libgfortran.so SONAME and powerpc64le-linux ABI changes

[David Edelsohn]

On Wed, Oct 6, 2021 at 12:21 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Wed, Oct 06, 2021 at 11:42:17AM -0400, David Edelsohn wrote:
> > On Wed, Oct 6, 2021 at 11:19 AM Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > >
> > > On Wed, Oct 06, 2021 at 08:59:53AM +0200, Thomas Koenig wrote:
> > > > On 05.10.21 23:54, Segher Boessenkool wrote:
> > > > >>There is also the issue of binary data.  If some user has written
> > > > >>out data in double double and wants to read it in as IEEE quad,
> > > > >>the results are going to be garbage.  Another option for CONVERT
> > > > >>might be the solution to that, or, as you wrote, having a
> > > > >>REAL(KIND=15).  It should be inaccessible via SELECTED_REAL_KIND,
> > > > >>though.
> > > > >
> > > > >That means flipping the default on all PowerPC to no longer be double-
> > > > >double.  This means that you should have IEEE QP work everywhere, or the
> > > > >people who do need more than double precision will have no recourse.
> > > >
> > > > I think we can exclude big-endian POWER from this - they do not have
> > > > IEEE QP support, correct?  So, exclude that from the SONAME change.
> > >
> > > Not correct, no.  IEEE QP works fine in either endianness.
> >
> > This needs to be described with more granularity.  IEEE QP
> > instructions work with either endianness.
> >
> > IEEE QP is enabled and supported for PPC64 LE Linux on Power.  The
> > transition is under discussion.
>
> IEEE QP insns are enabled for BE as well:
>
> powerpc64-linux-gcc -Wall -W -O2 -S qp.c -mcpu=power9
>
> === qp.c ===
> #define QP _Float128
> QP f(QP x) { return x*x; }
> ===
>
> results in
>
> .L.f:
>         xsmulqp 2,2,2
>         blr
>
> > PPC64 BE Linux on Power does not define IEEE QP.  The ABI could be
> > updated and IEEE QP could be enabled, but PPC64 BE is not planning
> > future releases from Linux distros.
>
> This is a different thing: on BE (and on LE by default as well) we use
> double-double for long double.
>
> > IEEE QP for PPC64 FreeBSD on Power is an open question for the FreeBSD
> > community.
>
> Yes.
>
> > AIX on Power will continue to use double-double long double format.
> > GCC, LLVM, IBM Open XL and IBM XL compilers will continue to implement
> > and support the double-double format on AIX.
>
> Yes.  But this wasn't about what to use for long double -- it was about
> Fortran even :-)
>
> The actual IEEE QP float types work fine on BE.  I suspect they do on
> AIX as well for that matter?
>
> Fwiw, with -mcpu=power8 we get
>
> .L.f:
>         mflr 0
>         xxlor 35,34,34
>         std 0,16(1)
>         stdu 1,-112(1)
>         bl __mulkf3
>         nop
>         addi 1,1,112
>         ld 0,16(1)
>         mtlr 0
>         blr

GCC can generate the IEEE QP instructions on AIX.  The AIX Assembler
is aware of the instructions.  libquadmath is neither built nor
installed on AIX.  The AIX C library and AIX Math library are not
aware of the IEEE QP type nor how to work with it, e.g., printf.

Thanks, David