  Enabling On-Device Learning (ODL) for Ultra-Low-Power Micro-Controller Units
(MCUs) is a key step for post-deployment adaptation and fine-tuning of Deep
Neural Network (DNN) models in future TinyML applications. This paper tackles
this challenge by introducing a novel reduced precision optimization technique
for ODL primitives on MCU-class devices, leveraging the State-of-Art
advancements in RISC-V RV32 architectures with support for vectorized 16-bit
floating-point (FP16) Single-Instruction Multiple-Data (SIMD) operations. Our
approach for the Forward and Backward steps of the Back-Propagation training
algorithm is composed of specialized shape transform operators and Matrix
Multiplication (MM) kernels, accelerated with parallelization and loop
unrolling. When evaluated on a single training step of a 2D Convolution layer,
the SIMD-optimized FP16 primitives result up to 1.72$\times$ faster than the
FP32 baseline on a RISC-V-based 8+1-core MCU. An average computing efficiency
of 3.11 Multiply and Accumulate operations per clock cycle (MAC/clk) and 0.81
MAC/clk is measured for the end-to-end training tasks of a ResNet8 and a DS-CNN
for Image Classification and Keyword Spotting, respectively -- requiring 17.1
ms and 6.4 ms on the target platform to compute a training step on a single
sample. Overall, our approach results more than two orders of magnitude faster
than existing ODL software frameworks for single-core MCUs and outperforms by
1.6 $\times$ previous FP32 parallel implementations on a Continual Learning
setup.
