CPU ISA Hints {#dev_guide_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
`DNNL_ENABLE_CPU_ISA_HINTS`.

| CMake Option                | Supported values (defaults in bold) | Description
| :---                        | :---                                | :---
| DNNL_ENABLE_CPU_ISA_HINTS   | **ON**, OFF                         | Enables CPU ISA hints

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

## Run-time Controls

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

| Environment variable | Value            | Description
| :---                 | :---             | :---
| DNNL_CPU_ISA_HINTS   | NO_HINTS         | Use default configuration for ISA
|                      | PREFER_YMM       | Prefer to use YMM registers for vector operations

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

* @ref 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.
* @ref 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.
