VCOP memory is arranged as lines of 8 32-bit banks, for a total of 256 bits per line. This layout complements the 8-way SIMD width of table lookup operations: each SIMD lane corresponds to a lookup in one memory bank. The table layout in VCOP memory reflects this architecture.

The table layout cannot change between 8-way and 16-way SIMD. The producer of the table is independent of the consumer, so the producer need not be aware of whether the table will be read using 8-way SIMD or 16-way SIMD. So, for 16-way SIMD, only the lookup operation itself changes; the source table layout in L2 does not.

These considerations result in the following default scheme to support 16-way lookup and histogram:

• The table is assumed to be laid out in L2 memory according to the num_tables parameter of the _LOOKUP directive. This layout is dictated by VCOP’s memory architecture and num_tables, and is independent of the SIMD width that will be used to read the table.
• If vcop_simd=16 is specified, the lookup table operation is assumed to use twice the number of lanes as that specified by num_tables. For example, if num_tables==8, the lookup operation used 16 lanes. If num_tables==2, the lookup operation uses 4 lanes.
• Doubling the lookup width is enabled by duplicating the table in L1D as it is copied from L2. In effect, the N-way table becomes a 2N-way table, with the data from lanes [0,N-1] duplicated in lanes [N, 2N-1] (where N is num_tables).
• For a histogram operation, when the table is copied back to L2, the accumulated sums from the duplicated tables must be added together (with saturation).
• In this way the lookup throughput is doubled without requiring a change to the table layout in L2.

A few considerations apply to this scheme.

• First, the user should be aware that the table requires twice as much memory in L1 as in L2. Therefore the maximum table size that can be supported in 16-way SIMD is 16K bytes.
• Second, because of the duplication, the throughput of the copy-in/out operations is 256 bits per cycle instead of 512 bits as in 8-way mode.
• Third, like arithmetic kernels, in order to take advantage of 16-way SIMD, the kernel must be written in a SIMD-width independent way. That is, loop control (trip counts) and addressing must be in terms of VCOP_SIMD_WIDTH, rather than being hard-coded, or is more commonly the case, in terms of num_tables.

This scheme is the default, but is not beneficial for all cases of lookup operations. For this reason, _LOOKUP has been given an additional parameter to control this table duplication behavior, duplication:

_LOOKUP(num_tables, num_pts, duplication, table_size)

By default, duplication is VCOP_SIMD_WIDTH/8. That is, for 8-way it is 1 and for 16-way it is 2. In addition, num_pts may be up to VCOP_SIMD_WIDTH. This additional parameter is particularly useful for one table configurations. Consider the case of:

_LOOKUP(1, 8)

If the desired effect is to look up as many points as possible, this specification may be rewritten as:

_LOOKUP(1, VCOP_SIMD_WIDTH, 1)

This configuration will look up 8 points in 8 way mode and 16 points in 16 way mode from a single table in L1D. If the desired effect is to lookup sets of 8 points, the specification may instead be rewritten to:

_LOOKUP(1, 8, VCOP_SIMD_WIDTH/8)

This configuration will look up 8 points using one index from one table in 8 way mode. In 16 way mode, the table will be duplicated in memory, two index values will be used, and two sets of 8 points will be returned.