Re: [Qemu-devel] [PATCH RFC v0 5/6] target-ppc: add vsrv instruction

[Nikunj A Dadhania]

David Gibson <address@hidden> writes:

> [ Unknown signature status ]
> On Wed, Jul 27, 2016 at 12:56:57AM +0530, Nikunj A Dadhania wrote:
>> From: Vivek Andrew Sha <address@hidden>
>> 
>> Adds Vector Shift Right Variable instruction.
>> 
>> Signed-off-by: Vivek Andrew Sha <address@hidden>
>> Signed-off-by: Nikunj A Dadhania <address@hidden>
>> ---
>>  target-ppc/helper.h     |  1 +
>>  target-ppc/int_helper.c | 17 +++++++++++++++++
>>  target-ppc/translate.c  |  2 ++
>>  3 files changed, 20 insertions(+)
>> 
>> diff --git a/target-ppc/helper.h b/target-ppc/helper.h
>> index 9703f85..8eada2f 100644
>> --- a/target-ppc/helper.h
>> +++ b/target-ppc/helper.h
>> @@ -211,6 +211,7 @@ DEF_HELPER_3(vslw, void, avr, avr, avr)
>>  DEF_HELPER_3(vsld, void, avr, avr, avr)
>>  DEF_HELPER_3(vslo, void, avr, avr, avr)
>>  DEF_HELPER_3(vsro, void, avr, avr, avr)
>> +DEF_HELPER_3(vsrv, void, avr, avr, avr)
>>  DEF_HELPER_3(vslv, void, avr, avr, avr)
>>  DEF_HELPER_3(vaddcuw, void, avr, avr, avr)
>>  DEF_HELPER_3(vsubcuw, void, avr, avr, avr)
>> diff --git a/target-ppc/int_helper.c b/target-ppc/int_helper.c
>> index 412398f..f4776f0 100644
>> --- a/target-ppc/int_helper.c
>> +++ b/target-ppc/int_helper.c
>> @@ -1722,6 +1722,23 @@ void helper_vslv(ppc_avr_t *r, ppc_avr_t *a, 
>> ppc_avr_t *b)
>>      }
>>  }
>>  
>> +void helper_vsrv(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b)
>> +{
>> +    int i;
>> +    unsigned int shift, bytes, src[ARRAY_SIZE(r->u8) + 1];
>> +
>> +    src[0] = 0;
>> +    for (i = 0; i < ARRAY_SIZE(r->u8); i++) {
>> +        src[i + 1] = a->u8[i];
>> +    }
>> +
>> +    for (i = 0; i < ARRAY_SIZE(r->u8); i++) {
>> +        shift = b->u8[i] & 0x7;               /* extract shift value */
>> +        bytes = (src[i] << 8) + src[i + 1];     /* extract adjacent bytes */
>
> I think you should be able to construct bytes on the fly without
> pre-generating teh whole of src, as you already did for vslv.

Had done that, but that introduces a bug like this, for eg:

    vslv vra,vra,vrb

So modified vra->u8[i] is used during subsequent operation as input.

Assuming I take care or special casing "0":

bytes = ((vra->u8[i - 1]  << 8) | (vra->u8[i])) 
vra->u8[i] = (bytes >> shift) & 0xFF;

when i = 1, bytes will ((vra->u8[0]  << 8) | (vra->u8[1])). But vra->u8[0],
was changed in the previous operation.

Thats the reason src[] is needed

Regards
Nikunj