faster expf128

[Joseph Myers]

On Mon, 22 Jun 2020, Paul E Murphy via Libc-alpha wrote:

> On 6/22/20 6:02 AM, Paul Zimmermann wrote:
> > I have written some expf128 for x86_64 that is more than 10 times faster
> > than
> > the current glibc/libquadmath code [1] (see slide 21 of [2]).
> 
> I would highly recommend running the benchmarks against ppc64le or s390x
> before replacing the existing implementation.  I think it would improve the

Specificall, ppc64le *on POWER9*, as POWER9 and s390x are the two 
supported configurations with hardware support for binary128, and where 
it's thus plausible that an implementation based on floating-point 
operations is faster than one using only integer operations.

As documented in the manual, glibc supports _Float128 for powerpc64le, 
x86_64, x86, ia64, aarch64, alpha, mips64, riscv, s390 and sparc (most of 
those are architectures where long double has binary128 format; for 
powerpc64le the long double format depends on compiler options and 
configuration, and for x86_64, x86 and ia64 _Float128 is always a 
different format from long double).

-- 
Joseph S. Myers
joseph@codesourcery.com