Comment by hajile | Hacker News Reader

Comment by hajile | Hacker News Reader

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There seem to be very real differences between x86 and ARM not only in the designs they make easy, but also in the difficulty of making higher-performance designs.

It's telling that ARM, Apple, and Qualcomm have all shipped designs that are physically smaller, faster, and consume way less power vs AMD and Intel. Even ARM's medium cores have had higher IPC than same-generation x86 big cores since at least A78. SiFive's latest RISC-V cores are looking to match or exceed x86 IPC too. x86 is quickly becoming dead last which should be possible if ISA doesn't matter at all given AMD and Intel's budgets (AMD for example spends more in R&D than ARM's entire gross revenue).

ISA matters.

x86 is quite constrained by its decoders with Intel's 6 and 8-wide cores being massive and sucking an unbelievable amount of power and AMD choosing a hyper-complex 2x4 decoder implementation with a performance bottleneck in serial throughput. Meanwhile, we see 6-wide

32-bit ARM is a lot more simple than x86, but ARM claimed a massive 75% reduction in decoder size switching to 64-bit-only in A715 while increasing throughput. Things like uop cache aren't free. They take die area and power. Even worse, somebody has to spend a bunch of time designing and verifying these workarounds which balloons costs and increases time to market.

Another way the ISA matters is memory models. ARM uses barriers/fences which are only added where needed. x86 uses much tighter memory model that implies a lot of things the developers and compiler didn't actually need/want and that impact performance. The solution (not sure if x86 actually does this) is doing deep analysis of which implicit barriers can be provably ignored and speculating on the rest. Once again though, wiring in all these various proofs into the CPU is complicated and error-prone which slows things down while bloating circuitry, using extra die area/power, and sucking up time/money that could be spent in more meaningful ways.

While the theoretical performance mountain is the same, taking the stairs with ARM or RISC-V is going to be much easier/faster than trying to climb up the cliff faces.

> It's telling that ARM, Apple, and Qualcomm have all shipped designs that are physically smaller, faster, and consume way less power vs AMD and Intel.

These companies target different workloads. ARM, Apple, and Qualcomm are all making processors primarily designed to be run in low power applications like cell phones or laptops, whereas Intel and AMD are designing processors for servers and desktops.

> x86 is quickly becoming dead last which should be possible if ISA doesn't matter at all given AMD and Intel's budgets (AMD for example spends more in R&D than ARM's entire gross revenue).

My napkin math is that Apple’s transistor volumes are roughly comparable to the entire PC market combined, and they’re doing most of that on TSMC’s latest node. So at this point, I think it’s actually the ARM ecosystem that has the larger R&D budget.

> These companies target different workloads.

This hasn't been true for at least half of a decade.

The latest generation of phone chips run from 4.2GHz all the way up to 4.6GHz with even just a single core using 12-16 watts of power and multi-core hitting over 20w.

Those cores are designed for desktops and happen to work in phones, but the smaller, energy-efficient M-cores and E-cores still dominate in phones because they can't keep up with the P-cores.

ARM's Neoverse cores are mostly just their normal P-cores with more validation and certification. Nuvia (designers of Qualcomm's cores) was founded because the M-series designers wanted to make a server-specific chip and Apple wasn't interested. Apple themselves have made mind-blowingly huge chips for their Max/Ultra designs.

"x86 cores are worse because they are server-grade" just isn't a valid rebuttal. A phone is much more constrained than a watercooled server in a datacenter. ARM chips are faster and consume less power and use less die area.

> So at this point, I think it’s actually the ARM ecosystem that has the larger R&D budget.

Apple doesn't design ARM's chips and we know ARM's peak revenue and their R&D spending. ARM pumps out several times more cores per year along with every other thing you would need to make a chip (and they announced they are actually making their own server chips). ARM does this with an R&D budget that is a small fraction of AMD's budget to do the same thing.

What is AMD's excuse? Either everybody at AMD and Intel suck or all the extra work to make x86 fast (and validating all the weirdness around it) is a ball and chain slowing them down.

How does that square with the fact that there is no dramatic performance loss for x86 emulation on ARM?

But only on Apple ARM implementations which have specific hardware built into the chip to do so (emulate the memory model), which won't be available in the future because they're dropping rosetta2..

Apple isn't dropping Rosetta 2. They say quite clearly that it's sticking around indefinitely for older applications and games.

It seems to me that Apple is simply going to require native ARM versions of new software if you want it to be signed and verified by them (which seems pretty reasonably after 5+ years).

Probably because must “emulation” is more like “transpilation” these days - there is a hit up front to translate into native instructions, but they are then cached and repeatedly executed like any other native instructions.

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