MAC on 8x8 bits - 2021.2 English

AI Engine Kernel Coding Best Practices Guide (UG1079)

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2021.2 English

The following figures show MAC with int8 X buffer and int8 Z buffer. The first figure shows how data is permuted and the second figure shows how coefficients are permuted. Note that the permute granularity for X buffer and Z buffer are 32 bits and 16 bits, respectively. The xoffsets parameter comes in pair. The first hex value is an absolute 32 bits offset and pick up 4 x 8 bits values (index, index+1, index+2, index+3). The second hex value is offset from the first value + 1 (32 bits offset) and picks up 4 x 8 bits values. For example, 0x00 selects index 0, 1, 2, 3 as well as 4, 5, 6, 7, and 0x24 selects index 16, 17, 18, 19 as well as 28, 29, 30, 31.

There is another xsquare parameter to do 8 bits granularity twiddling after main permute. How xsquare parameter works in this example can be seen in the center of the following figure.

The start (xstart, zstart) and step (xstep, zstep) parameters are always in terms of data type granularity. Hence, a value of 2 for 16 bits is 2 * 16 bits away, while a value of 2 for 8 bits is 2 * 8 bits away. The step parameter applies to the next block of selected data. So, if a pair of offset parameters select a 2 * 2 block, the step applies to the next 2 * 2 block. The step added to the index value must be aligned to the permute granularity (32 bits for data, 16 bits for coefficient). For example, when working with 8-bit data, xstep needs to be multiples of four. When working with 8-bit coefficient, zstep needs to be multiples of two. The following two figures show how step works for data and coefficients.

Note that for the coefficient in int8 * int8 types, the 2 * 2 index block is duplicated to construct a 4 * 2 block. See how index 0, 1, 2, and 3 are duplicated in Figure 2.

Figure 1. MAC8 on int8 x int8 Type (X Part)
Figure 2. MAC8 on int8 x int8 Type (Z Part)