Re: [Qemu-devel] [PATCH] target-ppc: fix nego and subf*o instructions

[Peter Maydell]

On 1 April 2013 00:19, Richard Henderson <address@hidden> wrote:
> On 2013-03-30 16:54, Aurelien Jarno wrote:
>>
>> The overflow computation of nego and subf*o instructions has been broken
>> in commit ffe30937. This patch fixes it.
>>
>> With this change the PPC emulation passes the Gwenole Beauchesne
>> testsuite again.
>>
>> Cc: Alexander Graf <address@hidden>
>> Cc: Richard Henderson <address@hidden>
>> Signed-off-by: Aurelien Jarno <address@hidden>
>> ---
>>   target-ppc/translate.c |    2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
>> index 5e741d1..062493a 100644
>> --- a/target-ppc/translate.c
>> +++ b/target-ppc/translate.c
>> @@ -749,7 +749,7 @@ static inline void
>> gen_op_arith_compute_ov(DisasContext *ctx, TCGv arg0,
>>       tcg_gen_xor_tl(cpu_ov, arg0, arg1);
>>       tcg_gen_xor_tl(t0, arg1, arg2);
>>       if (sub) {
>> -        tcg_gen_and_tl(cpu_ov, cpu_ov, t0);
>> +        tcg_gen_andc_tl(cpu_ov, t0, cpu_ov);
>>       } else {
>>           tcg_gen_andc_tl(cpu_ov, cpu_ov, t0);
>>       }
>
>
> I'm a bit confused.  This is the exact same algorithm that's used on ARM and
> i386.  And as far as I can determine, all three platforms have the same
> definition of "overflow".

I think it's not quite the same as ARM because the two arguments
to subtract are reversed for PPC (ie PPC is 'subtract from', not
'subtract'). I think that means you want 'xor_tl(cpu_ov, arg0, arg2)'
for your first xor, maybe? (untested)

-- PMM