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AMD claims Arm ISA doesn't offer efficiency advantage over x86

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Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#31

Post by mananaysiempre »

YetAnotherNick wrote: post #11
mananaysiempre wrote: post #8

Earlier quoted context omitted.

> x86 didn't dominate in low power because Intel had the resources to care but never did Remember Atom tablets (and how they sucked)?

> how they sucked Care to elaborate. I had the 9" mini laptop kind of device based on Atom and don't remember Atom to be the issue.

I had a Atom-based netbook (in the early days when they were 32-bit-only and couldn’t run up-to-date Windows). It didn’t suck, as such, but it was definitely resource-starved.

However, what I meant is Atom-based Android tablets. At about the same time as the netbook craze (late 2000s to early 2010s) there was a non-negligible number of Android tablets, and a noticeable fraction of them was not ARM- but Atom-based. (The x86 target in the Android SDK wasn’t only there to support emulators, originally.) Yet that stopped pretty quickly, and my impression is that that happened because, while Intel would certainly have liked to hitch itself to the Android train, they just couldn’t get Atoms fast enough at equivalent power levels (either at all or quickly enough). Could have been something else, e.g. perhaps they didn’t have the expertise to build SoCs with radios?

Either way, it’s not that Intel didn’t want to get into consumer mobile devices, it’s that they tried and did not succeed.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#32

Post by [deleted] »

Findecanor wrote: post #22
mananaysiempre wrote: post #8

Earlier quoted context omitted.

> x86 didn't dominate in low power because Intel had the resources to care but never did Remember Atom tablets (and how they sucked)?

You mean Atom tablets running Android ? I have a ten-year old Lenovo Yoga Tab 2 8" Windows tablet, which I still use at least once every week. It is still useful. Who can say that they are still using a ten-year old Android tablet?

[deleted]

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#33

Post by whynotminot »

exmadscientist wrote: post #2

This is an entirely uncontroversial take among experts in the space. x86 is an old CISC-y hot mess. RISC-V is a new-school hyper-academic hot mess. Recent ARM is actually pretty good. And none of it matters, because the uncore and the fabrication details (in particular, whether things have been tuned to run full speed demon or full power sipper) completely dominate the ISA. In the past x86 didn't dominate in low powe…

An annoying thing people have done since Apple Silicon is claim that its advantages were due to Arm.

No, not really. The advantage is Apple prioritizing efficiency, something Intel never cared enough about.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#34

Post by kccqzy »

gigatexal wrote: post #12

You’ll pry the ARM M series chips of my Mac from my cold dead hands. They’re a game changer in the space and one of the best reasons to use a Mac. I am not a chip expert it’s just so night and day different using a Mac with an arm chip compared to an Intel one from thermals to performance and battery life and everything in between. Intel isn’t even in the same ballpark imo. But competition is good and let’s hope they…

That's not mostly because of a better ISA. If Intel and Apple had a chummier relationship you could imagine Apple licensing the Intel x86 ISA and the M series chips would be just as good but running x86. However I suspect no matter how chummy that relationship was, business is business and it is highly unlikely that Intel would give Apple such a license.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#35

Post by wtallis »

devnullbrain wrote: post #25
WithinReason wrote: post #10

Since newer CPUs have heterogeneous cores (high performance + low power), I'm wondering if it makes sense to drop legacy instructions from the low power cores, since legacy code can still be run on the other cores. Then e.g. an OS compiled the right way can take advantage of extra efficiency without the CPU losing backwards compatibility

Interesting but it would be pretty rough to implement. If you take a binary now and run it on a core without the correct instructions, it will SIGILL and probably crash. So you have these options: Create a new compilation target - You'll probably just end up running a lot of current x86 code exclusively on performance cores to a net loss. This is how RISC-V deals with optional extensions. Emulate - This already happe…

IIRC, there were several smartphone SoCs that dropped 32-bit ARM support from most but not all of their CPU cores. That was straightforward to handle because the OS knows which instruction set a binary wants to use. Doing anything more fine-grained would be a nightmare, as Intel found out with Alder Lake.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#36

Post by toast0 »

WithinReason wrote: post #10

Since newer CPUs have heterogeneous cores (high performance + low power), I'm wondering if it makes sense to drop legacy instructions from the low power cores, since legacy code can still be run on the other cores. Then e.g. an OS compiled the right way can take advantage of extra efficiency without the CPU losing backwards compatibility

Like o11c says, that's setting everyone up for a bad time. If the heterogenous cores are similar, but don't all support all the instructions, it's too hard to use. You can build legacy instructions in a space optimized way though, but there's no reason not to do that for the high performance cores too --- if they're legacy instructions, one expects them not to run often and perf doesn't matter that much.

