Re: [PATCH 15/20] target/arm: Fix sve_uzp_p vs odd vector lengths

[Richard Henderson <richard.henderson@linaro.org>]

On 8/25/20 6:43 AM, Peter Maydell wrote:
> On Sat, 15 Aug 2020 at 02:32, Richard Henderson
> <richard.henderson@linaro.org> wrote:
>>
>> Missed out on compressing the second half of a predicate
>> with length vl % 512 > 256.
>>
>> Adjust all of the x + (y << s) to x | (y << s) as a
>> general style fix.
>>
>> Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>  target/arm/sve_helper.c | 30 +++++++++++++++++++++---------
>>  1 file changed, 21 insertions(+), 9 deletions(-)
>>
>> diff --git a/target/arm/sve_helper.c b/target/arm/sve_helper.c
>> index 4758d46f34..fcb46f150f 100644
>> --- a/target/arm/sve_helper.c
>> +++ b/target/arm/sve_helper.c
>> @@ -1938,7 +1938,7 @@ void HELPER(sve_uzp_p)(void *vd, void *vn, void *vm, uint32_t pred_desc)
>>      if (oprsz <= 8) {
>>          l = compress_bits(n[0] >> odd, esz);
>>          h = compress_bits(m[0] >> odd, esz);
>> -        d[0] = extract64(l + (h << (4 * oprsz)), 0, 8 * oprsz);
>> +        d[0] = l | (h << (4 * oprsz));
> 
> Why did we drop the extract64() here ? This doesn't seem
> to correspond to either of the things the commit message
> says we're doing.

Indeed, the commit message could use expansion.

> Also, if oprsz is < 8, don't we need to mask out the high
> bits in l that would otherwise overlap with h << (4 * oprsz) ?
> Are they guaranteed zeroes somehow?

They are guaranteed zeros.  See aarch64_sve_narrow_vq.

>>              for (i = 0; i < oprsz_16; i++) {
>>                  l = m[2 * i + 0];
>>                  h = m[2 * i + 1];
>>                  l = compress_bits(l >> odd, esz);
>>                  h = compress_bits(h >> odd, esz);
>> -                tmp_m.p[i] = l + (h << 32);
>> +                tmp_m.p[i] = l | (h << 32);
>>              }
>> -            tmp_m.p[i] = compress_bits(m[2 * i] >> odd, esz);
>> +            l = m[2 * i + 0];
>> +            h = m[2 * i + 1];
>> +            l = compress_bits(l >> odd, esz);
>> +            h = compress_bits(h >> odd, esz);
>> +            tmp_m.p[i] = l | (h << final_shift);
>>
>>              swap_memmove(vd + oprsz / 2, &tmp_m, oprsz / 2);
> 
> Aren't there cases where the 'n' part of the result doesn't
> end up a whole number of bytes and we have to do a shift as
> well as a byte copy?

No, oprsz will always be a multiple of 2 for predicates.
Just like oprsz will always be a multiple of 16 for sve vectors.


r~