The `inference/analysis` module is used to analyze and optimize the inference program,
it references some philosophy from `LLVM/analysis`,
and make the various optimization features be pluggable and co-exist in a pipeline.
We borrowed some concepts from LLVM, such as
-[Pass](./pass.h)es to implement optimization that traverse the inference program,
-[DataFlowGraph](./data_flow_graph.h) to represent the data flow graph built from a program,
-[PassManager](./pass_manager.h) to manage a sequence of `Pass`es over a graph.
There are some other basic concepts here
-[Node](./node.h), the node in a `DataFlowGraph`,
-`Function`, the Operator in Fluid,
-`Value`, the Variable in Fluid;
-[Argument](./argument.h), the argument that treat as the input and output of all `Pass`es in the pipeline,
## How it works
The `inference/analysis` module make all the passes in a pipeline, and works in such way:
1. Build a `DataFlowGraph` from a Fluid inference ProgramDesc,
2. Call the middle passes one by one, the same `DataFlowGraph` is passed across all the passes,
3. Transform a new ProgramDesc from the modified `DataFlowGraph`.
The new optimization features can be added as an independent `Pass` and controlled by gflags,
each pass will generate unified debug information or visualization for better debugging.
## Supported Passes
### `FluidToDataFlowGraphPass`
Transform the fluid `ProgramDesc` to a `DataFlowGraph` to give an abstract representation for all the middle passes,
this should be the first pass of the pipeline.
### `DataFlowGraphToFluidPass`
Generate a final `ProgramDesc` from a data flow graph, this should be the last pass of the pipeline.
### `TensorRTSubgraphNodeMarkPass`
Mark the `Node` that are supported by TensorRT,
this pass will generate a visualization file which can be used for debugging.
### `TensorRTSubGraphPass`
Split the sub-graph that are can be accelerated by TensorRT.
### `DFG_GraphvizDrawPass`
This pass is just for debug, it will visualize the `DataFlowGraph` using the [graphviz](http://www.graphviz.org) tool.
It can be used as a helper class that draws the modified graph after each pass.
## Utilities
There is some helper function/class for analysis.
-[dot.h](./dot.h) give a easy to use interface for generating `DOT` codes,
-[graph_traits.h](./graph_traits.h) contains the graph traversal algorithms, it uses `iterator` to make the algorithms easy to share across different passes.
