pnext

SVE Find Next Active Predicate

PNEXT <Pdn>.<T>, <Pg>, <Pdn>.<T>

Finds the next active predicate bit.

Pseudocode Operation

next_pos ← -1
for i = 0 to VL-1:
  if Pg[i] ∧ Pdn[i] then
    for j = i+1 to VL-1:
      if Pg[j] ∧ Pdn[j] then
        next_pos ← j
        break
    break
if next_pos ≥ 0 then
  Pdn ← (1 << next_pos)
  Z ← 0
else
  Pdn ← 0
  Z ← 1

Example

PNEXT p0.T, p0/m, p0.T

Encoding

Binary Layout

00100101

31:24

size

23:22

011001110001

21:10

8:5

Pdn

3:0

Format SVE Predicate

Opcode 0x2519C400

Extension SVE

Operands

Pdn
Dest/Src
Pg
Governing Pred

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0x2519C400	PNEXT <Pdn>.<T>, <Pv>, <Pdn>.<T>		A64		00100101 \| size \| 011001110001 \| 0 \| Pv \| 0 \| Pdn

Description

An instruction used to construct a loop which iterates over all true elements in the vector select predicate register. If all elements in the first source predicate register are false it determines the first true element in the vector select predicate register, otherwise it determines the next true element in the vector select predicate register that follows the last true element in the first source predicate register. All elements of the destination predicate register are set to false, except the element corresponding to the determined vector select element, if any, which is set to true. Sets the First (N), None (Z), !Last (C) condition flags based on the predicate result, and the V flag to zero.

Operation

CheckSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
bits(PL) mask = P[v, PL];
bits(PL) operand = P[dn, PL];
bits(PL) result;
constant integer psize = esize DIV 8;

integer next = LastActiveElement(operand, esize) + 1;

while next < elements && (!ActivePredicateElement(mask, next, esize)) do
    next = next + 1;

result = Zeros(PL);
if next < elements then
    Elem[result, next, psize] = ZeroExtend('1', psize);

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