Convolution

Intel® oneAPI Deep Neural Network Developer Guide and Reference

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Date 12/16/2022

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Convolution

API Reference

General

The convolution primitive computes forward, backward, or weight update for a batched convolution operation on 1D, 2D, or 3D spatial data with bias.

The convolution operation is defined by the following formulas. We show formulas only for 2D spatial data which are straightforward to generalize to cases of higher and lower dimensions. Variable names follow the standard Naming Conventions.

NOTE:

Mathematical operation commonly called “convolution” in the context of deep learning workloads is actually cross-correlation.

Let , and be , , and tensors respectively. Let be a 1D tensor with elements.

Furthermore, let the remaining convolution parameters be:

Parameter	Depth	Height	Width	Comment
Padding: Front, top, and left				In the API we use `padding_l` to indicate the corresponding vector of paddings ( `_l` in the name stands for left )
Padding: Back, bottom, and right				In the API we use `padding_r` to indicate the corresponding vector of paddings ( `_r` in the name stands for right )
Stride				Convolution without strides is defined by setting the stride parameters to 1
Dilation				Non-dilated convolution is defined by setting the dilation parameters to 0

The following formulas show how oneDNN computes convolutions. They are broken down into several types to simplify the exposition, but in reality the convolution types can be combined.

To further simplify the formulas, we assume that if , or , or , or .

Forward

Regular Convolution

Here:

Convolution with Groups

In the API, oneDNN adds a separate groups dimension to memory objects representing tensors and represents weights as 5D tensors for 2D convolutions with groups.

where

,
, and

The case when is also known as a depthwise convolution.

Convolution with Dilation

Here:

where , and
where .

Deconvolution (Transposed Convolution)

Deconvolutions (also called fractionally strided convolutions or transposed convolutions) work by swapping the forward and backward passes of a convolution. One way to put it is to note that the weights define a convolution, but whether it is a direct convolution or a transposed convolution is determined by how the forward and backward passes are computed.

Difference Between Forward Training and Forward Inference

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

Backward

The backward propagation computes based on and .

The weights update computes and based on and .

NOTE:

The optimized memory formats

and

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
	DNNL_ARG_SRC
	DNNL_ARG_WEIGHTS
	DNNL_ARG_BIAS
	DNNL_ARG_DST
	DNNL_ARG_DIFF_SRC
	DNNL_ARG_DIFF_WEIGHTS
	DNNL_ARG_DIFF_BIAS
	DNNL_ARG_DIFF_DST
	DNNL_ARG_ATTR_POST_OP_DW
	DNNL_ARG_ATTR_MULTIPLE_POST_OP(binary_post_op_position) \| DNNL_ARG_SRC_1

Implementation Details

General Notes

N/A.

Data Types

Convolution primitive supports the following combination of data types for source, destination, and weights memory objects:

Propagation	Source	Weights	Destination	Bias
forward	f32	f32	f32, u8, s8	f32
forward	f16	f16	f16, f32, u8, s8	f16, f32
forward	u8, s8	s8	u8, s8, s32, f32, f16, bf16	u8, s8, s32, f32, f16, bf16
forward	bf16	bf16	f32, bf16	f32, bf16
forward	f64	f64	f64	f64
backward	f32, bf16	bf16	bf16
backward	f32, f16	f16	f16
backward	f32	f32	f32	f32
backward	f64	f64	f64	f64
weights update	bf16	f32, bf16	bf16, s8, u8	f32, bf16
weights update	f16	f32, f16	f16	f32, f16

WARNING:

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

Data Representation

Like other CNN primitives, the convolution primitive expects the following tensors:

Spatial	Source / Destination	Weights
1D
2D
3D

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

While convolution primitives can be created with memory formats specified explicitly, the performance may be suboptimal. The table below shows the combinations of memory formats the convolution primitive is optimized for.

Source / Destination	Weights	Limitations
`any`	`any`	N/A
dnnl_nwc , dnnl_nhwc , dnnl_ndhwc	`any`	N/A
dnnl_nwc , dnnl_nhwc , dnnl_ndhwc	dnnl_wio , dnnl_hwio , dnnl_dhwio	Only on GPUs with Xe-HPC architecture only
dnnl_ncw , dnnl_nchw , dnnl_ncdhw	`any`	Only on CPU

Post-ops and Attributes

Post-ops and attributes enable you to modify the behavior of the convolution primitive by applying the output scale to the result of the primitive and by chaining certain operations after the primitive. The following attributes and post-ops are supported:

Propagation	Type	Operation	Description	Restrictions
forward	attribute	Scale	Scales the result of convolution by given scale factor(s)	int8 convolutions only
forward	attribute	Zero points	Sets zero point(s) for the corresponding tensors	int8 convolutions 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	Depthwise	Applies a Convolution operation to the result	See a separate section

The following masks are supported by the primitive:

0, which applies one zero point value to an entire tensor, and
2, which applies a zero point value per each element in a IC or OC dimension for DNNL_ARG_SRC or DNNL_ARG_DST arguments respectively.

When scales and/or zero-points masks are specified, the user must provide the corresponding scales and/or zero-points as additional input memory objects with argument DNNL_ARG_ATTR_SCALES | DNNL_ARG_${MEMORY_INDEX} or DNNL_ARG_ATTR_ZERO_POINTS | DNNL_ARG_${MEMORY_INDEX} during the execution stage. For instance, a source tensor zero points memory argument would be passed with index (DNNL_ARG_ATTR_ZERO_POINTS | DNNL_ARG_SRC).

