Re: [Qemu-devel] [PATCH v2 1/2] target/mips: Make the results of DIV_<U|S>.<B|H|W|D> the same as on hardware

[Mateja Marjanovic]

On 2.4.19. 17:08, Philippe Mathieu-Daudé wrote:
> Hi Mateja,
>
> On 4/2/19 2:11 PM, Mateja Marjanovic wrote:
> > From: Mateja Marjanovic <address@hidden>
> >
> > MSA instructions DIV_<U|S>.<B|H|W|D> when dividing by zero,
> > didn't return the same value when executed on a referent hardware
> > (FPGA MIPS 64 r6, little endian) and when executed on QEMU, which
> > is not a real bug, because the result when dividing by zero is
> > UNPREDICTABLE [1] (page 141, 142).
> I'm surprised by the arch, I'd have expected a MSA floating point
> exception instead of UNPREDICTABLE.

Our architecture documents do not list any exceptions related to
division by zero, for instructions DIV_<U|S>.<B|H|W|D> and
MOD_<U|S>.<B|H|W|D>.

The only exceptions that are mentioned are:
Reserved Instruction Exception, MSA Disabled Exception. [1] (page 141, 
142, 254, 255).

> So here we decide to follow the FPGA model behavior rather than the
> architecture...
> If the community agree, my only request is to add a comment in the code
> that this is the "FPGA MIPS 64 r6" behavior (else while looking at this
> code later I'd be tempted to revert to a 0 return value instead of -1/1).
>
> > [1] MIPS Architecture for Programmers
> >      Volume IV-j: The MIPS64 SIMD
> >      Architecture Module, Revision 1.12
> >
> > Signed-off-by: Mateja Marjanovic <address@hidden>
> > ---
> >   target/mips/msa_helper.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/target/mips/msa_helper.c b/target/mips/msa_helper.c
> > index 655148d..8c77f12 100644
> > --- a/target/mips/msa_helper.c
> > +++ b/target/mips/msa_helper.c
> > @@ -641,14 +641,15 @@ static inline int64_t msa_div_s_df(uint32_t df, int64_t 
> > arg1, int64_t arg2)
> >       if (arg1 == DF_MIN_INT(df) && arg2 == -1) {
> >           return DF_MIN_INT(df);
> >       }
> > -    return arg2 ? arg1 / arg2 : 0;
> > +    return arg2 ? arg1 / arg2
> > +                : arg1 >= 0 ? -1 : 1;
> >   }
> >   
> >   static inline int64_t msa_div_u_df(uint32_t df, int64_t arg1, int64_t arg2)
> >   {
> >       uint64_t u_arg1 = UNSIGNED(arg1, df);
> >       uint64_t u_arg2 = UNSIGNED(arg2, df);
> > -    return u_arg2 ? u_arg1 / u_arg2 : 0;
> > +    return arg2 ? u_arg1 / u_arg2 : -1;
> >   }
> >   
> >   static inline int64_t msa_mod_s_df(uint32_t df, int64_t arg1, int64_t arg2)
Thanks,
Mateja