Re: [PATCH 0/2] Floating-point OE/UE exception bug

[Lucas Mateus Martins Araujo e Castro]

On 05/08/2022 13:20, Alex Bennée wrote:

> "Lucas Mateus Castro(alqotel)" <lucas.araujo@eldorado.org.br> writes:
>
> > From: "Lucas Mateus Castro (alqotel)" <lucas.araujo@eldorado.org.br>
> >
> > Changes in v2:
> >     - Completely reworked the solution:
> >         * Created re_bias in FloatFmt, it is 3/4 of the total exponent
> >           range of a FP type
> >       
> I thought this might have an effect on the efficiency of the FloatFmt
> extraction/packing but I couldn't see any real difference in fpbench. I
> doubt the compiler can dead code it away if not used by a front-end.
>
> Anyway have a:
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>
> for the series.
>
> >         * Added rebias bools that dictates if the result should have
> >           its exponent add/subtract the re_bias value if an
> >           overflow/underflow occurs.
> >         * ppc_store_fpscr sets/unsets rebias if OE/UE is set/unset
> >
> > The PowerISA defines that if an overflow exception happen with FPSCR.OE
> > set, the exponent of the intermediate result is subtracted 1536 in
> > double precision operations and is added 1536 in an underflow exception,
> > currently this behavior is not QEMU's behavior, this patch series fixes
> > that.
> >
> > Currently there's no test in this patch series as there's no way to
> > disable MSR.FE0 and MSR.FE1 in linux user, so any overflow/underflow
> > exception with OE/UE set causes a trapping exception.
> >       
> Could you do it with a system mode test? Probably overkill for this
> though. I suspect tweaking testfloat would be tricky.
>

Matheus currently has a patch to enable prctl, it still needs to change some places that have hardcoded values for linux-user (https://github.com/PPC64/qemu/tree/alqotel-ferst-prctl-patch has Matheus' patch and a patch to remove the ifdef from fp_exceptions_enabled).

With that change it should be possible to create a normal test for this (I sent a basic one along with v1, Message-Id=20220803124324.23593-1-lucas.araujo@eldorado.org.br"><20220803124324.23593-1-lucas.araujo@eldorado.org.br>) built on top of that branch.

Thanks,

> > Lucas Mateus Castro (alqotel) (2):
> >   fpu: Add rebias bool, value and operation
> >   target/ppc: Bugfix FP when OE/UE are set
> >
> >  fpu/softfloat-parts.c.inc     | 21 +++++++++++++++++++--
> >  fpu/softfloat.c               |  2 ++
> >  include/fpu/softfloat-types.h |  4 ++++
> >  target/ppc/cpu.c              |  2 ++
> >  target/ppc/fpu_helper.c       |  2 --
> >  5 files changed, 27 insertions(+), 4 deletions(-)
> >       
>
> --
> Alex Bennée
>

--
Lucas Mateus M. Araujo e Castro
Instituto de Pesquisas ELDORADO
Departamento Computação Embarcada
Analista de Software Trainee
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