Comment by klelatti | Hacker News Reader

Comment by klelatti | Hacker News Reader

From the SiFive website [1]

> The Performance P550 scales up to four-core complex configurations while delivering 30% higher performance in less than half the area of a comparable Arm® Cortex®-A75.

Dylan Patel wasn't impressed by these comparisons with A75 [2]

> @SiFive is claiming half the area and higher perf/GHz, but they are using 7nm and 100ns memory latency. Choosing to compare to the 10nm A75 on S845, notorious for its high latency at over 200ns. Purposely ignoring iso-node or other A75 comparisons.

And this analysis seems to be borne out in this Chips and Cheese post.

> As a step along that journey, P550 feels more comparable to one of Arm’s early out-of-order designs like Cortex A57. By the time A75 came out, Arm already accumulated substantial experience in designing out-of-order CPUs. Therefore, A75 is a well polished and well rounded core, aside from obvious sacrifices required for its low power and thermal budgets. P550 by comparison is rough around the edges.

So what to make of SiFive's claims? It seems quite an important claim / comparison.

[1] https://www.sifive.com/cores/performance-p550

[2] https://x.com/dylan522p/status/1415395415000817664

Even if i try to avoid the hyperbole by saying by the time SiFive nodes will reach maturity with the current cortex, GPT-30 will be out etc ...

The fact is that the node difference (7nm vs. 10nm) is critical here, SiFive’s area/power efficiency gains aren’t purely architectural but partly process-driven. Even with that advantage, matching a 2018 A75 (designed for mobile thermal/power limits) in 2024 feels like catching up to ARM’s rearview mirror. ARM’s A720 today benefits from years of iterative refinement (cache hierarchies, branch predictors, memory subsystems) that aren’t easily replicated overnight.

Scaling beyond cores is another hurdle, interconnects, memory controllers, and accelerators matter just as much as raw IPC. RISC-V’s ecosystem (tools, firmware, software optimization) also lags ARM’s, which could limit adoption even if the P550 were competitive.

SiFive’s claims highlight RISC-V’s potential, but until they benchmark against modern cores on the same node and demonstrate system-level competitiveness (not just microarchitecture wins), the gap will persist. That said, disruption takes time—ARM wasn’t born polished either. The real test is whether SiFive can close the maturity deficit before ARM’s roadmap (and AI-driven heterogeneity) leaves them behind. I doubt it, the GAP in GPU cores alone between Cortex and M Series is so huge, and then there are accelerators like NPU cores, which SiFive havent even started working on yet.

Even Cortex NPU's are behind Apple M Series, and if a large companies liek Samsung, Qualcomm, Mediatek lag behind Apple is Quality ARM chips with Decent GPU, NPU on Board memory, what hope does SiFive Have? At Worse Burn Investor money and die. At Best supply chips for your remote control, washing machine etc ... competing with mainstream applications would not be wise by any standards.

It's also worth noting that they're claiming a high perf/GHz, not a high perf. It's easy to shrink a chip by using slower but more compact libraries that don't increase gate size as much for larger fanout at the cost of a lower maximum frequency, like AMD's compact cores do. And lowering clock speeds mean that your main memory latency, measured in clock cycles, goes down increasing perf/GHz too.

> As a step along that journey, P550 feels more comparable to one of Arm’s early out-of-order designs like Cortex A57.

If it is as fast as a A57 on similar node, that would still be a major win for RISC-V which so far has been incredibly slow. The Nintendo Switch 1 uses Cortex-A57.

(author here) I compared it to the A75 on the Snapdragon 670, not the 845. I chose that comparison because I have a Pixel 3a (my previous daily driver cell phone), and that's the only A75 core I had access to.

Hello Chester! Thanks for clarifying and for a terrific post.

TSMC 7nm is 91MTr/mm2 and 10nm is 53MTr/mm2. That's a 1.72x increase in density while SiFive is claiming a 2x density advantage which still puts it pretty far ahead if the claim is accurate and that's without discussing the 30% IPC advantage (though final clockspeed equivalence from their claims would still put it 35% slower than the S845 at 2.8GHz). The real question is about how much more dense could A75 be if they lowered target clockspeeds.

Dylan's complaint about comparing to the S845 is mystifying ignorance as he should know better.

What other A75 SoCs are there? Exynos used it for their mid cores, but the SoC sucked. MediaTek had the Helio P65, but it was announced in late 2019 which was basically 2 years after S845 was announced at the end of 2017. There were some other smaller suppliers from China, but I have no idea who they are. S850 existed, but as I recall, it was just a better binning of the S845 announced months after the original.

S845 is the ONLY A75 design worth comparing.

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