CPU ISA Hints

For performance reasons, extra hints may be provided to oneDNN which enable the just-in-time (JIT) code generation to prefer or avoid certain CPU ISA features.

Build-time Controls

At build-time, support for this feature is controlled via cmake option ONEDNN_ENABLE_CPU_ISA_HINTS.

CMake Option

Supported values (defaults in bold)

Description

ONEDNN_ENABLE_CPU_ISA_HINTS

ON , OFF

Enables CPU ISA hints

This results in making oneDNN aware of ONEDNN_CPU_ISA_HINTS environment variable and corresponding setter utility namely, dnnl::set_cpu_isa_hints routine.

Run-time Controls

During run-time the ONEDNN_CPU_ISA_HINTS environment variable can be used to specify ISA specific hint to enable oneDNN to dispatch appropriate kernel.

Environment variable

Value

Description

ONEDNN_CPU_ISA_HINTS

NO_HINTS PREFER_YMM

Use default configuration for ISA Prefer to use YMM registers for vector operations

This feature can also be managed at run-time with the following functions:

  • dnnl::set_cpu_isa_hints function allows changing the CPU ISA hint at run-time. The limitation is that it is possible to set the value only once. In addition, it is advised to call this function before any other oneDNN API. This is because the first internal query of CPU ISA hints will disable the ability to change it. Once disabled, changing the CPU ISA hint will return an error.

  • dnnl::get_cpu_isa_hints function returns the currently used CPU ISA hints and by default poses no restrictions.

Function settings take precedence over environment variables. Moreover, if the hint is not applicable then it would be silently ignored. For instance with SSE41 there are no YMM registers and so hint to prefer YMM registers would be silently bypassed.