emx-onnx-cgen 0.3.3

emmtrix ONNX-to-C Code Generator

emmtrix ONNX-to-C Code Generator (emx-onnx-cgen)

emx-onnx-cgen compiles ONNX models to portable, deterministic C code for deeply embedded systems. The generated code is designed to run without dynamic memory allocation, operating-system services, or external runtimes, making it suitable for safety-critical and resource-constrained targets.

Key characteristics:

• No dynamic memory allocation (malloc, free, heap usage)
• Static, compile-time known memory layout for parameters, activations, and temporaries
• Deterministic control flow (explicit loops, no hidden dispatch or callbacks)
• No OS dependencies, using only standard C headers (for example, stdint.h and stddef.h)
• Single-threaded execution model
• Bitwise-stable code generation for reproducible builds
• Readable, auditable C code suitable for certification and code reviews
• Designed for bare-metal and RTOS-based systems

Goals

• Correctness-first compilation with outputs comparable to ONNX Runtime.
• Deterministic and reproducible C code generation.
• Clean, pass-based compiler architecture (import → normalize → optimize → lower → emit).
• Minimal C runtime with explicit, predictable data movement.

Non-goals

• Aggressive performance optimizations in generated C.
• Implicit runtime dependencies or dynamic loading.
• Training/backpropagation support.

Features

• CLI for ONNX-to-C compilation and verification.
• Deterministic codegen with explicit tensor shapes and loop nests.
• Minimal C runtime templates in templates/.
• ONNX Runtime comparison for end-to-end validation.
• Official ONNX operator coverage tracking.
• Support for a wide range of ONNX operators (see OFFICIAL_ONNX_FILE_SUPPORT.md).
• Supported data types:
  • float, double, float16
  • int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t
  • bool
• Optional support for dynamic dimensions using C99 variable-length arrays (VLAs), when the target compiler supports them.

Installation

Install the package directly from PyPI (recommended):

pip install emx-onnx-cgen

Optional for verification and tests:

• onnxruntime
• numpy
• A C compiler (cc, gcc, clang or via --cc)

Quickstart

Compile an ONNX model into a C source file:

emx-onnx-cgen compile path/to/model.onnx build/model.c

Verify an ONNX model end-to-end against ONNX Runtime (default):

emx-onnx-cgen verify path/to/model.onnx

CLI Reference

emx-onnx-cgen provides two subcommands: compile and verify.

compile

emx-onnx-cgen compile <model.onnx> <output.c> [options]

Options:

• --template-dir: Directory containing the C templates (default: templates).
• --model-name: Override the generated model name (default: output file stem).
• --emit-testbench: Emit a JSON-producing main() testbench for validation.
• --emit-data-file: Emit constant data arrays into a companion _data C file.
• --large-weight-threshold: Store weights larger than this element count in a binary file (default: 1048576; set to 0 to disable).
• --large-temp-threshold-bytes: Mark temporary buffers larger than this threshold as static (default: 1024).
• --no-restrict-arrays: Disable restrict qualifiers on generated array parameters.

verify

emx-onnx-cgen verify <model.onnx> [options]

Options:

• --template-dir: Directory containing the C templates (default: templates).
• --model-name: Override the generated model name (default: model file stem).
• --cc: Explicit C compiler command for building the testbench binary.
• --large-weight-threshold: Store weights larger than this element count in a binary file (default: 1024).
• --large-temp-threshold-bytes: Mark temporary buffers larger than this threshold as static (default: 1024).
• --max-ulp: Maximum allowed ULP distance for floating outputs (default: 100).
• --runtime: Runtime backend for verification (onnxruntime or onnx-reference, default: onnx-reference).

How verification works:

1. Compile with a testbench: the compiler is invoked with --emit-testbench, generating a C program that runs the model and prints inputs/outputs as JSON.
2. Build and execute: the testbench is compiled with the selected C compiler (--cc, CC, or a detected cc/gcc/clang) and executed in a temporary directory.
3. Run runtime backend: the JSON inputs from the testbench are fed to the selected runtime (onnxruntime or onnx-reference) using the same model.
4. Compare outputs: floating outputs are compared by maximum ULP distance (see https://www.emmtrix.com/wiki/ULP_Difference_of_Float_Numbers for the ULP definition and algorithm); non-floating outputs must match exactly. Missing outputs or mismatches are treated as failures.
5. ORT unsupported models: when using onnxruntime, if ORT reports NOT_IMPLEMENTED, verification is skipped with a warning (exit code 0).

Output

By default, the compiler emits a single C source file that includes:

• A generated entry point that mirrors the ONNX graph inputs/outputs.
• Tensor buffers for constants and temporaries.

When --emit-data-file is enabled, the main C source declares constant arrays as extern, and a second file named like the output with a _data suffix contains the constant definitions.

When --large-weight-threshold is set and a weight exceeds the threshold, the compiler emits a <model>.bin file with weights packed contiguously and generates a <model>_load helper that loads weights from the binary file at runtime.

Official ONNX test coverage

See OFFICIAL_ONNX_FILE_SUPPORT.md for the generated support matrix. See SUPPORT_OPS.md for operator-level support derived from the expectation JSON files.

Maintained by

This project is maintained by emmtrix.