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
#2This 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 power because Intel had the resources to care but never did, and AMD never had the resources to try. Other companies stepped in to full that niche, and had to use other ISAs. (If they could have used x86 legally, they might well have done so. Oops?) That may well be changing. Or perhaps AMD will let x86 fade away.
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#3I'd be interested to hear someone with more experience talk about this or if there's more recent research, but in school I read this paper: https://research.cs.wisc.edu/vertical/papers/2013/hpca13-isa...> that seems to agree that x86 and ARM as instruction sets do not differ greatly in power consumption. They also found that GCC picks RISC-like instructions when compiling for x86 which meant the number of micro-ops was similar between ARM and x86, and that the x86 chips were optimized well for those RISC-like instructions and so were similarly efficient to ARM chips. They have a quote that "The microarchitecture, not the ISA, is responsible for performance differences."
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#4And 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 decoding would be a deciding factor.
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#5IIRC There was a Jim Keller interview a few years ago where he said basically the same thing (I think it was from right around when he joined Tenstorrent?). The ISA itself doesn't matter, it's just instructions. The way the chip interprets those instructions is what makes the difference. ARM was designed from the beginning for low powered devices whereas x86 wasn't. If x86 is gonna compete with ARM (and RISC-V) then the chips are gonna need to also be optimized for low powered devices, but that can break decades of compatibility with older software.
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#6Ok. When will we get the laptop with AMD CPU that is on par with a Macbook regarding battery life?
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#7[deleted]
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#8This 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…
> x86 didn't dominate in low power because Intel had the resources to care but never did
Remember Atom tablets (and how they sucked)?
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#9Ok. When will we get the laptop with AMD CPU that is on par with a Macbook regarding battery life?
How much of the Mac's impressive battery life is due purely to CPU efficiency, and how much is due to great vertical integration and the OS being tuned for power efficiency?
It's a genuine question; I'm sure both factors make a difference but I don't know their relative importance.
Re: AMD claims Arm ISA doesn't offer efficiency advantage over x86
#10Since 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