The interesting thing about the ESP32 is that it has a co-processor that can perform measurements using ADC, temperature sensor, and external I2C sensors, while main processors are in deep sleep mode.
I also read about a tiny open source cpu with a GCC toolchain called ZPU. While slow, it doesn’t use much space or code size.
I wonder if this could serve as a suitable co-processor for a RSIC-V for power savings. Another interesting concept is “soft peripherals” like a bit-banged SPI bus, so the main cpu doesn’t have to poll etc…
Reference about ULP in ESP32: http://esp-idf.readthedocs.io/en/latest/api-guides/ulp.html