onnx2c

Onnx2c is a ONNX to C compiler. It will read an ONNX file, and generate C code to be included in your project.

Onnx2c's target is "Tiny ML", meaning running the inference on microcontrollers. To make this easier, the generated code:

• Does not #include <stdio.h> (i.e. no printf()s)
• Compile-time allocates buffers. Does not use dynamic memory allocation or (much) stack memory
• Has no library requirements except standard C maths library. (Floating point hardware recommended!)
• Should be compiler-friendly allowing the C compiler optimize the output as well as it can
• Is contained in one single C file for easier project management

The idea behind onnx2c is to be an easy-to-use tool with no learning curve. If you can export your trained neural network to an ONNX file (e.g. PyTorch and Tensorflow both can) and you have a working microcontroller project, then joining the two with onnx2c should be easy.

To make all of the above easier to achieve, there are some non-goals for onnx2c:

• ONNX specification coverage. (For now, 91 out of 166 ONNX Operands are at least partially implemented).
• accelerators
• backpropagation (i.e. training)

Building

Make sure you have ProtocolBuffers libraries installed, e.g.:

• Ubuntu: apt install libprotobuf-dev protobuf-compiler
• MacOS: brew install protobuf

Get the sources:

git clone https://github.com/kraiskil/onnx2c.git
cd onnx2c
git submodule update --init

then run a standard CMake build

mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make onnx2c

FAQ

Getting error: ‘class onnx::ModelProto’ has no member named ‘ParseFromIstream’; ?

If you have ProtoBuf 3.6 or earlier, you need the following modification to onnx/onnx/onnx.proto

• remove the last lines (i.e. option optimize_for = LITE_RUNTIME;)

With ProtoBuf 3.12 (e.g. Ubuntu 20.10 onwards) this modification is not needed.

Versions between 3.6 and 3.12 are uninvestigated.

Seeing build error void* __builtin_memset ... is out of the bounds ... ?

On (at least) protobuf 3.6, which ships as default on Ubuntu 20.04, the build fails when onnx2c is build in Release mode.

Change the buildstep above to cmake -DCMAKE_BUILD_TYPE=Debug .. Or update your protobuf. See kraiskil/onnx2c#39 and onnx/onnx#4756.

Usage

The build creates onnx2c binary. Run

./onnx2c [your ONNX model file] > model.c

At the end of the model.c there is a function called 'void entry(...)'. Call that from your main program to run inference. Function parameters are named as in your ONNX model.

Using the compiler -ffast-math (or equivalent) when compiling onnx2c-generated code increases computation speed. See the GCC wiki on floating point maths for details.

Onnx2c has a few optimization passes that modify the generated output:

• Tensor unionization to wrap intermediate tensors in unions to help the compiler re-use the heap memory.
• Removing Cast nodes, by modifying their predecessor node's output tensor.
• Optimization for AVR processors to put constants into instruction memory.

Floating-point output precision can be configured with --precision N. If omitted, onnx2c uses a precision of 20.

./onnx2c -h prints out all available command line options.

onnx2c prints a log on stdout. Log level can be given with the -l N command line option. Logging levels are

• 0 Fatal errors only
• 1 Warnings where onnx2c might not be correctly implemented
• 2 Generic info (default level in the Release build)
• 3 Debug: high level trace of what onnx2c does useful debugging the model
• 4 Trace: detailed info useful for debugging onnx2c

There is a helper script to initially run any .onnx on a MCU development board. This is intended as a tool when designing the network to see if it will fit the target, before starting training the network. See the script sources and the onnx2c development documentation for instructions.

Development

Tips for development of onnx2c, including testing is described in a separate file.

On-target performance

or, how to extrapolate from incomplete data.

At the time of writing this, a single ONNX neural net has been benchmarked with onnx2c - the "Hello World"-sine generating example from TensorFlow Lite micro and compiled to ONNX with keras2onnx.

That ONNX file was compiled with STM32CubeAI and onnx2c to a STM32F411 running STM32Cube HAL with a clock speed of 84 or 96MHz. With same project and optimization settings (gcc -O4), measuring inference time by toggling GPIO pins, the STMCubeAI-generated version ran at 490us, while the onnx2c one took 20us.

See Notes below for a description of the RAM optimized version.

Memory consumption was about similar:

platform	text	data	bss	runtime
STM HAL + onnx2c @96MHz	8276	1300	3060	20us
STM HAL + CubeAI @96MHz	14372	1696	2808	490us
OpenCM3 + onnx2c @84MHz	8236	1296	388	25us
--"-- (onnx2c RAM opt)	8236	12	388	29us

Comparison

The same NN model was measured on a youtube video by Shawn Hymel, run both via TFL and STM32CubeAI. The device used was a STM32L4 at 80MHz. There the TFL version took 104us, while the STM32CubeAI one took 74us.

The STM32L4 used by Hymel is a low-power version of the STM32F4, so the L4 certainly should not be faster than the F4. Same versions of CubeAI were used. The only difference was that Hymel fed the TFL model to CubeAI, not the ONNX model as in the above measurement. I am not sure if this is relevant, but so far it is the only thing I can think of that could explain the difference. Also the measured ONNX model was not converted from the TFL model that Hymel used, but re-trained using the tutorial. But this most likely is not the cause for the execution speed difference.

More datapoints are definitely needed...

Notes

The above values are made with an older version of onnx2c. Later versions have added a "mark constant tensors as 'const'" optimisation, that significantly reduces RAM usage, but has a small performance penalty (4us in the above case).

This is because when marked const, GCC generates code that reads the 'const' vectors from flash (as opposed to copying them to RAM). Reading flash is, of course, slower than RAM.

Disabling of this optimisation should be added as a command-line option to onnx2c.

16 bit Floating Point Support

onnx2c supports the ONNX float16 and bfloat16 data types. float16 is the IEEE half-precision floating point format with 1 sign bit, 5 exponent bits and 10 mantissa bits. bfloat16 is the Brain Floating Point format with 1 sign bit, 8 exponent bits and 7 mantissa bits. See Wikipedia for more details.

They are mapped to the _Float16 and __bf16 C types respectively. While these types are not part of the C standard, both GCC and Clang support them depending on the target architecture:

• GCC Documentation
• Clang Documentation

If your model does not use these types, onnx2c will not generate any code using them.