tbl

SVE Table Lookup

TBL <Zd>.<T>, <Zn>.<T>, <Zm>.<T>

Looks up elements in a vector table using indices.

Pseudocode Operation

integer esize = 8 << UInt(sz);
integer elements = VL / esize;
for e = 0 to elements-1
  integer index = UInt(Zm[e * esize +: esize]);
  if index < elements then
    Zd[e * esize +: esize] = Zn[index * esize +: esize];
  else
    Zd[e * esize +: esize] = 0;

Example

TBL z0.s.T, z1.s.T, z2.s.T

Encoding

Binary Layout

00000101

31:24

size

23:22

20:16

001100

15:10

9:5

4:0

Format SVE Permute

Opcode 0x05203000

Extension SVE

Operands

Zd
Destination scalable vector register (SVE)
Zn
Table
Zm
Indices

Other forms of tbl

tbl Vector Table Lookup

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction	ISA	Bit pattern
0x0E000000	TBL <Vd>.<Ta>, { <Vn>.16B }, <Vm>.<Ta>	A64	0 \| Q \| 001110 \| 00 \| 0 \| Rm \| 0 \| 00 \| 0 \| 00 \| Rn \| Rd
0x0E002000	TBL <Vd>.<Ta>, { <Vn>.16B, <Vn+1>.16B }, <Vm>.<Ta>	A64	0 \| Q \| 001110 \| 00 \| 0 \| Rm \| 0 \| 01 \| 0 \| 00 \| Rn \| Rd
0x0E004000	TBL <Vd>.<Ta>, { <Vn>.16B, <Vn+1>.16B, <Vn+2>.16B }, <Vm>.<Ta>	A64	0 \| Q \| 001110 \| 00 \| 0 \| Rm \| 0 \| 10 \| 0 \| 00 \| Rn \| Rd
0x0E006000	TBL <Vd>.<Ta>, { <Vn>.16B, <Vn+1>.16B, <Vn+2>.16B, <Vn+3>.16B }, <Vm>.<Ta>	A64	0 \| Q \| 001110 \| 00 \| 0 \| Rm \| 0 \| 11 \| 0 \| 00 \| Rn \| Rd
0x05203000	TBL <Zd>.<T>, { <Zn>.<T> }, <Zm>.<T>	A64	00000101 \| size \| 1 \| Zm \| 001100 \| Zn \| Zd
0x05202800	TBL <Zd>.<T>, { <Zn1>.<T>, <Zn2>.<T> }, <Zm>.<T>	A64	00000101 \| size \| 1 \| Zm \| 00101 \| 0 \| Zn \| Zd

Description

Reads each element of the second source (index) vector and uses its value to select an indexed element from a table of elements consisting of one or two consecutive vector registers, where the first or only vector holds the lower numbered elements, and places the indexed table element in the destination vector element corresponding to the index vector element. If an index value is greater than or equal to the number of vector elements then it places zero in the corresponding destination vector element. Since the index values can select any element in a vector this operation is not naturally vector length agnostic.

Operation

CheckSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
bits(VL) indexes = Z[m, VL];
bits(VL) result;
constant integer table_size = if double_table then VL*2 else VL;
constant integer table_elems = table_size DIV esize;
bits(table_size) table;

if double_table then
    bits(VL) top = Z[(n + 1) MOD 32, VL];
    bits(VL) bottom = Z[n, VL];
    table = (top:bottom)<table_size-1:0>;
else
    table = Z[n, table_size];

for e = 0 to elements-1
    integer idx = UInt(Elem[indexes, e, esize]);
    Elem[result, e, esize] = if idx < table_elems then Elem[table, idx, esize] else Zeros(esize);

Z[d, VL] = result;

tbl