Paddle/doc/fluid/design/ir/draft.md

Motivation

There is a gap between the Program defined by user and the Executable that can be scheduled efficiently on heterogeneous hardware, either locally or distributedly.

Usually, the gap is bridged by

• A serious transformations with defined order.

• These transformations usually involve insert, delete, clustering, split, dependency analysis.

• Has a simple way to verify and debug each transformation.

• Flexible to add, remove or customize transformations to fit the requirements of various algorithms (models) and hardware secenarios.

Some other events also push us to a better unified pattern.

• The deep learning framework is built around the concepts of graphs. To leverage tools such as compilation (e.g. TVM and nGraph) or cross-framework conversion (e.g. ONNX), we also need a intermediate representation that can be connected to the rest of the ecosystem.

We need a unified pattern to naturally support the requirements described above. The pattern should fit both training, inference and other offline serielized model transformations. Learned from LLVM and other deep learning framework, we draft the design below.

Design

Major Concepts

Node

Node represents an operation that performs some computation or a variable that is input or output of operation.

Nodes are connected to other Nodes via inputs and outputs.

Other properties (maybe device placement information) can be added to Node in the future if it's a common requirement of many other Passes. Otherwise, it should live in a Node wrapper class that is private to some Pass or be a local member of a Pass.

Graph

Graph contains a list of Nodes, which are connected to each other via inputs and outputs.

TODO: Better definitions for the graph.

Graph can also contain Attributes. Attributes can be any thing. For example, it can be a list of "wraper" nodes. The wrapper nodes compose Nodes and provide helper method for execution or transformation. Attribute can also contain other things that describe some properties of the Graph or Graph nodes. Attribute can be passed across Pass. However, it should be used with care.

Pass

Pass represents a transformation of Graph. Its input is a Graph and its output is also a Graph. For example, a Pass can simply print out the Graph. A Pass can also fuse some Graph's Nodes.

Optimize

Optimize contains a series of Pass with defined order. Optimize transforms a Graph that only contains raw modeling logic to a Graph that can be run efficiently while maintaining the original modeling logic.

Optimize Process

• Program is first converted to Graph.
• Graph goes through a series of Pass
• Graph is transformed from raw model logic to a form that is efficient to execute.

Program->ProgramToGraph->Graph->Pass1->Graph->Pass2->Graph->Pass3->Graph->Executor