Intel dropped their x86-S proposal; but I guess something like that could work for low power cores. If you provide a way for a 64-bit OS to start application processors directly in 64-bit mode, you could setup low power cores so that they could only run in 64-bit mode. I'd be surprised if the juice is worth the squeeze, but it'd be reasonable --- it's pretty rare to be outside 64-bit mode, and systems that do run outside 64-bit mode probably don't need all the cores on a modern processor. If you're running in a 64-bit OS, it knows which processes are running in 32-bit mode, and could avoid scheduling them on reduced functionality cores; If you're running a 32-bit OS, somehow or another the OS needs to not use those cores... either the ACPI tables are different and they don't show up for 32-bit, init fails and the OS moves on, or the there is a firmware flag to hide them that must be set before running a 32-bit OS.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#37

Post by cptskippy »

YetAnotherNick wrote: post #11
mananaysiempre wrote: post #8

Earlier quoted context omitted.

> x86 didn't dominate in low power because Intel had the resources to care but never did Remember Atom tablets (and how they sucked)?

> how they sucked Care to elaborate. I had the 9" mini laptop kind of device based on Atom and don't remember Atom to be the issue.

Atom used an in-order execution model so it's performance was always going to be lacking. Because it was in-order it had a much simpler decoder and much smaller die size, which meant you could crap the chipset and CPU on a single die.

Atom wasn't about power efficiency or performance, it was about cost optimization.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#38

Post by yndoendo »

wlesieutre wrote: post #15
exmadscientist wrote: post #2

This is an entirely uncontroversial take among experts in the space. x86 is an old CISC-y hot mess. RISC-V is a new-school hyper-academic hot mess. Recent ARM is actually pretty good. And none of it matters, because the uncore and the fabrication details (in particular, whether things have been tuned to run full speed demon or full power sipper) completely dominate the ISA. In the past x86 didn't dominate in low powe…

VIA used to make low power x86 processors

Fun fact. The idea of strong national security is the reason why there are three companies with access to the x86 ISA.

DoD originally required all products to be sourced by at least three companies to prevent supply chain issues. This required Intel to allow AMD and VIA to produce products based on ISA.

For me this is good indicator if someone that talks about good national security knows what they are talking about or are just spewing bullshit and playing national security theatre.

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#39

Post by bfrog »

tinktank wrote: post #17
bfrog wrote: post #4

And yet... the world keeps proving Intel and AMD wrong on this premise with highly efficient Arm parts. While sure, there's bound to be improvements to make on x86 ultimately its a variable length opcode encoding with a complex decoder path. If nothing else, this is likely a significant issue in comparison to the nicely word aligned op code encoding arm has and surely given apples to apples core designs, the opcode d…

PPA results comparing x86 to ARM say otherwise; take a look at Ryzen's embedded series and Intel's latest embedded cores.

Have they reached apple m level of performance/watt after half a decade of the apple m parts being out yet? Do either AMD or Intel beat Apple in any metric in mobile?

Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86

#40

Post by Findecanor »

WithinReason wrote: post #10

Since newer CPUs have heterogeneous cores (high performance + low power), I'm wondering if it makes sense to drop legacy instructions from the low power cores, since legacy code can still be run on the other cores. Then e.g. an OS compiled the right way can take advantage of extra efficiency without the CPU losing backwards compatibility

I think it is not really the execution units for simple instructions that take up much chip area on application-class CPUs these days, but everything around them.

I think support in the OS/runtime environment* would be more interesting for chips where some cores have larger execution units such as those for vector and matmul units. Especially for embedded / low power systems.

Maybe x87/MMX could be dropped though.

*. BTW. If you want to find research papers on the topic, a good search term is "partial-ISA migration".

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