SEL

Multi-vector conditionally select elements from two vectors

This instruction selects consecutive elements from the two or four first source vectors where predicate elements are true, and places them in the corresponding elements of the two or four destination vectors. The corresponding elements from the two or four second source vectors are placed in the remaining elements of the destination vectors.

Encoding: Two registers

Variants: FEAT_SME2 (ARMv9.3)

313029282726252423222120191817161514131211109876543210
110000011010000
sizeZmPNgZnZd

SEL { <Zd1>.<T>-<Zd2>.<T> }, <PNg>, { <Zn1>.<T>-<Zn2>.<T> }, { <Zm1>.<T>-<Zm2>.<T> }

Decoding algorithm

if !IsFeatureImplemented(FEAT_SME2) then EndOfDecode(Decode_UNDEF);
constant integer esize = 8 << UInt(size);
constant integer n = UInt(Zn:'0');
constant integer m = UInt(Zm:'0');
constant integer d = UInt(Zd:'0');
constant integer g = UInt('1':PNg);
constant integer nreg = 2;

Encoding: Four registers

Variants: FEAT_SME2 (ARMv9.3)

313029282726252423222120191817161514131211109876543210
110000011011000000
sizeZmPNgZnZd

SEL { <Zd1>.<T>-<Zd4>.<T> }, <PNg>, { <Zn1>.<T>-<Zn4>.<T> }, { <Zm1>.<T>-<Zm4>.<T> }

Decoding algorithm

if !IsFeatureImplemented(FEAT_SME2) then EndOfDecode(Decode_UNDEF);
constant integer esize = 8 << UInt(size);
constant integer n = UInt(Zn:'00');
constant integer m = UInt(Zm:'00');
constant integer d = UInt(Zd:'00');
constant integer g = UInt('1':PNg);
constant integer nreg = 4;

Operation

CheckStreamingSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
array [0..3] of bits(VL) results;
constant bits(PL) pred = P[g, PL];
constant bits(PL * nreg) mask = CounterToPredicate(pred<15:0>, PL * nreg);

for r = 0 to nreg-1
    constant bits(VL) operand1 = Z[n+r, VL];
    constant bits(VL) operand2 = Z[m+r, VL];
    for e = 0 to elements-1
        if ActivePredicateElement(mask, r * elements + e, esize) then
            Elem[results[r], e, esize] = Elem[operand1, e, esize];
        else
            Elem[results[r], e, esize] = Elem[operand2, e, esize];

for r = 0 to nreg-1
    Z[d+r, VL] = results[r];

Explanations

<Zd1>: For the "Two registers" variant: is the name of the first scalable vector register of the destination multi-vector group, encoded as "Zd" times 2.
<Zd1>: For the "Four registers" variant: is the name of the first scalable vector register of the destination multi-vector group, encoded as "Zd" times 4.
<T>: <Zd2>: Is the name of the second scalable vector register of the destination multi-vector group, encoded as "Zd" times 2 plus 1.
<PNg>: Is the name of the governing scalable predicate register PN8-PN15, with predicate-as-counter encoding, encoded in the "PNg" field.
<Zn1>: For the "Two registers" variant: is the name of the first scalable vector register of the first source multi-vector group, encoded as "Zn" times 2.
<Zn1>: For the "Four registers" variant: is the name of the first scalable vector register of the first source multi-vector group, encoded as "Zn" times 4.
<Zn2>: Is the name of the second scalable vector register of the first source multi-vector group, encoded as "Zn" times 2 plus 1.
<Zm1>: For the "Two registers" variant: is the name of the first scalable vector register of the second source multi-vector group, encoded as "Zm" times 2.
<Zm1>: For the "Four registers" variant: is the name of the first scalable vector register of the second source multi-vector group, encoded as "Zm" times 4.
<Zm2>: Is the name of the second scalable vector register of the second source multi-vector group, encoded as "Zm" times 2 plus 1.
<Zd4>: Is the name of the fourth scalable vector register of the destination multi-vector group, encoded as "Zd" times 4 plus 3.
<Zn4>: Is the name of the fourth scalable vector register of the first source multi-vector group, encoded as "Zn" times 4 plus 3.
<Zm4>: Is the name of the fourth scalable vector register of the second source multi-vector group, encoded as "Zm" times 4 plus 3.

Operational Notes

If PSTATE.DIT is 1: