LASTA (scalar)

Extract element after last to general-purpose register

If there is an active element then extract the element after the last active element modulo the number of elements from the final source vector register. If there are no active elements, extract element zero. Then zero-extend and place the extracted element in the destination general-purpose register.

Encoding: SVE

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

313029282726252423222120191817161514131211109876543210
00000101100000101
sizeBPgZnRd

LASTA <R><d>, <Pg>, <Zn>.<T>

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 8 << UInt(size);
constant integer rsize = if esize < 64 then 32 else 64;
constant integer g = UInt(Pg);
constant integer n = UInt(Zn);
constant integer d = UInt(Rd);
constant boolean isBefore = FALSE;

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(VL) operand = Z[n, VL];
bits(rsize) result;
integer last = LastActiveElement(mask, esize);

if isBefore then
    if last < 0 then last = elements - 1;
else
    last = last + 1;
    if last >= elements then last = 0;
result = ZeroExtend(Elem[operand, last, esize], rsize);

X[d, rsize] = result;

Explanations

<R>: <d>: Is the number [0-30] of the destination general-purpose register or the name ZR (31), encoded in the "Rd" field.
<Pg>: Is the name of the governing scalable predicate register P0-P7, encoded in the "Pg" field.
<Zn>: Is the name of the source scalable vector register, encoded in the "Zn" field.
<T>:

Operational Notes

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.