On Wed, Oct 06, 2021 at 11:42:17AM -0400, David Edelsohn wrote:
> On Wed, Oct 6, 2021 at 11:19 AM Segher Boessenkool
> <seg...@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
Segher
