Deep Neural Network Library (DNNL)
1.2.0

Performance library for Deep Learning

Softmax

The softmax primitive performs softmax along a particular axis on data with arbitrary dimensions. All other axes are treated as independent (batch).

In general form, the operation is defined by the following formulas:

\[ dst(\overline{ou}, c, \overline{in}) = \frac {e^{src(\overline{ou}, c, \overline{in}) - \nu(\overline{ou}, \overline{in})}} { \sum\limits_{ic} e^{src(\overline{ou}, ic, \overline{in}) - \nu(\overline{ou}, \overline{in})} }, \]

where

- \(c\) axis over which the softmax computation is computed on,
- \(\overline{ou}\) is the outermost index (to the left of softmax axis),
- \(\overline{in}\) is the innermost index (to the right of softmax axis), and
\(\nu\) is used to produce more accurate results and defined as:

\[ \nu(\overline{ou}, \overline{in}) = \max\limits_{ic} src(\overline{ou}, ic, \overline{in}) \]

There is no difference between the dnnl_forward_training and dnnl_forward_inference propagation kinds.

The backward propagation computes \(diff\_src(ou, c, in)\), based on \(diff\_dst(ou, c, in)\) and \(dst(ou, c, in)\).

- Both forward and backward propagation support in-place operations, meaning that
`src`

can be used as input and output for forward propagation, and`diff_dst`

can be used as input and output for backward propagation. In case of in-place operation, the original data will be overwritten.

The softmax primitive doesn't support any post-ops or attributes.

The softmax primitive supports the following combinations of data types:

Propagation | Source / Destination |
---|---|

forward / backward | f32 |

forward | f16 |

The softmax primitive works with arbitrary data tensors. There is no special meaning associated with any logical dimensions. However, the softmax axis is typically referred to as channels (hence in formulas we use \(c\)).

- No primitive specific limitations. Refer to Data Types for limitations related to data types support.

- Use in-place operations whenever possible.
- Currently the softmax primitive is optimized for the cases where the dimension of the softmax axis is physically dense. For instance:
- Optimized: 2D case, tensor \(A \times B\), softmax axis 1 (B), format tag dnnl_ab
- Optimized: 4D case, tensor \(A \times B \times C \times D\), softmax axis 3 (D), format tag dnnl_abcd
- Optimized: 4D case, tensor \(A \times B \times C \times D\), softmax axis 1 (B), format tag dnnl_abcd, and \(C = D = 1\)
- Optimized: 4D case, tensor \(A \times B \times C \times D\), softmax axis 1 (B), format tag dnnl_acdb or dnnl_aBcd16b, and \(C \cdot D \ne 1\)
- Non-optimized: 2D case, tensor \(A \times B\), softmax axis 0 (A), format tag dnnl_ab, and \(B \ne 1\)
- Non-optimized: 2D case, tensor \(A \times B\), softmax axis 1 (B), format tag dnnl_ba, and \(A \ne 1\)
- Non-optimized: 4D case, tensor \(A \times B \times C \times D\), softmax axis 2 (C), format tag dnnl_acdb, and and \(D \cdot B \ne 1\)