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Arm CPUs Approach Intel Complexity

June 4, 2019

Author: Linley Gwennap

Despite the slowdown in Moore’s Law, smartphone makers are determined to deliver better performance each year. To do so, they’re investing more transistors in CPUs, either of their own design or licensed from Arm. When normalized to the same IC process (a proxy for transistor count), Samsung’s custom M4 is about 70% of the size of Intel’s Skylake, the most powerful CPU in common use today. Arm’s high-end Cortex CPUs are considerably smaller but are quickly catching up: Cortex-A76 is more than twice the size of Cortex-A73, which came out just two years earlier.

Bigger isn’t always better. Although Samsung has blown past Apple in CPU size, it still trails in performance and performance per clock (IPC). In fact, the custom Vortex CPU in Apple’s A12 processor has a greater IPC than the latest Skylake-based products (including Coffee Lake). This comparison isn’t exactly fair, as the Intel CPU is designed to reach twice the A12’s top speed despite its lagging manufacturing process. The longer pipeline required to operate at 5GHz depresses IPC but increases peak performance, albeit at higher power. In mobile devices, Skylake operates at much lower clock speeds yet still delivers competitive performance.

Many of the newest smartphone processors move to a three-tier CPU cluster. Instead of four identical big cores, they have one or two cores that deliver maximum performance plus a second tier of “middle” cores. This approach can reduce die area and power while optimizing for typical use cases that have only one or two heavy threads. These processors also have a set of small cores to efficiently handle low-performance tasks. The recent Exynos 9820, for example, features two custom M4 CPUs, two Cortex-A75s, and four Cortex-A55s. Qualcomm and Huawei use Cortex-A76 for both the high and middle tiers by creating two different physical designs.

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