vtbl

Vector Table Lookup

VTBL<c>.8 <Dd>, <list>, <Dm>

Look up elements in a vector table.

Pseudocode Operation

for i ← 0 to (size_of_Dd / 8) - 1:
  index ← Dm[i*8 +: 8]
  if index < (len + 1) * 16:
    Dd[i*8 +: 8] ← table[index]
  else:
    Dd[i*8 +: 8] ← 0

Example

VTBL.8 d0, {r0-r3}, d2

Encoding

Binary Layout

111100111

31:23

21:20

19:16

15:12

11:10

len

9:8

3:0

Format NEON Table

Opcode 0xF3B00800

Extension NEON (SIMD)

Operands

Dd
Destination 64-bit SIMD/FP register
list
Table
Dm
Indices

Reference

View in Arm A64 ISA Reference ↗ Arm AArch32 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0xF3B00800	VTBL{<c>}{<q>}.8 <Dd>, <list>, <Dm>		A32		111100111 \| D \| 11 \| Vn \| Vd \| 10 \| len \| N \| 0 \| M \| 0 \| Vm
0xFFB00800	VTBL{<c>}{<q>}.8 <Dd>, <list>, <Dm>		T32		111111111 \| D \| 11 \| Vn \| Vd \| 10 \| len \| N \| 0 \| M \| 0 \| Vm

Description

Vector Table Lookup uses byte indexes in a control vector to look up byte values in a table and generate a new vector. Indexes out of range return 0. Vector Table Extension works in the same way, except that indexes out of range leave the destination element unchanged. Depending on settings in the CPACR, NSACR, and HCPTR registers, and the Security state and PE mode in which the instruction is executed, an attempt to execute the instruction might be undefined, or trapped to Hyp mode. For more information see Enabling Advanced SIMD and floating-point support.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();  CheckAdvSIMDEnabled();

    // Create 256-bit = 32-byte table variable, with zeros in entries that will not be used.
    table3 = if length == 4 then D[n+3] else Zeros(64);
    table2 = if length >= 3 then D[n+2] else Zeros(64);
    table1 = if length >= 2 then D[n+1] else Zeros(64);
    table = table3 : table2 : table1 : D[n];

    for i = 0 to 7
        index = UInt(Elem[D[m],i,8]);
        if index < 8*length then
            Elem[D[d],i,8] = Elem[table,index,8];
        else
            if is_vtbl then
                Elem[D[d],i,8] = Zeros(8);
            // else Elem[D[d],i,8] unchanged