Re: [PATCH] PPC64 IEEE128 bit FP support

[David Edelsohn <dje.gcc@gmail.com>]

I don't believe that musl currently supports IBM double-double long
double format, only IEEE 754 64 bit double precision as long double.

IBM is working with other toolchains to support both the original
double-double and the new 128 bit quad precision through compiler
options, compiler macros, header definitions, and symbol compatibility
in libraries.  musl still would not support IBM double-double format.

I don't know if it would be necessary for musl libc long double to be
selectable for backward compatibility or one can implement a "flag
day" where a musl libc release switches to only 128 bit quad precision
long double.

Thanks, David
On Fri, Nov 2, 2018 at 12:13 PM Rich Felker <dalias@libc.org> wrote:
>
> On Fri, Oct 26, 2018 at 03:01:20PM +0200, Szabolcs Nagy wrote:
> > * Markus Wichmann <nullplan@gmx.net> [2018-10-26 06:28:29 +0200]:
> > > Now you just need to look through all the maths code to find all the
> > > places that need changing. __floatscan() comes to mind immediately. And
> > > I don't know if any of the libm functions needs adjustment for this new
> > > format.
> >
> > generic c code in musl should work for all supported
> > floating-point formats, which includes ieee binary128
> > for long double.
> >
> > only float.h needs to be set up according to the abi.
> >
> > some long double math functions don't have high quality
> > implementations for ieee binary128 format though.
>
> Yes. Assuming there aren't other problems revealed by my questions
> about argument passing and ISA levels, I think the only blocking issue
> here is naming the ABI. I forgot to mention but that should also
> involve a gcc patch that we can put in mcm and eventually upstream.
>
> QoI issues for IEEE-quad-based [sub]archs can be improved later;
> aarch64 and s390x are already affected IIRC.
>
> Rich