PFIRST

Set the first active predicate element to true

Sets the first active element in the destination predicate to true, otherwise elements from the source predicate are passed through unchanged. Sets the First (N), None (Z), !Last (C) condition flags based on the predicate result, and the V flag to zero.

Encoding: SVE

Variants: FEAT_SVE || FEAT_SME (FEAT_SVE || FEAT_SME)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	1	0	0	1	0	1	0	1	0	1	1	0	0	0	1	1	0	0	0	0	0					0
								op	S														Pg					Pdn

PFIRST <Pdn>.B, <Pg>, <Pdn>.B

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 8;
constant integer g = UInt(Pg);
constant integer dn = UInt(Pdn);

Operation

CheckSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
constant bits(PL) mask = P[g, PL];
bits(PL) result = P[dn, PL];
integer first = -1;
constant integer psize = esize DIV 8;

for e = 0 to elements-1
    if ActivePredicateElement(mask, e, esize) && first == -1 then
        first = e;

if first >= 0 then
    Elem[result, first, psize] = ZeroExtend('1', psize);

PSTATE. = PredTest(mask, result, esize);
P[dn, PL] = result;

Explanations

<Pdn>: Is the name of the source and destination scalable predicate register, encoded in the "Pdn" field.
<Pg>: Is the name of the governing scalable predicate register, encoded in the "Pg" field.

Operational Notes

If FEAT_SME is implemented and the PE is in Streaming SVE mode, then any subsequent instruction which is dependent on the NZCV condition flags written by this instruction might be significantly delayed.