Arm® A64 Instruction Set Architecture: Armv8, for Armv8-A architecture profile

CLASTA (vectors)

Conditionally extract element after last to vector register.

From the second 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 replicate that element to destructively fill the destination and first source vector.

If there are no active elements then leave the destination and source vector unmodified.

if !HaveSVE() then UNDEFINED;
integer esize = 8 << UInt(size);
integer g = UInt(Pg);
integer dn = UInt(Zdn);
integer m = UInt(Zm);
boolean isBefore = FALSE;

Assembler Symbols

CheckSVEEnabled();
integer elements = VL DIV esize;
bits(PL) mask = P[g];
bits(VL) operand1 = Z[dn];
bits(VL) operand2 = Z[m];
bits(VL) result;
integer last = LastActiveElement(mask, esize);

if last < 0 then
    result = operand1;
else
    if !isBefore then
        last = last + 1;
        if last >= elements then last = 0;
    for e = 0 to elements-1
        Elem[result, e, esize] = Elem[operand2, last, esize];

Z[dn] = result;

Operational information

This instruction might be immediately preceded in program order by a MOVPRFX instruction. The MOVPRFX instruction must conform to all of the following requirements, otherwise the behavior of the MOVPRFX and this instruction is unpredictable:

• The MOVPRFX instruction must be unpredicated.
• The MOVPRFX instruction must specify the same destination register as this instruction.
• The destination register must not refer to architectural register state referenced by any other source operand register of this instruction.