Re: [Qemu-devel] [PATCH v1 31/41] s390x/tcg: Implement VECTOR SHIFT LEFT DOUBLE BY BYTE

[Richard Henderson]

On 4/15/19 11:45 PM, David Hildenbrand wrote:
> On 13.04.19 02:54, Richard Henderson wrote:
>> On 4/11/19 12:08 AM, David Hildenbrand wrote:
>>> +static DisasJumpType op_vsldb(DisasContext *s, DisasOps *o)
>>> +{
>>> +    int src_idx = get_field(s->fields, i4) & 0xf;
>>> +
>>> +    if (src_idx == 0) {
>>> +        gen_gvec_mov(get_field(s->fields, v1), get_field(s->fields, v2));
>>> +    } else {
>>> +        gen_gvec_3_ool(get_field(s->fields, v1), get_field(s->fields, v2),
>>> +                       get_field(s->fields, v3), src_idx,
>>> +                       gen_helper_gvec_vsldb);
>>> +        return DISAS_NEXT;
>>
>> You could also expand this inline using your new extract2 primitive.
>>
>>   int i4 = get_field(s->fields, i4);
>>   int left_shift, right_shift;
>>
>>   left_shift = (i4 & 7) * 8;
>>   right_shift = 64 - left_shift;
>>
>>   if ((i4 & 8) == 0) {
>>       read_vec_element_i64(t0, get_field(s->fields, v2), 0, ES_64);
>>       read_vec_element_i64(t1, get_field(s->fields, v2), 1, ES_64);
>>       read_vec_element_i64(t2, get_field(s->fields, v3), 0, ES_64);
>>   } else {
>>       read_vec_element_i64(t0, get_field(s->fields, v2), 1, ES_64);
>>       read_vec_element_i64(t1, get_field(s->fields, v3), 0, ES_64);
>>       read_vec_element_i64(t2, get_field(s->fields, v3), 1, ES_64);
>>   }
>>   tcg_gen_extract2_i64(t0, t1, t0, right_shift);
>>   tcg_gen_extract2_i64(t1, t2, t1, right_shift);
> 
> Trying to understand the magic, left_shift is really only used to to
> calculate right_shift, right?

Yes.  I thought that was clearer as a separate step.


r~