lasta

SVE Extract Last Element After

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

Extracts element after last active (SIMD scalar destination).

Pseudocode Operation

activecount ← CountActiveLanes(Pg, esize)
if activecount == VL/esize then
  Vd ← Zn[0]
else if activecount > 0 then
  Vd ← Zn[activecount]
else
  Vd ← Vd

Example

LASTA 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 0x05228000

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
0x0520A000	LASTA <R><d>, <Pg>, <Zn>.<T>		A64		00000101 \| size \| 10000 \| 0 \| 101 \| Pg \| Zn \| Rd
0x05228000	LASTA <V><d>, <Pg>, <Zn>.<T>		A64		00000101 \| size \| 10001 \| 0 \| 100 \| Pg \| Zn \| Vd

Description

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 place the extracted element in the destination SIMD&FP scalar 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];