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TBL

Programmable table lookup in one or two vector table (zeroing).

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.

It has encodings from 2 classes: SVE and SVE2

SVE

313029282726252423222120191817161514131211109876543210
00000101size1Zm001100ZnZd

SVE

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

if !HaveSVE() then UNDEFINED;
integer esize = 8 << UInt(size);
integer n = UInt(Zn);
integer m = UInt(Zm);
integer d = UInt(Zd);
boolean double_table = FALSE;

SVE2

313029282726252423222120191817161514131211109876543210
00000101size1Zm001010ZnZd

SVE2

TBL <Zd>.<T>, { <Zn1>.<T>, <Zn2>.<T> }, <Zm>.<T>

if !HaveSVE2() then UNDEFINED;
integer esize = 8 << UInt(size);
integer n = UInt(Zn);
integer m = UInt(Zm);
integer d = UInt(Zd);
boolean double_table = TRUE;

Assembler Symbols

<Zd>

Is the name of the destination scalable vector register, encoded in the "Zd" field.

<T> Is the size specifier, encoded in size:
size <T>
00 B
01 H
10 S
11 D
<Zn>

Is the name of the first source scalable vector register, encoded in the "Zn" field.

<Zn1>

Is the name of the first scalable vector register of a multi-vector sequence, encoded in the "Zn" field.

<Zn2>

Is the name of the second scalable vector register of a multi-vector sequence, encoded in the "Zn" field.

<Zm>

Is the name of the second source scalable vector register, encoded in the "Zm" field.

Operation

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

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

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();

Z[d] = result;
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