clasta

SVE Conditional Last Element After

CLASTA <Rdn>, <Pg>, <Rdn>, <Zm>.<T>

Extracts element after the last active element.

Pseudocode Operation

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

Example

CLASTA r0, p0/m, r0, z2.s.T

Encoding

Binary Layout

00000101

31:24

size

23:22

11000

21:17

101

15:13

12:10

9:5

Rdn

4:0

Format SVE Extract

Opcode 0x0530A000

Extension SVE

Operands

Rdn
Dest/Fallback
Pg
Predicate
Zm
Vector

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction	ISA	Bit pattern
0x0530A000	CLASTA <R><dn>, <Pg>, <R><dn>, <Zm>.<T>	A64	00000101 \| size \| 11000 \| 0 \| 101 \| Pg \| Zm \| Rdn
0x052A8000	CLASTA <V><dn>, <Pg>, <V><dn>, <Zm>.<T>	A64	00000101 \| size \| 10101 \| 0 \| 100 \| Pg \| Zm \| Vdn
0x05288000	CLASTA <Zdn>.<T>, <Pg>, <Zdn>.<T>, <Zm>.<T>	A64	00000101 \| size \| 10100 \| 0 \| 100 \| Pg \| Zm \| Zdn

Description

From the source vector register extract the element after the last active element, or if the last active element is the final element extract element zero, and then zero-extend that element to destructively place in the destination and first source general-purpose register. If there are no active elements then destructively zero-extend the least significant element-size bits of the destination and first source general-purpose 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(esize) operand1 = X[dn, esize];
bits(VL) operand2 = Z[m, VL];
bits(csize) result;
integer last = LastActiveElement(mask, esize);

if last < 0 then
    result = ZeroExtend(operand1, csize);
else
    if !isBefore then
        last = last + 1;
        if last >= elements then last = 0;
    result = ZeroExtend(Elem[operand2, last, esize], csize);

X[dn, csize] = result;