openvino/docs/Extensibility_UG/ov_transformations.md

Overview of Transformations API

@sphinxdirective

.. toctree:: :maxdepth: 1 :hidden:

openvino_docs_Extensibility_UG_model_pass openvino_docs_Extensibility_UG_matcher_pass openvino_docs_Extensibility_UG_graph_rewrite_pass

@endsphinxdirective

OpenVINO Transformation mechanism allows to develop transformation passes to modify ov::Model. You can use this mechanism to apply additional optimizations to the original Model or transform unsupported subgraphs and operations to new operations which are supported by the plugin. This guide contains all necessary information that you need to start implementing OpenVINO™ transformations.

Working with Model

Before the moving to transformation part it is needed to say several words about functions which allow to modify ov::Model. This chapter extends the model representation guide and shows an API that allows us to manipulate with ov::Model.

Working with node input and output ports

First of all let's talk about ov::Node input/output ports. Each OpenVINO™ operation has input and output ports except cases when operation has Parameter or Constant type.

Every port belongs to its node, so using a port we can access parent node, get shape and type for particular input/output, get all consumers in case of output port, and get producer node in case of input port. With output port we can set inputs for newly created operations.

Lets look at the code example.

@snippet ov_model_snippets.cpp ov:ports_example

Node replacement

OpenVINO™ provides two ways for node replacement: via OpenVINO™ helper function and directly via port methods. We are going to review both of them.

Let's start with OpenVINO™ helper functions. The most popular function is ov::replace_node(old_node, new_node).

We will review real replacement case where Negative operation is replaced with Multiply.

@snippet ov_model_snippets.cpp ov:replace_node

ov::replace_node has a constraint that number of output ports for both of ops must be the same; otherwise, it raises an exception.

The alternative way to do the same replacement is the following:

@snippet ov_model_snippets.cpp ov:manual_replace

Another transformation example is insertion.

@snippet ov_model_snippets.cpp ov:insert_node

The alternative way to the insert operation is to make a node copy and use ov::replace_node():

@snippet ov_model_snippets.cpp ov:insert_node_with_copy

Node elimination

Another type of node replacement is its elimination.

To eliminate operation, OpenVINO™ has special method that considers all limitations related to OpenVINO™ Runtime.

@snippet ov_model_snippets.cpp ov:eliminate_node

ov::replace_output_update_name() in case of successful replacement it automatically preserves friendly name and runtime info.

Transformations types

OpenVINO™ Runtime has three main transformation types:

• Model pass - straightforward way to work with ov::Model directly
• Matcher pass - pattern-based transformation approach
• Graph rewrite pass - container for matcher passes needed for efficient execution

Transformation conditional compilation

Transformation library has two internal macros to support conditional compilation feature.

• MATCHER_SCOPE(region) - allows to disable the MatcherPass if matcher isn't used. The region name should be unique. This macro creates a local variable matcher_name which you should use as a matcher name.
• RUN_ON_MODEL_SCOPE(region) - allows to disable run_on_model pass if it isn't used. The region name should be unique.

Transformation writing essentials

When developing a transformation, you need to follow these transformation rules:

###1. Friendly Names

Each ov::Node has an unique name and a friendly name. In transformations we care only about friendly name because it represents the name from the model. To avoid losing friendly name when replacing node with other node or subgraph, set the original friendly name to the latest node in replacing subgraph. See the example below.

@snippet ov_model_snippets.cpp ov:replace_friendly_name

In more advanced cases, when replaced operation has several outputs and we add additional consumers to its outputs, we make a decision how to set friendly name by arrangement.

###2. Runtime Info

Runtime info is a map std::map<std::string, ov::Any> located inside ov::Node class. It represents additional attributes in ov::Node. These attributes can be set by users or by plugins and when executing transformation that changes ov::Model we need to preserve these attributes as they will not be automatically propagated. In most cases, transformations have the following types: 1:1 (replace node with another node), 1:N (replace node with a sub-graph), N:1 (fuse sub-graph into a single node), N:M (any other transformation). Currently, there is no mechanism that automatically detects transformation types, so we need to propagate this runtime information manually. See the examples below.

@snippet ov_model_snippets.cpp ov:copy_runtime_info

When transformation has multiple fusions or decompositions, ov::copy_runtime_info must be called multiple times for each case.

Note: copy_runtime_info removes rt_info from destination nodes. If you want to keep it, you need to specify them in source nodes like this: copy_runtime_info({a, b, c}, {a, b})

###3. Constant Folding

If your transformation inserts constant sub-graphs that need to be folded, do not forget to use ov::pass::ConstantFolding() after your transformation or call constant folding directly for operation. The example below shows how constant subgraph can be constructed.

@snippet ov_model_snippets.cpp ov:constant_subgraph

Manual constant folding is more preferable than ov::pass::ConstantFolding() because it is much faster.

Below you can find an example of manual constant folding:

@snippet src/transformations/template_pattern_transformation.cpp manual_constant_folding

Common mistakes in transformations

In transformation development process:

• Do not use deprecated OpenVINO™ API. Deprecated methods has the OPENVINO_DEPRECATED macros in its definition.
• Do not pass shared_ptr<Node> as an input for other node if type of node is unknown or it has multiple outputs. Use explicit output port.
• If you replace node with another node that produces different shape, remember that new shape will not be propagated until the first validate_nodes_and_infer_types call for ov::Model. If you are using ov::pass::Manager, it will automatically call this method after each transformation execution.
• Do not forget to call the ov::pass::ConstantFolding pass if your transformation creates constant subgraphs.
• Use latest OpSet if you are not developing downgrade transformation pass.
• When developing a callback for ov::pass::MatcherPass, do not change nodes that come after the root node in topological order.

Using pass manager

ov::pass::Manager is a container class that can store the list of transformations and execute them. The main idea of this class is to have high-level representation for grouped list of transformations. It can register and apply any transformation pass on model. In addition, ov::pass::Manager has extended debug capabilities (find more information in the how to debug transformations section).

The example below shows basic usage of ov::pass::Manager

@snippet src/transformations/template_pattern_transformation.cpp matcher_pass:manager3

Another example shows how multiple matcher passes can be united into single GraphRewrite.

@snippet src/transformations/template_pattern_transformation.cpp matcher_pass:manager2

How to debug transformations

If you are using ngraph::pass::Manager to run sequence of transformations, you can get additional debug capabilities by using the following environment variables:

OV_PROFILE_PASS_ENABLE=1 - enables performance measurement for each transformation and prints execution status
OV_ENABLE_VISUALIZE_TRACING=1 -  enables visualization after each transformation. By default, it saves dot and svg files.

Note

: Make sure that you have dot installed on your machine; otherwise, it will silently save only dot file without svg file.

See Also

• OpenVINO™ Model Representation
• OpenVINO™ Extensions