Arm's New Cortex-A78 and Cortex-X1 Microarchitectures: An Efficiency and Performance Divergence

Implementations Choices & Customers

Naturally, the Cortex-X1 is expected to be quite bigger than a Cortex-A78, but not dramatically more. Arm does warn though that for mobile designs it’s extremely unlikely that we’ll see implementations with more than two X1 cores. The company here is essentially embracing the industry trend of going for a three tier core hierarchy, and with the introduction of the A78 and X1, they’re allowing customers to build such systems with much more flexibility and more differentiation than the frequency and process library differentiation we’ve been seeing on today’s “mid” and performance cores.

There’s still going to be customers who may be cost averse or simply not take part in the “Cortex-X Program”, who might just avoid the X1 and just go with A78 cores. The comparison Arm is making here is against an equivalent A77 setup, and the A78 cores would indeed bring a good amount of area savings all while improving performance.

Cortex-X1 implementers would very likely go for a hybrid cluster implementation with X1, A78 and A55 cores in a DSU. Arm here depicts Qualcomm’s favorite 1+3+4 configuration, and it's a logical setup that we’d expect to see in a future Snapdragon chip.

Today’s announcement of the Arm cores also came with an unusual quote from Samsung LSI:

“Samsung and Arm have a strong technology partnership and we are very excited to see the new direction Arm is taking with Cortex-X Custom program, enabling innovation in the Android ecosystem for next-gen user experiences.”

- Joonseok Kim, vice president of SoC design team at Samsung Electronics

It’s extremely rare to hear Samsung talk about a new Arm IP like this during a launch, and I think it’s pretty safe to say that this is very much an indirect confirmation that they’re a licensee of the X1 cores. In which case, we’ll be seeing the core in the next generation of flagship Exynos chipsets. Looking back at what happened with Samsung’s custom CPU design team last year as well as their lackluster performance of their custom cores, the very existence of the X1 probably further sealed the fate for their custom core efforts. The only remaining questions for me is whether they’ll go for a 1+3+4, or a 2+2+4 setup, and if Samsung’s 5nm will showcase better competitiveness compared to their lagging 7nm node.

Meanwhile HiSilicon, being in the middle of political turmoil, probably won't get to produce an X1 chip; plus the vendor has a tendency not always use the latest CPU IPs anyhow. MediaTek would be the last candidate licensee for the X1 – but here I’m also relatively uncertain if the company’s cost-oriented mantra actually fits well with the X1’s philosophy of going all out on area, with the likelihood that it’s also more expensive to license.

First Impressions - Arm Finally Going For Pure Performance

Today’s reveal of the Cortex-A78 and Cortex-X1 brought both the expected and the unexpected. I've had relatively modest expectations of the A78, as for years we had been told it would be the smallest upgrade amongst the new Austin family of Arm CPU microarchitectures. The A76 and A77 were after all both big leaps in performance and IPC. What I didn’t expect was for Arm to really focus on maximizing the PPA of the design, with efficiency being a first-class citizen in terms of design priorities. In that sense, the A78’s performance improvements might be a little tame compared to previous generations, but seemingly it’s still going to be an excellent core that is going to continue Arm's recent strides in outstandingly efficient computing.

Meanwhile the Cortex-X1 is a big change for Arm. And that change has less to do with the technology of the cores, and more with the business decisions that it now opens up for the company, although both are intertwined. For years many people were wondering why the company didn't design a core that could more closely compete with what Apple had built. In my view, one of the reasons for that was that Arm has always been constrained by the need to create a “one core fits all” design that could fit all of their customers’ needs – and not just the few flagship SoC designs.

The Cortex-X program here effectively unshackles Arm from these business limitations, and it allows the company to provide the best of both worlds. As a result, the A78 continues the company’s bread & butter design philosophy of power-performance-area leadership, whilst the X1 and its successors can now aim for the stars in terms of performance, without such strict area usage or power consumption limitations.

In this regard, the X1 seems really, really impressive. The 30% IPC improvement over the A77 is astounding and not something I had expected from the company this generation. The company has been incessantly beating the drum of their annual projected 20-25% improvements in performance – a pace which is currently well beyond what the competition has been able to achieve. These most recent projected performance figures are getting crazy close to the best that what we’ve seeing from the x86 players out there right now. That’s exciting for Arm, and should be worrying for the competition.