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 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 dnnl::fpmath_mode primitive attribute specifies which implicit down-conversions are allowed for that given primitive. Only down-conversions from f32 to narrower data-types (f16, bf16, or tf32) are currently allowed. Furthermore these down-conversions are allowed only during computation, and do not affect the storage datatype (which must remain f32).

The dnnl::fpmath_mode 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 (f16, bf16, or tf32).

  • a specific datatype (f16, bf16, or tf32) 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.

Enforcing the floating-point math mode to an integral primitive.

A user can enforce an integral primitive to comply with the floating-point math mode by setting the second argument of dnnl_primitive_attr_set_fpmath_mode_v2 (C API) or dnnl::primitive_attr::set_fpmath_mode (C++ API) functions to 1 or true.

In this case, an integral primitive will be forced to add an implicit weights up-conversion to the data type specified in the dnnl::fpmath_mode in addition to down-conversions described for the primitives with floating-point computation data-types. The de-quantization parameters of this up-conversion can be specified using quantization attributes. For more details, refer to Matmul Tutorial: weights decompression.

A note on the 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 ONEDNN_DEFAULT_FPMATH_MODE environment variable, the dnnl_set_default_fpmath_mode (C API) or the dnnl::set_default_fpmath_mode (C++ API) functions.


For builds where Arm Compute Library is enabled, setting ONEDNN_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.