Re: [Qemu-devel] [PATCH 2/6] target-ppc: add vextu[bhw]lx instructions

[Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>]

On 09/28/2016 10:24 PM, Richard Henderson wrote:
> On 09/27/2016 10:45 PM, Rajalakshmi Srinivasaraghavan wrote:
>> +#if defined(HOST_WORDS_BIGENDIAN)
>> +#define VEXTULX_DO(name, elem)                                  \
>> +target_ulong glue(helper_, name)(target_ulong a, ppc_avr_t *b)  \
>> +{                                                               \
>> +    target_ulong r = 0;                                         \
>> +    int i;                                                      \
>> +    int index = a & 0xf;                                        \
>> +    for (i = 0; i < elem; i++) {                                \
>> +        r = r << 8;                                             \
>> +        if (index + i <= 15) {                                  \
>> +            r = r | b->u8[index + i];                           \
>> +        }                                                       \
>> +    }                                                           \
>> +    return r;                                                   \
>> +}
>> +#else
>> +#define VEXTULX_DO(name, elem)                                  \
>> +target_ulong glue(helper_, name)(target_ulong a, ppc_avr_t *b)  \
>> +{                                                               \
>> +    target_ulong r = 0;                                         \
>> +    int i;                                                      \
>> +    int index = 15 - (a & 0xf);                                 \
>> +    for (i = 0; i < elem; i++) {                                \
>> +        r = r << 8;                                             \
>> +        if (index - i >= 0) {                                   \
>> +            r = r | b->u8[index - i];                           \
>> +        }                                                       \
>> +    }                                                           \
>> +    return r;                                                   \
>> +}
>> +#endif
>> +
>> +VEXTULX_DO(vextublx, 1)
>> +VEXTULX_DO(vextuhlx, 2)
>> +VEXTULX_DO(vextuwlx, 4)
>> +#undef VEXTULX_DO
> Ew.
>
> This should be one 128-bit shift and one and.
>
> Since the shift amount is a multiple of 8, the 128-bit shift for vextub[lr]x
> does not need to cross a double-word boundary, and so can be decomposed into
> one 64-bit shift of (count & 64 ? hi : lo).
>
> For vextu[hw]lr]x, you'd need to do the whole left-shift, right-shift, or thing.
>
> But still, fantastically better than a loop.
Ack. Will send an updated patch.
>
>
> r~
>
>

-- 
Thanks
Rajalakshmi S