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0 pointsusefulcat5y ago0 comments

I'm not sure that's entirely true. According to this (see "Why can’t Intel and AMD add more instruction decoders?"):

https://debugger.medium.com/why-is-apples-m1-chip-so-fast-32...

..a big part of the reason the M1 is so fast is the large reorder buffer, which is enabled by the fact that arm instructions are all the same size, which makes parallel instruction decoding far easier. Because x86 instructions are variable length, the processor has to do some amount of work to even find out where the next instruction starts, and I can see how it would be difficult to do that work in parallel, especially compared to an architecture with a fixed instruction size.

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AnimalMuppet5y ago

Well, if we can have speculative execution, why not speculative decode? You could decode the stream as if the next instruction started at $CURRENT_PC+1, $CURRENT_PC+2, etc. When you know how many bytes the instruction at $CURRENT_PC takes, you could keep the right decode and throw the rest away.

Sure, it would mean multiple duplicate decoders, which eats up transistors. On the other hand, we've got to find something useful for all those transistors to do, and this looks useful...

usefulcatOP5y ago

According to the article I linked, that's basically how they do it:

"The brute force way Intel and AMD deal with this is by simply attempting to decode instructions at every possible starting point. That means x86 chips have to deal with lots of wrong guesses and mistakes which has to be discarded. This creates such a convoluted and complicated decoder stage that it is really hard to add more decoders. But for Apple, it is trivial in comparison to keep adding more.

In fact, adding more causes so many other problems that four decoders according to AMD itself is basically an upper limit for them."

exmadscientist5y ago

That doesn't make any sense. The ROB is after instructions have been cracked into uops; the internal format and length of uops is "whatever is easiest for the design", since it's not visible to the outside world.

This argument does apply to the L1 cache, which sits before decode. (It does not apply to uop caches/L0 caches, but is related to them anyway, as they are most useful for CISCy designs, with instructions that decode in complicated ways into many uops.)

usefulcatOP5y ago

Maybe it wasn't clear, but the article I linked is saying that compared to M1, x86 architectures are decode-limited, because parallel decoding with variable-length instructions is tricky. Intel and AMD (again according to the linked article) have at most 4 decoders, while M1 has 8.

So yes the ROB is after decoding, but surely there's little point in having the ROB be larger than can be kept relatively full by the decoders.

jabl5y ago

Well intentioned as that article may be, it makes plenty of mistakes. For a rather glaring one, no, uops are not linked to OoO.

Secondly, it ignores the existence of uop caches that x86 designs use in order to not need such wide decoders. Some ARM designs also use uop caches, FWIW, since it can be more power efficient.

That doesn't mean that fixed width decoding like on aarch64 isn't an advantage; it certainly is. Also, M1 is certainly a very impressive design, though of course it also helps that it's fabbed on the latest and greatest process.

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0 pointsusefulcat5y ago0 comments

I'm not sure that's entirely true. According to this (see "Why can’t Intel and AMD add more instruction decoders?"):

https://debugger.medium.com/why-is-apples-m1-chip-so-fast-32...

0 comments

AnimalMuppet5y ago

Sure, it would mean multiple duplicate decoders, which eats up transistors. On the other hand, we've got to find something useful for all those transistors to do, and this looks useful...

usefulcatOP5y ago

According to the article I linked, that's basically how they do it:

In fact, adding more causes so many other problems that four decoders according to AMD itself is basically an upper limit for them."

exmadscientist5y ago

usefulcatOP5y ago

So yes the ROB is after decoding, but surely there's little point in having the ROB be larger than can be kept relatively full by the decoders.

jabl5y ago

Well intentioned as that article may be, it makes plenty of mistakes. For a rather glaring one, no, uops are not linked to OoO.

Secondly, it ignores the existence of uop caches that x86 designs use in order to not need such wide decoders. Some ARM designs also use uop caches, FWIW, since it can be more power efficient.

j / k navigate · click thread line to collapse