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Comment by dhatch387 | Hacker News Reader

From the OP:

> The entire x86 computer architecture is living on borrowed time. It’s a dead platform walking. The future belongs to ARM, and Apple’s A-series SoC’s are leading the way.

> The A9X didn’t come out of nowhere. Watching Apple’s A-series chips gain on x86 over the past five years, we’ve all been speculating about whether Apple might someday start using ARM chips in MacBooks. As of now, it’s only a question of whether they want to.

The most interesting point of this review in my opinion. x86 is hampered by backwards-compatibility. Could anyone comment on some of the more technical CPU architecture reasons for this? Do A64 CPUs have potential to outpace x86_64 processors in power consumption and performance simply because of the architecture?

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ARM's 64 bit is pretty much always [1] dragging just as much legacy baggage as Intel's 64 bit. However, that's fairly irrelevant once you get everything decoded into micro-ops and sent into the pipeline. There, branch prediction, pipeline depth, number of pipelines, etc all matter a lot more than the opcodes the CPU sees.

Also, you have to keep in mind that Gruber's been banging the single core benchmark drum pretty heavily, and rather loudly ignoring that Apple's ARM chips are very inefficient in multiple core benchmarks. In Geekbench's tests, Intel chips scale at nearly 1:1 with additional cores [2], while dual core performance of the ipad is only 70% better [3]. So, any sort of operations where you're wanting multiple intensive threads going on, the A9X falls on its face. So when you start looking at the hardware in the light of a heavier workload, Apple's not gaining that much ground on Intel, who has a lot more experience going broad than Apple does.

[1] AArch32 is optional, but pretty much on every ARM chip for compatibility

[2] http://browser.primatelabs.com/geekbench3/search?q=6600u

[3] http://browser.primatelabs.com/geekbench3/search?q=iPad6%2C8...

>Could anyone comment on some of the more technical CPU architecture reasons for this?

>x86 is hampered by backwards-compatibility.

Those two sentences don't belong in a single comment.

I assume the question was about _how_ backwards compatibility is holding x86 back and what features are problematic.

x86 is hampered by backwards-compatibility. Could anyone comment on some of the more technical CPU architecture reasons for this?

The reasons are mostly (1) x86 has strong memory ordering which is slower than weak ordering and (2) x86 needs a "complex" decoder to parse all the prefixes and crack instructions into uops (but ARMv8 cores require two decoders and may crack shifts) and (3) x86 is so hairy that it requires a massive test suite and dozens of errata still slip through, but the overhead of these is really small compared to Intel's advantages in money, manpower, and accumulated expertise.

Do A64 CPUs have potential to outpace x86_64 processors in power consumption and performance simply because of the architecture?

IMO no. It's cheaper to reach parity (Apple's processor R&D is much lower than Intel's), but I don't see why they'd outpace.

No one beats Intel at their own game.

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