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Arm-Compatible CPUs Get Bigger

July 10, 2018

Author: Linley Gwennap

Sometimes big isn’t big enough. In the latest round of mobile SoCs, Apple and Samsung have expanded their CPU transistor budgets to gain a performance advantage over other smartphone makers. Their custom cores are bigger than Arm’s “big” cores but still run the same software. Bigger isn’t always better: although Apple’s A11 processor delivers excellent power and area efficiency, Samsung’s Exynos 9810 lags standard Cortex CPUs on these metrics.

Qualcomm’s Snapdragon 845, which debuted earlier this year, was the first processor to implement Arm’s new DynamIQ architecture, which allows designers to combine different CPU types (e.g., big and little) in the same cluster. DynamIQ CPUs also include private L2 caches, a shift from the shared L2 caches in previous designs. This change restructures the SoC layout while encouraging larger on-chip memories.

Despite sharing Arm compatibility, these leading-edge 10nm CPUs range in size by nearly 10x, from a 0.36mm2 Cortex-A53 (in the Huawei Kirin 970) to the massive 3.43mm2 Samsung Meerkat (in the Exynos 9810). The switch from shared to private L2 cache complicates this comparison, but the Apple and Samsung designs are far bigger than the Cortex models. This size advantage helps Apple achieve more than twice the per-clock performance (IPC) of Cortex-A73, while Samsung chalks up a 67% IPC advantage.

Samsung’s design shows the greatest growth, tripling the size of the previous-generation Mongoose core. Apple’s Monsoon blew up 15% relative to the Hurricane CPU in the A10 processor. Even the largest Arm CPUs are much more compact than the Intel Skylake core, which measures 8.73mm2 in that company’s 14nm process. (We estimate Skylake would consume 6.91mm2 in TSMC’s 10nm technology, still more than twice the size of Meerkat.) Intel’s recent Kaby Lake and Coffee Lake processors continue to use the same 14nm Skylake CPU.

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