LASTB (scalar)

Extract last element to general-purpose register

If there is an active element then extract the last active element from the final source vector register. If there are no active elements, extract the highest-numbered element. 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
00000101100001101
sizeBPgZnRd

LASTB <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 = 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(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.