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Yesterday, Arm launched two new IPs (Cortex-M55 and Ethos-U55) to expand its AI-related product offerings.
Cortex-M55
The Cortex-M55 CPU brings many new features announced by Arm over the past year. The first new feature is support for custom instructions. Arm first announced custom instructions at the 2019 Techcon and subsequently launched them with the Cortex-M33. This feature follows similar capabilities provided by companies like SiFive and Andes based on RISC-V IP cores. The goal is to favorably fold tight instruction sequences into a single instruction in heavily executed cores to save power and potentially increase throughput.
In addition to custom instructions, Arm also refers to the Cortex-M55 as the “most AI-capable Cortex-M processor” because it is the first processor to implement Helium functionality. Helium, also known as the M-Profile Vector Extension (MVE), is an extension of the Armv8.1-M architecture that introduces new SIMD 128-bit vector operations designed to enhance DSP and ML applications and performance. It relies more on existing registers rather than vector registers like NEON and introduces new support for features such as lane prediction, loop prediction, and complex operations like scatter-gather. More importantly, it achieves this by executing smaller data blocks (32 bits) to keep the silicon area of the IP minimal. For machine learning, Helium provides instructions such as VMLAV horizontal vector multiply-accumulate instructions.
Ethos-U55
Launched alongside the Cortex-M55 is the Ethos-U55, an extension of the Ethos family. The Ethos-U55 is a dedicated microNPU inference accelerator that can be used in conjunction with Cortex-M series CPUs to provide performance and energy efficiency specifically aimed at NPUs. The Ethos-U55 differs slightly from the Ethos-N series as it is specifically designed for Cortex-M CPUs. Firstly, unlike the Ethos-N, the Ethos-U is a standalone IP block that can be integrated into the SoC CCN-500 network, designed to work closely with a paired Cortex-M CPU and leverage its processing capabilities. Due to the Helium extension, the Cortex-M55 is particularly well-suited for use with the U55, but it can also work with older Cortex-M processors like the M7, M4, and M33.
Performance Comparison
Arm claims that for typical voice assistant workloads, the inference performance of the Cortex-M55 can be improved by up to 6 times due to the Helium extension, with power efficiency improved by 7 times. When used in conjunction with the Ethos-U55, Arm claims this improvement results in performance increases of 50 times and 25 times, respectively. These comparisons are made against the Cortex-M7. It is worth noting that to achieve this performance boost, the code must be recompiled to fully utilize the new M-Profile vector extensions and the processing capabilities of the Ethos MAC engine.
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