doc/microkernel-naming-conventions.md

Microkernel naming conventions

This documents deciphers XNNPACK's microkernels naming convention.

General conventions

Microkernel function names follow this convention:

xnn_<datatype>_<microkernel><activation?>_ukernel_<parameters>__<arch>

Where <datatype> can be:

• cs16
• f16 - 16-bit half precision float
• f32 - 32-bit single precision float
• qc8
• qs8 - quantized signed 8 bit
• qu8 - quantized unsigned 8 bit
• s16
• u32
• x8
• x16
• x24
• x32
• xx

<microkernel> is the type of microkernel, such as:

• gemm
• igemm
• avgpool

<activation> if supported for the microkernel is activation that is fused into the microkernel:

• linear
• minmax
• relu

<parameters> are microkernel specific, and can mean different things depending on the microkernel (see below for details).

<arch> is the architecture the microkernel is optimized for, and can contain further subdivisions for additional instruction sets supported on the specified architecture, or processor information:

• scalar
• aarch32_neon_cortex_a55
• neonv8_mlal
• wasm
• avx512
• avx512skx

GEMM and IGEMM microkernels

The <parameters> for GEMM and IGEMM microkernels represent the mr and nr of the microkernel. You can think of it as the number of rows and columns of the output calculated by the microkernel.

E.g. xnn_f32_gemm_minmax_ukernel_4x8__aarch32_neon_cortex_a7 processes 32 elements of the output matrix.

DWCONV microkernels

These microkernels come in 2 varieties, uni-pass and multi-pass.

Uni-pass have XpYc in their name, where X is the kernel tile, and Y is the channel tile. p stands for primary, c for channel.

Multi-pass have UfVmWlXcYsZr in their name, where U is the first pass tile, V is the middle pass tile, W is the last pass tile, X is the channel tile, Y is the channel subtile, and Z is the channel round. f stands for first, m for middle, l for last, c for channel, s for subtile, r for round. The kernel size must be at least W+1, the middle pass runs for as many iterations as possible, and the last pass handles the remainder (at least 1). c, s, r, affects the tiling of channels. We run as many tiles of c as possible, followed by rounds of s. We determine how many tiles of c to run based on rounding the number of channels up to r. r is determined based on the natural tiling size of the microarchitecture (e.g. SSE/AVX) and the number of elements we can read OOB (XNN_EXTRA_BYTES).

Average Pooling and Global Average Pooling

These microkernels come in 2 varieties, uni-pass and multi-pass.

Uni-pass have Cx in their name, where C is a number. This microkernel processes up to and including C elements.

Multi-pass have CpDx in their name, where C and D are numbers. This microkernel processes D elements in the first pass, and middle pass (which can run multiple times), and up to C elements in the last pass.

E.g. xnn_f32_avgpool_minmax_ukernel_9x__neon_c4 can process up to 9 elements.