lastb

SVE Extract Last Element Before

LASTB <Vd>.<T>, <Pg>, <Zn>.<T>

Extracts last active element (SIMD scalar destination).

Pseudocode Operation

activecount ← CountActiveLanes(Pg, esize)
if activecount > 0 then
  Vd ← Zn[activecount - 1]
else
  Vd ← Vd

Example

LASTB v0.4s.T, p0/m, z1.s.T

Encoding

Binary Layout

00000101

31:24

size

23:22

10001

21:17

100

15:13

12:10

9:5

4:0

Format SVE Extract

Opcode 0x05238000

Extension SVE

Operands

Vd
Dest Scalar
Pg
Predicate
Zn
Vector

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0x0521A000	LASTB <R><d>, <Pg>, <Zn>.<T>		A64		00000101 \| size \| 10000 \| 1 \| 101 \| Pg \| Zn \| Rd
0x05238000	LASTB <V><d>, <Pg>, <Zn>.<T>		A64		00000101 \| size \| 10001 \| 1 \| 100 \| Pg \| Zn \| Vd

Description

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 place the extracted element in the destination SIMD&FP register.

Operation

CheckSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
bits(PL) mask = P[g, PL];
bits(VL) operand = Z[n, VL];
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;
V[d, esize] = Elem[operand, last, esize];