Re: [Qemu-devel] [RFC v1 05/13] target-ppc: add modulo word operations

[Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>]

David Gibson <david@gibson.dropbear.id.au> writes:

> [ Unknown signature status ]
> On Fri, Jul 22, 2016 at 12:24:55PM +0530, Nikunj A Dadhania wrote:
>> David Gibson <david@gibson.dropbear.id.au> writes:
>> 
>> > [ Unknown signature status ]
>> > On Fri, Jul 22, 2016 at 10:59:18AM +0530, Nikunj A Dadhania wrote:
>> >> David Gibson <david@gibson.dropbear.id.au> writes:
>> >> 
>> >> > [ Unknown signature status ]
>> >> > On Mon, Jul 18, 2016 at 10:35:09PM +0530, Nikunj A Dadhania wrote:
>> >> >> Adding following instructions:
>> >> >> 
>> >> >> moduw: Modulo Unsigned Word
>> >> >> modsw: Modulo Signed Word
>> >> >> 
>> >> >> Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
>> >> >
>> >> > As rth has already mentioned this many branches probably means this
>> >> > wants a helper.
>> >> >
>> >> >> ---
>> >> >>  target-ppc/translate.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>> >> >>  1 file changed, 48 insertions(+)
>> >> >> 
>> >> >> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
>> >> >> index d44f7af..487dd94 100644
>> >> >> --- a/target-ppc/translate.c
>> >> >> +++ b/target-ppc/translate.c
>> >> >> @@ -1178,6 +1178,52 @@ GEN_DIVE(divde, divde, 0);
>> >> >>  GEN_DIVE(divdeo, divde, 1);
>> >> >>  #endif
>> >> >>  
>> >> >> +static inline void gen_op_arith_modw(DisasContext *ctx, TCGv ret, TCGv arg1,
>> >> >> +                                     TCGv arg2, int sign)
>> >> >> +{
>> >> >> +    TCGLabel *l1 = gen_new_label();
>> >> >> +    TCGLabel *l2 = gen_new_label();
>> >> >> +    TCGv_i32 t0 = tcg_temp_local_new_i32();
>> >> >> +    TCGv_i32 t1 = tcg_temp_local_new_i32();
>> >> >> +    TCGv_i32 t2 = tcg_temp_local_new_i32();
>> >> >> +
>> >> >> +    tcg_gen_trunc_tl_i32(t0, arg1);
>> >> >> +    tcg_gen_trunc_tl_i32(t1, arg2);
>> >> >> +    tcg_gen_brcondi_i32(TCG_COND_EQ, t1, 0, l1);
>> >> 
>> >> Result for:
>> >> <anything> % 0 and ...
>> >> 
>> >> >> +    if (sign) {
>> >> >> +        TCGLabel *l3 = gen_new_label();
>> >> >> +        tcg_gen_brcondi_i32(TCG_COND_NE, t1, -1, l3);
>> >> >> +        tcg_gen_brcondi_i32(TCG_COND_EQ, t0, INT32_MIN, l1);
>> >> >> +        gen_set_label(l3);
>> >> >
>> >> > It's not really clear to be what the logic above is doing.
>> >> 
>> >> ... For signed case
>> >> 0x8000_0000 % -1
>> >> 
>> >> Is undefined, addressing those cases.
>> >
>> > Do you mean the tcg operations have undefined results or that the ppc
>> > instructions have undefined results?
>> 
>> TCG side, I haven't tried.
>> 
>> > If the latter, then why do you care about those cases?
>> 
>> Thats how divd is implemented as well, i didn't want to break that. I am
>> looking at doing both div and mod as helpers.
>> 
>> >> >> +        tcg_gen_rem_i32(t2, t0, t1);
>> >> >> +    } else {
>> >> >> +        tcg_gen_remu_i32(t2, t0, t1);
>> >> >> +    }
>> >> >> +    tcg_gen_br(l2);
>> >> >> +    gen_set_label(l1);
>> >> >> +    if (sign) {
>> >> >> +        tcg_gen_sari_i32(t2, t0, 31);
>> >> >
>> >> > AFAICT this sets t2 to either 0 or -1 depending on the sign of t0,
>> >> > which seems like an odd thing to do.
>> >> 
>> >> Extending the sign later ...
>> >
>> > Right, so after sign extension you have a 64-bit 0 or -1.  Still not
>> > seeing what that 0 or -1 result is useful for.
>> 
>> Oh ok, i got why you got confused. I am re-writing all of it though, but
>> for understanding:
>> 
>>   if (divisor == 0)
>>      goto l1;
>> 
>>   if (signed) {
>>      if (divisor == -1 && dividend == INT_MIN)
>>         goto l1;
>>      compute_signed_rem(t2, t0, t1);
>>   } else {
>>      compute_unsigned_rem(t2, t0, t1);  
>>   }
>>   goto l2; /* jump to setting extending result and return */
>> 
>> l1: /* in case of invalid input set values */
>>   if (signed)
>>      t2 = -1 or 0;
>>   else
>>      t2 = 0;
>
> Ok, so why do you ever need different result values in the case of
> invalid input?  Why is always returning 0 not good enough?

Let me go through the spec, as divd does the same thing.

Regards
Nikunj