Re: [PATCH] PPC64 IEEE128 bit FP support

[Rich Felker <dalias@libc.org>]

On Fri, Nov 02, 2018 at 02:00:36PM -0400, David Edelsohn wrote:
> IEEE binary64 still will be available for long double.
> 
> Current musl libc has not been compatible with the ABI and with
> current GLIBC because it did not implement IBM double-double format.
> 
> Musl libc can call it an ABI break, but musl libc has not been compatible.

We do not implement the same ABI as glibc on all systems. On a very
few, it's mostly or entirely the same, but at least on PowerPC and
MIPS there are significant differences.

> And the new configuration in GCC, LLVM and GLIBC allow both IBM
> double-double and IEEE 754 binary128 to co-exist.  Changing the musl
> libc dynamic linker name doesn't fix that and actually makes musl libc
> even less compatible with the rest of the PPC64LE Linux Ecosystem.

I don't know what you mean by "allow double-double and IEEE [quad] to
co-exist". Object files/shared libraries built for one obviously
cannot be linked with the other. The way incompatible archs/ABIs
co-exist on musl is always via different ldso names and library paths;
this is regardless of arch (and even across different archs). If
there's some other way of doing it on glibc/powerpc, like having a
shared dynamic linker that selects the right-ABI libraries to load,
that's fine for glibc but unrelated to musl.

Rich


> On Fri, Nov 2, 2018 at 1:55 PM Szabolcs Nagy <nsz@port70.net> wrote:
> >
> > * David Edelsohn <dje.gcc@gmail.com> [2018-11-02 13:32:55 -0400]:
> > > 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.
> >
> > since currently long double is ieee-binary64 in musl,
> > if we want it to be ieee-binary128 then that's a new
> > abi and in musl new abi means a new dynamic linker name:
> > current toolchain uses /lib/ld-musl-powerpc64le.so.1, a new
> > toolchain would use e.g. /lib/ld-musl-powerpc64le_f128.so.1
> >
> > i think even if the 64bit long double is deprecated and not
> > selectable in gcc anymore, we would use a new dynamic
> > linker name (but i think in musl we can easily keep supporting
> > two long double variants, the difficulty is in the gcc config
> > machinery and ux issues if there are no abi markers for
> > relocatable objects to detect abi mismatch at link time).
> >
> > >
> > > 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