pfirst

SVE Predicate First Active

PFIRST <Pd>.B, <Pg>, <Pn>.B

Sets destination predicate to true only at the first active element.

Pseudocode Operation

for i = 0 to VL/8-1
  if Pg[i] == 1 and Pn[i] == 1 then
    Pd[i] ← 1
    break
  else
    Pd[i] ← 0
  endif
endfor
for j = i+1 to VL/8-1
  Pd[j] ← 0
endfor

Example

PFIRST p0.B, p0/m, p1.B

Encoding

Binary Layout

00100101

31:24

011000110000

21:10

8:5

Pdn

3:0

Format SVE Predicate

Opcode 0x2558C000

Extension SVE

Operands

Pd
Dest Pred
Pg
Mask
Pn
First source predicate register (SVE)

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0x2558C000	PFIRST <Pdn>.B, <Pg>, <Pdn>.B		A64		00100101 \| 0 \| 1 \| 011000110000 \| 0 \| Pg \| 0 \| Pdn

Description

Sets the first active element in the destination predicate to true, otherwise elements from the source predicate are passed through unchanged. 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[g, PL];
bits(PL) result = P[dn, PL];
integer first = -1;
constant integer psize = esize DIV 8;

for e = 0 to elements-1
    if ActivePredicateElement(mask, e, esize) && first == -1 then
        first = e;

if first >= 0 then
    Elem[result, first, psize] = ZeroExtend('1', psize);

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