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Cortex-X1: A Bigger Arm

June 2, 2020

Author: Linley Gwennap

To help its smartphone customers develop three-tier CPU clusters, Arm rolled out two new high-end cores: the big Cortex-A78 and the bigger Cortex-X1. The X1 is the first in a new line of more-powerful smartphone CPUs, targeting an impressive 30% gain in per-clock performance (IPC) over the previous Cortex-A77. To achieve this gain, the microarchitecture increases the fetch bandwidth, reorder window, and other critical parameters. Two new Neon units (for a total of four) double peak throughput for AI and other math-intensive computations.

After a big IPC gain in the previous generation, the A78 focuses on improving area and power efficiency. It provides a 7% IPC gain over the A77 even while trimming area and power. The company splurged on a third load unit and on doubling the branch-prediction bandwidth, but most other changes were minor, evolutionary upgrades. The focus on power efficiency extends battery life and enables sustained performance, whereas the X1 targets applications that need a quick burst of maximum performance.

In their high-end smartphone processors, Huawei, Qualcomm, and Samsung employ various approaches to boost the performance of the top one or two CPUs. These processors typically add a few standard big cores and four little cores, such as Cortex-A55, to extend Arm’s Big.Little approach to three tiers. We expect the X1 will become a standard solution for the high-performance core in these premium products, creating what could be called Bigger.Big.Little clusters. Production RTL for both the A78 and X1 is already available, and the first processors using them should enter production late this year.

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