LASTP

Scalar index of last true predicate element (predicated)

Determines the index of the last active and true element of the source predicate, or minus one if there are no active and true elements, and places the scalar result in the destination general-purpose register.

Encoding: SVE2

Variants: FEAT_SVE2p2 || FEAT_SME2p2 (FEAT_SVE2p2 || FEAT_SME2p2)

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	0	0	0	1	0	1	0					0
								size					opc					Pg					Pn				Rd

LASTP <Xd>, <Pg>, <Pn>.<T>

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE2p2) && !IsFeatureImplemented(FEAT_SME2p2) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 8 << UInt(size);
constant integer g = UInt(Pg);
constant integer n = UInt(Pn);
constant integer d = UInt(Rd);

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];
integer index = -1;

for e = elements-1 downto 0
    if (index == -1 && ActivePredicateElement(mask, e, esize) &&
          ActivePredicateElement(operand, e, esize)) then
        index = e;

X[d, 64] = index<63:0>;

Explanations

<Xd>: Is the 64-bit name of the destination general-purpose register, encoded in the "Rd" 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.
<T>:

Operational Notes

If PSTATE.DIT is 1:

• The execution time of this instruction is independent of:
  • The values of the data supplied in any of its operand registers when its governing predicate register contains the same value for each execution.
  • The values of the NZCV flags.
• The response of this instruction to asynchronous exceptions does not vary based on:
  • The values of the data supplied in any of its operand registers when its governing predicate register contains the same value for each execution.
  • The values of the NZCV flags.

If FEAT_SME is implemented and the PE is in Streaming SVE mode, then any subsequent instruction which is dependent on the general-purpose register written by this instruction might be significantly delayed.