patch.  The glibc <math.h> says the
> >values I used have enough digits for IEEE quad-precision:
> >
> >/* The above constants are not adequate for computation using `long 
> >double's.
> > Therefore we provide as an extension constants with similar names as a
> > GNU extension.  Provide enough digits for the 128-bit IEEE quad.  */
> >
> >I don't know if we need more accuracy for IBM double double.
> 
> double double has less precision than quad so it should be fine.

The *precision* (as defined in IEEE 754) of double double is much higher
than that of IEEE quad precision float: "the maximum number of
significant digits that can be represented in a format" -- it's just
that most of those bits have to be zero!

Neither double-double is a subset of QP, nor the other way around.  This
is *the* problem we have in rs6000 with these two float formats: GCC
cannot handle if two FP formats are like that.

(The math constants here are fine for double double, of course).


Segher