NOTE:

The library does not prevent using post-ops in training, but note that not all post-ops are feasible for training usage. For instance, using ReLU with non-zero negative slope parameter as a post-op would not produce an additional output workspace that is required to compute backward propagation correctly. Hence, in this particular case one should use separate convolution and eltwise primitives for training.

The library supports any number and order of post operations, but only the following sequences deploy optimized code:

Type of convolutions	Post-ops sequence supported
f32, bf16 and f16 convolution	eltwise, sum, sum -> eltwise
int8 convolution	eltwise, sum, sum -> eltwise, eltwise -> sum

The operations during attributes and post-ops applying are done in single precision floating point data type. The conversion to the actual destination data type happens just before the actual storing.

Example 1

Consider the following pseudo-code:

primitive_attr attr;
attr.set_output_scale(mask=0, alpha);
attr.set_post_ops({
        { sum={scale=beta} },
        { eltwise={scale=gamma, type=tanh, alpha=ignore, beta=ignored } }
    });

convolution_forward(src, weights, dst, attr);

The would lead to the following:

Example 2

The following pseudo-code:

primitive_attr attr;
attr.set_output_scale(mask=0, alpha);
attr.set_post_ops({
        { eltwise={scale=gamma, type=relu, alpha=eta, beta=ignored } },
        { sum={scale=beta} }
    });

convolution_forward(src, weights, dst, attr);

That would lead to the following:

Example 3

The following pseudo-code:

primitive_attr attr;
attr.set_output_scale(mask=0, alpha);
attr.set_zero_point(src, mask=0, shift_src);
attr.set_zero_point(dst, mask=0, shift_dst);
attr.set_post_ops({
        { eltwise={scale=gamma, type=relu, alpha=eta, beta=ignored } }
    });

convolution_forward(src, weights, dst, attr);

That would lead to the following:

Algorithms

oneDNN implements convolution primitives using several different algorithms:

Direct. The convolution operation is computed directly using SIMD instructions. This is the algorithm used for the most shapes and supports int8, f32, bf16, f16 and f64 (only on GPU engine) data types.
Winograd. This algorithm reduces computational complexity of convolution at the expense of accuracy loss and additional memory operations. The implementation is based on the Fast Algorithms for Convolutional Neural Networks by A. Lavin and S. Gray. The Winograd algorithm often results in the best performance, but it is applicable only to particular shapes. Moreover, Winograd only supports f32 and f16 data types.
Implicit GEMM. The convolution operation is reinterpreted in terms of matrix-matrix multiplication by rearranging the source data into a scratchpad memory. This is a fallback algorithm that is dispatched automatically when other implementations are not available. GEMM convolution supports the int8, f32, and bf16 data types.

Direct Algorithm

oneDNN supports the direct convolution algorithm on all supported platforms for the following conditions:

Data and weights memory formats are defined by the convolution primitive (user passes any).
The number of channels per group is a multiple of SIMD width for grouped convolutions.
For each spatial direction padding does not exceed one half of the corresponding dimension of the weights tensor.
Weights tensor width does not exceed 14.

In case any of these constraints are not met, the implementation will silently fall back to an explicit GEMM algorithm.

Winograd Convolution

oneDNN supports the Winograd convolution algorithm on systems with Intel(R) Advanced Vector Extensions 512 (Intel(R) AVX-512) support and Intel Deep Learning Boost (Intel DL Boost) under the following conditions:

Source, weights and destination data type is f32
Data and weights memory formats are defined by the convolution primitive (user passes any as the data format).
The spatial domain is two-dimensional.
The weights shape is 3x3, there are no groups, dilation or strides (, , and ).

The Winograd convolution algorithm implementation additionally chooses tile size based on the problem shape and propagation kind :

For forward_inference oneDNN supports or
oneDNN supports only Winograd for all the training propagation kinds.

The following side effects should be weighed against the (potential) performance boost achieved from using the Winograd algorithm:

Memory consumption. Winograd implementation in oneDNN requires additional scratchpad memory to store intermediate results. As more convolutions using Winograd are added to the topology, the amount of memory required can grow significantly. This growth can be controlled if the scratchpad memory can be reused across multiple primitives. See Primitive Attributes: Scratchpad for more details.
Accuracy. In some cases Winograd convolution produce results that are significantly less accurate than results from the direct convolution.

Create a Winograd convolution by simply creating a convolution primitive descriptor (step 6 in simple network example specifying the Winograd algorithm. The rest of the steps are exactly the same.

auto conv1_pd = convolution_forward::primitive_desc(engine,
    prop_kind::forward_inference, algorithm::convolution_winograd,
    conv1_src_md, conv1_weights_md, conv1_bias_md, conv1_dst_md,
    conv1_strides, conv1_padding_l, conv1_padding_r);

Automatic Algorithm Selection

oneDNN supports dnnl::algorithm::convolution_auto algorithm that instructs the library to automatically select the best algorithm based on the heuristics that take into account tensor shapes and the number of logical processors available. (For automatic selection to work as intended, use the same thread affinity settings when creating the convolution as when executing the convolution.)

Implementation Limitations

Refer to Data Types for limitations related to data types support.
See Winograd Convolution section for limitations of Winograd algorithm implementations.
GPU
- Depthwise post-op is not supported

Performance Tips

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

Example

Convolution Primitive Example

This C++ API example demonstrates how to create and execute a Convolution primitive in forward propagation mode in two configurations - with and without groups.

Key optimizations included in this example:

Creation of optimized memory format from the primitive descriptor;
Primitive attributes with fused post-ops.

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Intel® oneAPI Deep Neural Network Developer Guide and Reference

Convolution

General

Execution Arguments

Implementation Details

Algorithms

Implementation Limitations

Performance Tips

Example