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

[David Gibson]

On Wed, Jul 27, 2016 at 12:27:27PM +0530, Nikunj A Dadhania wrote:
> David Gibson <address@hidden> writes:
> 
> > [ Unknown signature status ]
> > On Wed, Jul 27, 2016 at 12:01:33PM +0530, Nikunj A Dadhania wrote:
> >> 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.
> >
> > Ah, good point.
> >
> >> Thats the reason src[] is needed
> >
> > It's probably possible to avoid generating all of src by instead
> > generating the bytes one loop iteration ahead, but that sounds fiddly,
> > so the current approach is fine for now.
> 
> Or do the operation in the reverse: starting from size to 0. Let me try
> that.

Good idea, that should work.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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