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. The variable names follow the standard Conventions.

Forward

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

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:

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

Difference Between Forward Training and Forward Inference

There is no difference between the forward_training and forward_inference propagation kinds.

Backward

The backward propagation computes \(\diffsrc(ou, c, in)\), based on \(\diffdst(ou, c, in)\) and \(\dst(ou, c, in)\).

Execution Arguments

When executed, the inputs and outputs should be mapped to an execution argument index as specified by the following table.

Primitive input/output	Execution argument index
\(\src\)	DNNL_ARG_SRC
\(\dst\)	DNNL_ARG_DST
\(\diffsrc\)	DNNL_ARG_DIFF_SRC
\(\diffdst\)	DNNL_ARG_DIFF_DST

Operation Details

1. 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.

Post-ops and Attributes

The softmax primitive does not have to support any post-ops or attributes.

Data Types Support

The softmax primitive supports the following combinations of data types.

Note

Here we abbreviate data types names for readability. For example, dnnl::memory::data_type::f32 is abbreviated to f32.

Propagation	Source / Destination
forward / backward	bf16, f32
forward	f16

Data Representation

Source, Destination, and Their Gradients

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 \(c\)).

API

struct softmax_forward : public dnnl::primitive

Softmax forward propagation primitive.

Public Functions

softmax_forward()

Default constructor. Produces an empty object.

softmax_forward(const primitive_desc &pd)

Constructs a softmax forward propagation primitive.

Parameters

pd – Primitive descriptor for a softmax forward propagation primitive.

struct desc

Descriptor for a softmax forward propagation primitive.

Public Functions

desc()

Default constructor. Produces an empty object.

desc(prop_kind aprop_kind, const memory::desc &data_desc, int softmax_axis)

Constructs a descriptor for a softmax forward propagation primitive.

Parameters

• aprop_kind – Propagation kind. Possible values are dnnl::prop_kind::forward_training, and dnnl::prop_kind::forward_inference.

• data_desc – Source and destination memory descriptor.

• softmax_axis – Axis over which softmax is computed.

struct primitive_desc : public dnnl::primitive_desc

Primitive descriptor for a softmax forward propagation primitive.

Public Functions

primitive_desc()

Default constructor. Produces an empty object.

primitive_desc(const desc &adesc, const engine &aengine, bool allow_empty = false)

Constructs a primitive descriptor for a softmax forward propagation primitive.

Parameters

• adesc – descriptor for a softmax forward propagation primitive.

• aengine – Engine to use.

• allow_empty – A flag signifying whether construction is allowed to fail without throwing an exception. In this case an empty object will be produced. This flag is optional and defaults to false.

primitive_desc(const desc &adesc, const primitive_attr &attr, const engine &aengine, bool allow_empty = false)

Constructs a primitive descriptor for a softmax forward propagation primitive.

Parameters

• adesc – Descriptor for a softmax forward propagation primitive.

• aengine – Engine to use.

• attr – Primitive attributes to use.

• allow_empty – A flag signifying whether construction is allowed to fail without throwing an exception. In this case an empty object will be produced. This flag is optional and defaults to false.

memory::desc src_desc() const

Returns a source memory descriptor.

Returns

Source memory descriptor.

Returns

A zero memory descriptor if the primitive does not have a source parameter.

memory::desc dst_desc() const

Returns a destination memory descriptor.

Returns

Destination memory descriptor.

Returns

A zero memory descriptor if the primitive does not have a destination parameter.

struct softmax_backward : public dnnl::primitive

Softmax backward propagation primitive.

Public Functions

softmax_backward()

Default constructor. Produces an empty object.

softmax_backward(const primitive_desc &pd)

Constructs a softmax backward propagation primitive.

Parameters

pd – Primitive descriptor for a softmax backward propagation primitive.

struct desc

Descriptor for a softmax backward propagation primitive.

Public Functions

desc()

Default constructor. Produces an empty object.

desc(const memory::desc &diff_data_desc, const memory::desc &data_desc, int softmax_axis)

Constructs a descriptor for a softmax backward propagation primitive.

Parameters

• diff_data_desc – Diff source and diff destination memory descriptor.

• data_desc – Destination memory descriptor.

• softmax_axis – Axis over which softmax is computed.

struct primitive_desc : public dnnl::primitive_desc

Primitive descriptor for a softmax backward propagation primitive.

Public Functions

primitive_desc()

Default constructor. Produces an empty object.

primitive_desc(const desc &adesc, const engine &aengine, const softmax_forward::primitive_desc &hint_fwd_pd, bool allow_empty = false)

Constructs a primitive descriptor for a softmax backward propagation primitive.

Parameters

• adesc – Descriptor for a softmax backward propagation primitive.

• aengine – Engine to use.

• hint_fwd_pd – Primitive descriptor for a softmax forward propagation primitive. It is used as a hint for deciding which memory format to use.

• allow_empty – A flag signifying whether construction is allowed to fail without throwing an exception. In this case an empty object will be produced. This flag is optional and defaults to false.

primitive_desc(const desc &adesc, const primitive_attr &attr, const engine &aengine, const softmax_forward::primitive_desc &hint_fwd_pd, bool allow_empty = false)

Constructs a primitive descriptor for a softmax backward propagation primitive.

Parameters

• adesc – Descriptor for a softmax backward propagation primitive.

• attr – Primitive attributes to use.

• aengine – Engine to use.

• hint_fwd_pd – Primitive descriptor for a softmax forward propagation primitive. It is used as a hint for deciding which memory format to use.

• allow_empty – A flag signifying whether construction is allowed to fail without throwing an exception. In this case an empty object will be produced. This flag is optional and defaults to false.

memory::desc dst_desc() const

Returns a destination memory descriptor.

Returns

Destination memory descriptor.

Returns

A zero memory descriptor if the primitive does not have a destination parameter.

memory::desc diff_src_desc() const

Returns a diff source memory descriptor.

Returns

Diff source memory descriptor.

Returns

A zero memory descriptor if the primitive does not have a diff source memory with.

memory::desc diff_dst_desc() const

Returns a destination memory descriptor.

Returns

Destination memory descriptor.

Returns

A zero memory descriptor if the primitive does not have a destination parameter.