BRKBS

Break before first true condition, setting the condition flags

Sets destination predicate elements up to but not including the first active and true source element to true, then sets subsequent elements to false. Inactive elements in the destination predicate register are set to zero. Sets the First (N), None (Z), !Last (C) condition flags based on the predicate result, and the V flag to zero.

Encoding: Setting the condition flags

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	1	1	0	1	0	0	0	0	0	1					0					0
								B	S									Pg					Pn				M	Pd

BRKBS <Pd>.B, <Pg>/Z, <Pn>.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 n = UInt(Pn);
constant integer d = UInt(Pd);
constant boolean merging = FALSE;
constant boolean setflags = TRUE;

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];
constant bits(PL) operand = P[n, PL];
constant bits(PL) operand2 = if merging then P[d, PL] else Zeros(PL);
boolean break = FALSE;
bits(PL) result;
constant integer psize = esize DIV 8;

for e = 0 to elements-1
    constant boolean element = ActivePredicateElement(operand, e, esize);
    if ActivePredicateElement(mask, e, esize) then
        break = break || element;
        constant bit pbit = if !break then '1' else '0';
        Elem[result, e, psize] = ZeroExtend(pbit, psize);
    elsif merging then
        constant bit pbit = PredicateElement(operand2, e, esize);
        Elem[result, e, psize] = ZeroExtend(pbit, psize);
    else
        Elem[result, e, psize] = ZeroExtend('0', psize);

if setflags then
    PSTATE. = PredTest(mask, result, esize);
P[d, PL] = result;

Explanations

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