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Inner Product

API Reference

General

The inner product primitive (sometimes called fully connected) treats each activation in the minibatch as a vector and computes its product with a weights 2D tensor producing a 2D tensor as an output.

Forward

More precisely, let \(\src\), \(\weights\), \(\bias\) and \(\dst\) be \(N \times IC\), \(OC \times IC\), \(OC\), and \(N \times OC\) tensors, respectively (variable names follow the standard Naming Conventions). Then:

\[\dst(n, oc) = \bias(oc) + \sum_{ic=0}^{IC-1} \src(n, ic) \cdot \weights(oc, ic)\]

In cases where the \(\src\) and \(\weights\) tensors have spatial dimensions, they are flattened to 2D. For example, if they are 4D \(N \times IC' \times IH \times IW\) and \(OC \times IC' \times KH \times KW\) tensors, then the formula above is applied with \(IC = IC' \cdot IH \cdot IW\). In such cases, the \(\src\) and \(\weights\) tensors must have equal spatial dimensions (e.g. \(KH = IH\) and \(KW = IW\) for 4D tensors).

Difference Between Forward Training and Forward Inference

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

Backward

The backward propagation computes \(\diffsrc\) based on \(\diffdst\) and \(\weights\).

The weights update computes \(\diffweights\) and \(\diffbias\) based on \(\diffdst\) and \(\src\).

Note

The optimized memory formats \(\src\) and \(\weights\) might be different on forward propagation, backward propagation, and weights update.

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
\(\weights\)	DNNL_ARG_WEIGHTS
\(\bias\)	DNNL_ARG_BIAS
\(\dst\)	DNNL_ARG_DST
\(\diffsrc\)	DNNL_ARG_DIFF_SRC
\(\diffweights\)	DNNL_ARG_DIFF_WEIGHTS
\(\diffbias\)	DNNL_ARG_DIFF_BIAS
\(\diffdst\)	DNNL_ARG_DIFF_DST
\(\text{binary post-op}\)	DNNL_ARG_ATTR_MULTIPLE_POST_OP(binary_post_op_position) | DNNL_ARG_SRC_1
\(\text{prelu post-op}\)	DNNL_ARG_ATTR_MULTIPLE_POST_OP(prelu_post_op_position) | DNNL_ARG_WEIGHTS

Implementation Details

General Notes

N/A.

Data Types

Inner product primitive supports the following combination of data types for source, destination, weights, and bias:

Propagation	Source	Weights	Destination	Bias
forward / backward	f32	f32	f32	f32
forward	f16	f16	f32, f16, u8, s8	f16, f32
forward	u8, s8	s8	u8, s8, s32, bf16, f32	u8, s8, s32, bf16, f32
forward	bf16	bf16	f32, bf16	f32, bf16
backward	f32, bf16	bf16	bf16
backward	f32, f16	f16	f16
weights update	bf16	f32, bf16	bf16	f32, bf16
weights update	f16	f32, f16	f16	f32, f16

Data Representation

Like other CNN primitives, the inner product primitive expects the following tensors:

Spatial	Source	Destination	Weights
1D	\(N \times C \times W\)	\(N \times C\)	\(OC \times IC \times KW\)
2D	\(N \times C \times H \times W\)	\(N \times C\)	\(OC \times IC \times KH \times KW\)
3D	\(N \times C \times D \times H \times W\)	\(N \times C\)	\(OC \times IC \times KD \times KH \times KW\)

Memory format of data and weights memory objects is critical for inner product primitive performance. In the oneDNN programming model, inner product primitive is one of the few primitives that support the placeholder format dnnl::memory::format_tag::any (shortened to any from now on) and can define data and weight memory objects formats based on the primitive parameters. When using any it is necessary to first create an inner product primitive descriptor and then query it for the actual data and weight memory objects formats.

The table below shows the combinations of memory formats the inner product primitive is optimized for. For the destination tensor (which is always \(N \times C\)) the memory format is always dnnl::memory::format_tag::nc (dnnl::memory::format_tag::ab).

Source / Destination	Weights	Limitations
any	any	N/A
dnnl_nc , dnnl_nwc , dnnl_nhwc , dnnl_ndhwc	any	N/A
dnnl_nc , dnnl_ncw , dnnl_nchw , dnnl_ncdhw	any	N/A
dnnl_nc , dnnl_nwc , dnnl_nhwc , dnnl_ndhwc	dnnl_io , dnnl_wio , dnnl_hwio , dnnl_dhwio	N/A
dnnl_nc , dnnl_ncw , dnnl_nchw , dnnl_ncdhw	dnnl_oi , dnnl_oiw , dnnl_oihw , dnnl_oidhw	N/A

Post-Ops and Attributes

Post-ops and attributes enable you to modify the behavior of the inner product primitive by chaining certain operations after the inner product operation. The following post-ops are supported by inner product primitives:

Propagation	Type	Operation	Description	Restrictions
forward	attribute	Scales	Scales the result of inner product by given scale factor(s)	int8 inner products only
forward	post-op	Eltwise	Applies an Eltwise operation to the result
forward	post-op	Sum	Adds the operation result to the destination tensor instead of overwriting it
forward	post-op	Binary	Applies a Binary operation to the result	General binary post-op restrictions
forward	post-op	Prelu	Applies an PReLU operation to the result

The following masks are supported by the primitive:

• 0, which applies one scale value to an entire tensor, and

• 1, which applies a scale value per output channel for DNNL_ARG_WEIGHTS argument.

When scales masks are specified, the user must provide the corresponding scales as additional input memory objects with argument DNNL_ARG_ATTR_SCALES | DNNL_ARG_${MEMORY_INDEX} during the execution stage.

Implementation Limitations

1. Check Data Types.

2. The CPU engine does not support u8 or s8 data type for dst with f16 src and weights.

Performance Tips

• Use dnnl::memory::format_tag::any for source, weights, and destinations memory format tags when create an inner product primitive to allow the library to choose the most appropriate memory format.

Example

Inner Product Primitive Example

This C++ API example demonstrates how to create and execute an Inner Product primitive.

Key optimizations included in this example:

• Primitive attributes with fused post-ops;

• Creation of optimized memory format from the primitive descriptor.

Convolution Matrix Multiplication