ShapeBuilder

onnx.shape_inference.infer_shapes() tries to infer shapes and types based on input shapes. It does not supports formulas and introduces new symbols. Examples 201: Better shape inference compares shape inference from onnx and the shape inference provided by class BasicShapeBuilder.

This class walks through all nodes and looks into a list of functions computing the output shapes based on the node type. It tries as much as possible to express the new shape with formulas based on the dimensions used to defined the inputs. The list of functions is available in experimental_experiment.xshape.shape_type_compute called from class _InferenceRuntime.

While doing this, every function may try to compute some tiny constants in _BuilderRuntime. This is used by _ShapeRuntime to deduce some shapes.

For example, if X has shape ("d1", 2) then Shape(X, start=1) is constant [2]. This can be later used to infer the shape after a reshape.

After getting an expression, a few postprocessing are applied to reduce its complexity. This relies on ast. It is done by function simplify_expression. d + f - f is replaced by d.