  Mixed-precision quantization, where a deep neural network's layers are
quantized to different precisions, offers the opportunity to optimize the
trade-offs between model size, latency, and statistical accuracy beyond what
can be achieved with homogeneous-bit-width quantization. To navigate the
intractable search space of mixed-precision configurations for a given network,
this paper proposes a hybrid search methodology. It consists of a
hardware-agnostic differentiable search algorithm followed by a hardware-aware
heuristic optimization to find mixed-precision configurations latency-optimized
for a specific hardware target. We evaluate our algorithm on MobileNetV1 and
MobileNetV2 and deploy the resulting networks on a family of multi-core RISC-V
microcontroller platforms with different hardware characteristics. We achieve
up to 28.6% reduction of end-to-end latency compared to an 8-bit model at a
negligible accuracy drop from a full-precision baseline on the 1000-class
ImageNet dataset. We demonstrate speedups relative to an 8-bit baseline, even
on systems with no hardware support for sub-byte arithmetic at negligible
accuracy drop. Furthermore, we show the superiority of our approach with
respect to differentiable search targeting reduced binary operation counts as a
proxy for latency.
