brkb

SVE Break Before First True

BRKB <Pd>.B, <Pg>/Z, <Pn>.B

Sets predicates up to (but excluding) the first active element.

Pseudocode Operation

integer esize = 8;
integer elements = VL / esize;
integer g = 0;
for e = 0 to elements-1
  if Pg[e] == '1' and Pn[e] == '1' and g == 0 then
    g = 1;
  if g == 0 then
    Pd[e] = '1';
  else
    Pd[e] = '0';

Example

BRKB p0.B, p0/m/Z, p1.B

Encoding

Binary Layout

00100101

31:24

01000001

21:14

13:10

8:5

3:0

Format SVE Predicate

Opcode 0x25904000

Extension SVE

Operands

Pd
Destination predicate register (SVE)
Pg
Limit
Pn
First source predicate register (SVE)

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0x25904000	BRKB <Pd>.B, <Pg>/<ZM>, <Pn>.B		A64		00100101 \| 1 \| 0 \| 01000001 \| Pg \| 0 \| Pn \| M \| Pd

Description

Sets destination predicate elements up to but not including the first active and true source element to true, then sets subsequent elements to false. Inactive elements in the destination predicate register remain unmodified or are set to zero, depending on whether merging or zeroing predication is selected. Does not set the condition flags.

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(PL) operand  = P[n, PL];
bits(PL) operand2 = P[d, PL];
boolean break = FALSE;
bits(PL) result;
constant integer psize = esize DIV 8;

for e = 0 to elements-1
    boolean element = ActivePredicateElement(operand, e, esize);
    if ActivePredicateElement(mask, e, esize) then
        break = break || element;
        bit pbit = if !break then '1' else '0';
        Elem[result, e, psize] = ZeroExtend(pbit, psize);
    elsif merging then
        bit pbit = PredicateElement(operand2, e, esize);
        Elem[result, e, psize] = ZeroExtend(pbit, psize);
    else
        Elem[result, e, psize] = ZeroExtend('0', psize);

if setflags then
    PSTATE.<N,Z,C,V> = PredTest(mask, result, esize);
P[d, PL] = result;