.. index:: pair: page; Primitive Attributes: floating-point math mode
.. _doxid-dev_guide_attributes_fpmath_mode:

Primitive Attributes: floating-point math mode
==============================================

For some applications, it can be beneficial to allow down-conversions to speedup computations without noticeable impact on accuracy.

This section describes how the default numerical behavior of oneDNN (described in :ref:`Data Types <doxid-dev_guide_data_types>`) can be altered to allow implicit down-conversions of floating-point types.

The floating-point math mode attribute.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When passed to a primitive creation, the :ref:`dnnl::fpmath_mode <doxid-group__dnnl__api__attributes_1ga0ad94cbef13dce222933422bfdcfa725>` primitive attribute specifies which implicit down-conversions are allowed for that given primitive. Only down-conversions from f32 to narrower data-types (f16, bf16) are currently allowed. Furthermore these down-conversions are allowed only during computation, and do not affect the storage datatype (which must remain f32).

The :ref:`dnnl::fpmath_mode <doxid-group__dnnl__api__attributes_1ga0ad94cbef13dce222933422bfdcfa725>` primitive attribute can take 3 types of values:

* the ``strict`` mode disables any down-conversion.

* the ``any`` mode allows all conversions from f32 to a smaller floating-point datatype (bf16 or f16).

* a specific datatype (f16 or bf16) which specifically allows down-conversion only from f32 to a datatype at least as accurate as the specified data-type (at least same number of exponent and mantissa bits).

This attribute is ignored if a primitive computation data-type is integral.

A note on default floating-point math mode
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The default floating-point mode is ``strict``, which means no implicit down-conversion is allowed. However, this default behavior can be changed with the ``DNNL_DEFAULT_FPMATH_MODE`` environment variable, the :ref:`dnnl_set_default_fpmath_mode <doxid-group__dnnl__api__mathmode_1ga97dd535e43073cee2ebc4b709e42c3ca>` (C API) or the :ref:`dnnl::set_default_fpmath_mode <doxid-group__dnnl__api__service_1ga0d55da5f92d60a7324cfdc97004ad975>` (C++ API) functions.

.. note:: 

   For builds where Arm Compute Library is enabled, setting ``DNNL_DEFAULT_FPMATH_MODE`` to ``BF16`` or ``ANY`` will instruct Compute Library to dispatch bfloat16 kernels where available, provided the hardware supports bfloat16 instructions. Note: this may introduce a drop in accuracy.