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From: David Hildenbrand <david@redhat.com>
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Date: Tue, 16 Apr 2019 20:26:34 +0200
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Subject: Re: [Qemu-devel] [PATCH v1 37/41] s390x/tcg: Implement VECTOR
 SUBTRACT WITH BORROW COMPUTE BORROW INDICATION
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To: Richard Henderson <richard.henderson@linaro.org>, qemu-devel@nongnu.org
Cc: qemu-s390x@nongnu.org, Cornelia Huck <cohuck@redhat.com>, Thomas Huth <thuth@redhat.com>, Richard Henderson <rth@twiddle.net>

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 <richard.henderson@linaro.org>
> 
> 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