  Diffusion models have achieved great success in image synthesis through
iterative noise estimation using deep neural networks. However, the slow
inference, high memory consumption, and computation intensity of the noise
estimation model hinder the efficient adoption of diffusion models. Although
post-training quantization (PTQ) is considered a go-to compression method for
other tasks, it does not work out-of-the-box on diffusion models. We propose a
novel PTQ method specifically tailored towards the unique multi-timestep
pipeline and model architecture of the diffusion models, which compresses the
noise estimation network to accelerate the generation process. We identify the
key difficulty of diffusion model quantization as the changing output
distributions of noise estimation networks over multiple time steps and the
bimodal activation distribution of the shortcut layers within the noise
estimation network. We tackle these challenges with timestep-aware calibration
and split shortcut quantization in this work. Experimental results show that
our proposed method is able to quantize full-precision unconditional diffusion
models into 4-bit while maintaining comparable performance (small FID change of
at most 2.34 compared to >100 for traditional PTQ) in a training-free manner.
Our approach can also be applied to text-guided image generation, where we can
run stable diffusion in 4-bit weights with high generation quality for the
first time.
