Shuffle

API reference: C, C++

The shuffle primitive shuffles data along the shuffle axis (here is designated as \(C\)) with the group parameter \(G\). Namely, the shuffle axis is thought to be a 2D tensor of size \((\frac{C}{G} \times G)\) and it is being transposed to \((G \times \frac{C}{G})\).

The formal definition is shown below:

Forward

\[ dst(\overline{ou}, c, \overline{in}) = src(\overline{ou}, c', \overline{in}) \]

where

• \(c\) dimension is called a shuffle axis,
• \(G\) is a group_size,
• \(\overline{ou}\) is the outermost indices (to the left from shuffle axis),
• \(\overline{in}\) is the innermost indices (to the right from shuffle axis), and

• \(c'\) and \(c\) relate to each other as define by the system:

  \[ \begin{cases} c &= u + v\frac{C}{G}, \\ c' &= uG + v, \\ \end{cases} \]

Here, \(u \in [0, \frac{C}{G})\) and \(v \in [0, G)\).

Difference Between Forward Training and Forward Inference

There is no difference between the mkldnn_forward_training and mkldnn_forward_inference propagation kinds.

Backward

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

Essentially, backward propagation is the same as forward propagation with \(g\) replaced by \(C / g\).

Implementation Details

General Notes

1. The memory format and data type for src and dst are assumed to be the same, and in the API are typically referred as data (e.g., see data_desc in mkldnn::shuffle_forward::desc::desc()). The same holds for diff_src and diff_dst. The corresponding memory descriptors are referred to as diff_data_desc.

Data Types

The shuffle primitive supports the following combinations of data types:

Propagation	Sou
forward / backward	f32
forward	s32, s8, u8

Warning

There might be hardware and/or implementation specific restrictions. Check Implementation Limitations section below.

Data Layouts

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

Shuffle operation typically appear in CNN topologies. Hence, in the library the shuffle primitive is optimized for the corresponding memory formats:

Spatial	Logical tensor	Shuffle Axis	Implementations optimized for memory formats
2D	NCHW	1 (C)	mkldnn_nchw (mkldnn_abcd), mkldnn_nhwc (mkldnn_acdb), optimized^
3D	NCDHW	1 (C)	mkldnn_ncdhw (mkldnn_abcde), mkldnn_ndhwc (mkldnn_acdeb), optimized^

Here optimized^ means the format that comes out of any preceding compute-intensive primitive.

Post-ops and Attributes

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

Implementation Limitations

1. Refer to Data Types for limitations related to data types support.

Performance Tips

N/A