Module encoder 

ONNX protobuf encoding — the inverse of graph_builder and weights (§19.1, §55.4 dump path).

Serialises an onnx_runtime_ir::Graph (plus model-level metadata that the IR does not itself store) back into an ONNX ModelProto, then to protobuf bytes via prost. This is the foundational capability the EPContext writer (§55.4) builds on, but it is deliberately model-agnostic: it hardcodes no op type, vendor, or model name.

Round-trip contract

Everything the load path (decode → build → weights) preserves survives an encode → decode round-trip byte-for-byte:

• nodes: op_type, domain, input/output order (incl. skipped optional slots), attributes, doc_string;
• graph inputs / outputs / interior value_info (dtype + static & symbolic dims, symbols re-emitted by their interned name);
• initializers: all supported dtypes, raw little-endian bytes byte-exact (including STRING payloads);
• opset imports, ir_version, producer fields, model doc_string, metadata_props.

STRING attributes are byte-preserving (§55.3/§55.4)

ONNX STRING attributes are arbitrary byte strings — a compiled-vendor blob, a relative path, or text — that are not guaranteed to be valid UTF-8. The IR stores them as raw bytes (Attribute::String), so both decode and encode round-trip the bytes exactly with zero op or attribute-name knowledge. The EPContext ep_cache_context opaque blob is preserved purely by this generic mechanism — the encoder contains no op-specific branch.

Fields deliberately not encoded

The IR Graph does not model these, so they cannot be reproduced and are emitted empty / default:

• TrainingInfoProto, FunctionProto, sparse initializers, quantization annotations (not represented in the IR). Model-local functions ARE supported on the load path — crate::function_inline expands every function call into its primitive body before the IR is built — but the IR Graph does not retain the original FunctionProto declarations, so they are not re-emitted here;
• nested GraphProto.name values — the IR does not retain graph names for control-flow bodies, so nested graphs are emitted with an empty name.

Structs

Model

An IR Graph bundled with the model-level metadata and live weight bytes needed to encode a complete ONNX ModelProto.

ModelMetadata

Model-level metadata that the IR Graph does not itself carry.

Constants

DEFAULT_IR_VERSION

Default ONNX ir_version stamped when ModelMetadata does not override it (IR version 10, the version paired with opset 21).

Functions

encode_model

Encode model into serialized ONNX protobuf bytes.

encode_model_proto

Build the ModelProto for model without serialising it (useful when the caller wants to mutate the proto before encoding, e.g. the §55.4 writer splicing in EPContext nodes).

write_model

Encode model and write the serialized bytes to path.