Re: [qemu-s390x] [Qemu-devel] [PATCH v1 37/41] s390x/tcg: Implement VECTOR SUBTRACT WITH BORROW COMPUTE BORROW INDICATION

[David Hildenbrand]

On 13.04.19 08:11, Richard Henderson wrote:
> On 4/11/19 12:08 AM, David Hildenbrand wrote:
>> +static DisasJumpType op_vsbcbi(DisasContext *s, DisasOps *o)
>> +{
>> +    if (get_field(s->fields, m5) != ES_128) {
>> +        gen_program_exception(s, PGM_SPECIFICATION);
>> +        return DISAS_NORETURN;
>> +    }
>> +
>> +    gen_gvec_4_ool(get_field(s->fields, v1), get_field(s->fields, v2),
>> +                   get_field(s->fields, v3), get_field(s->fields, v4), 0,
>> +                   gen_helper_gvec_vsbcbi128);
>> +    return DISAS_NEXT;
>> +}
> 
> 
> Reviewed-by: Richard Henderson <address@hidden>
> 
> I'm sure this can be done similarly to add with carry compute carry, but it's
> harder to reason with sub2.  Something like
> 
>       tcg_gen_andi_i64(tl, cl, 1);
>       tcg_gen_sub2_i64(tl, th, al, zero, tl, zero);
>       tcg_gen_sub2_i64(tl, th, tl, th, bl, zero);
>       tcg_gen_andi_i64(tl, th, 1);
>       tcg_gen_sub2_i64(tl, th, ah, zero, tl, zero);
>       tcg_gen_sub2_i64(tl, th, tl, th, bh, zero);
>       tcg_gen_andi_i64(tl, th, 1);
>       /* result in tl */

Indeed, looks good to me. The only thing to care about is to convert -1
to 1, so we get a proper borrow.

Thanks!

-- 

Thanks,

David / dhildenb