Recently (around 14 December 2022), Apple’s Machine Learning Research team published “Stable Diffusion with Core ML on Apple Silicon” with Python and Swift source code optimized for Apple Silicon (M1/M2) on Github apple/ml-stable-diffusion. Here I’m trying it out on a MacBook (though the code also works on iPhones and iPads)...


First, the installation pre-requisites - a folder with the Python code and a virtual environment with dependencies installed:

git clone
cd ml-stable-diffusion
mkdir models outputs

python3 -m pip install virtualenv
python3 -m virtualenv v2
source v2/bin/activate

pip install -e .
pip install --upgrade torch==1.12.1
pip install --upgrade numpy==1.23.1

I downgraded two modules:

  • PyTorch 1.12.1 - to resolve this warning message, Torch version 1.13.1 has not been tested with coremltools. You may run into unexpected errors. Torch 1.12.1 is the most recent version that has been tested.
  • Numpy 1.23.1 - to resolve this error message, AttributeError: module 'numpy' has no attribute 'bool'. Did you mean: 'bool_'?

Model Conversion

Next, converting the models to Core ML - this step also downloads the models, which requires a login with a User Access Token. Currently, the code supports v1.4 and v1.5, each of which is about ~5.5GB total across 15-16 files.

huggingface-cli login

python -m python_coreml_stable_diffusion.torch2coreml --convert-unet --convert-text-encoder --convert-vae-decoder --convert-safety-checker -o models --bundle-resources-for-swift-cli 
python -m python_coreml_stable_diffusion.torch2coreml --convert-unet --convert-text-encoder --convert-vae-decoder --convert-safety-checker -o models --model-version 'runwayml/stable-diffusion-v1-5'

After some waiting... the converted Core ML models are availabe in the models folder. For each version, the converted models are stored in four folders, totaling ~2.5 GB per version:

  • Stable_Diffusion_version_CompVis_stable-diffusion-v1-4_safety_checker.mlpackage / Stable_Diffusion_version_runwayml_stable-diffusion-v1-5_safety_checker.mlpackage
  • Stable_Diffusion_version_CompVis_stable-diffusion-v1-4_text_encoder.mlpackage / Stable_Diffusion_version_runwayml_stable-diffusion-v1-5_text_encoder.mlpackage
  • Stable_Diffusion_version_CompVis_stable-diffusion-v1-4_unet.mlpackage / Stable_Diffusion_version_runwayml_stable-diffusion-v1-5_unet.mlpackage
  • Stable_Diffusion_version_CompVis_stable-diffusion-v1-4_vae_decoder.mlpackage / Stable_Diffusion_version_runwayml_stable-diffusion-v1-5_vae_decoder.mlpackage

Then, add --bundle-resources-for-swift-cli to generate a pre-compiled Resources bundle for Swift:

python -m python_coreml_stable_diffusion.torch2coreml --convert-unet --convert-text-encoder --convert-vae-decoder --convert-safety-checker -o models --bundle-resources-for-swift-cli 
mv models/Resources models/Resources14

python -m python_coreml_stable_diffusion.torch2coreml --convert-unet --convert-text-encoder --convert-vae-decoder --convert-safety-checker -o models --model-version 'runwayml/stable-diffusion-v1-5' --bundle-resources-for-swift-cli
mv models/Resources models/Resources15

The output always is placed in a Resources folder, so I rename it to avoid version collition. Each version is again to another ~2.5GB per version.

If you get an error xcrun: error: unable to find utility "coremlcompiler", not a developer tool or in PATH, and you are sure both XCode and XCode Command Line tools are installed (xcode-select --install), then maybe this will help: open > Settings... > Locations, click the Command Line Tools folder to “reconnect” the command line tools. From the terminal, you should be able to run xcrun coremlcompiler.

Xcode Command Line Tools location

Run Stable Diffusion in Python

At last, we can now try out the Apple-provided Python code to run Stable Diffusion. I notice it first loads the original PyTorch models, before replacing them with the .mlpackage Core ML models in compile-on-load mode. It sure takes a long time to run! For me, the run time is practically in the same ballpark as using the original Python model.

python -m python_coreml_stable_diffusion.pipeline --prompt "a photo of an astronaut riding a horse on mars" -i models -o outputs

You can also add TRANSFORMERS_OFFLINE=1 HF_DATASETS_OFFLINE=1 before running Python to use already downloaded models.

Run Stable Diffusion in Swift

However, Python does not support loading pre-compiled optimized models... so the final test is to run Apple’s Swift version which can load precompiled .mlmodelc models.

swift run StableDiffusionSample "a photo of an astronaut riding a horse on mars" --resource-path models/Resources14 --output-path outputs

swift run StableDiffusionSample "a photo of an astronaut riding a horse on mars" --resource-path models/Resources15 --output-path outputs --seed 123456

For me, the Swift version is an incredible 4x times faster (though not the first time as code needs to be compiled). Stable Diffusion on M1 is now really, really FAST!