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ARM chips have an instruction with JavaScript in the name

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Re: ARM chips have an instruction with JavaScript in the name

#41

Post by Someone »

orf wrote: post #23
est31 wrote: post #13

Earlier quoted context omitted.

Not surprising to me to see CPU instructions being added for widely used use cases. Basically, a CPU instruction set maker's job is to perform clustering of the tasks that the CPUs do and accelerate commonly used tasks. If people do a lot of AES, the instruction set maker adds AES extensions. If people do a lot of CRC, they add CRC instructions. If people convert doubles to integers all day in a very JS specific way,…

Can you name any other instructions with the name of a programming language in the actual instruction name? No? Then it seems way more specific than the other examples you listed. So specific that it’s only applicable to a single language and that language is in the instruction name. That’s surprising, like finding an instruction called “python GIL release”.

CPUs with the name of a programming language exist:

- https://en.wikipedia.org/wiki/Lisp_machine

- https://en.wikipedia.org/wiki/Java_processor

These are examples of https://en.wikipedia.org/wiki/High-level_language_computer_a....

Re: ARM chips have an instruction with JavaScript in the name

#42

Post by est31 »

diggan wrote: post #36
orf wrote: post #23

Earlier quoted context omitted.

Can you name any other instructions with the name of a programming language in the actual instruction name? No? Then it seems way more specific than the other examples you listed. So specific that it’s only applicable to a single language and that language is in the instruction name. That’s surprising, like finding an instruction called “python GIL release”.

Not necessarily programming languages (in similar vain as what est31 said) but cryptography has made its way into instructions, see SHA1RNDS4, SHA1NEXTE, SHA1MSG1 and more. They are not general cryptographic primitives but specific instructions for computing a specific cryptographic hash, just because SHA became popular. Also has SHA in it's name :)

Yes, the examples I quoted AES and CRC all have dedicated instructions in x86 processors (at least the ones that implement the extensions).

https://en.wikipedia.org/wiki/AES_instruction_set#Instructio...

https://www.felixcloutier.com/x86/crc32

As does ARM:

https://developer.arm.com/documentation/ddi0514/g/introducti...

https://developer.arm.com/documentation/dui0801/g/A32-and-T3...

You could say that these are language-agnostic, and indeed they are, but in a certain way they are more specific than the JavaScript operation, because the JavaScript operation is likely used in many different algorithms. I'd argue that from the point of view of a chip manufacturer, the difference matters only little. Both are tasks that the CPUs do often and thus cost energy. By implementing them natively, the CPU manufacturer reduces the cost of those workflows for their clients.

Re: ARM chips have an instruction with JavaScript in the name

#43

Post by klelatti »

olliej wrote: post #37
klelatti wrote: post #31

Given that the instruction set already has a float to integer conversion it seems likely that the overhead of implementing this would be small and so given the performance (and presumably energy) win quoted elsewhere seems like a good move. It would be interesting to know the back story on this: how did the idea feed back from JS implementation teams to ARM. Webkit via Apple or V8 via Google?

Correct, the overhead is minimal - it basically just makes the float->int conversion use a fixed set of rounding and clamping modes, irrespective of what the current mode flags are set to. The problem is JS's double->int conversion was effectively defined as "what wintel does by default", so on arm, ppc, etc you need a follow on branch that checks for the clamping requirements and corrects the result value to what x8…

Interesting. So it's as much an x86 legacy issue as JS and presumably JS followed x86 because it was more efficient to do so (or maybe by default).

Sounds too like performance gains will depend on how often the branch is taken which seems highly dependent on the values that are being converted?

Re: ARM chips have an instruction with JavaScript in the name

#44

Post by freeone3000 »

klelatti wrote: post #43
olliej wrote: post #37

Earlier quoted context omitted.

Correct, the overhead is minimal - it basically just makes the float->int conversion use a fixed set of rounding and clamping modes, irrespective of what the current mode flags are set to. The problem is JS's double->int conversion was effectively defined as "what wintel does by default", so on arm, ppc, etc you need a follow on branch that checks for the clamping requirements and corrects the result value to what x8…

Interesting. So it's as much an x86 legacy issue as JS and presumably JS followed x86 because it was more efficient to do so (or maybe by default). Sounds too like performance gains will depend on how often the branch is taken which seems highly dependent on the values that are being converted?

The branch always has to be taken if executing JavaScript, because otherwise how would you tell if the value was correct or not? You'd have to calculate it using this method regardless and then compare!

Re: ARM chips have an instruction with JavaScript in the name

#45

Post by userbinator »

alain94040 wrote: post #39
mhh__ wrote: post #2

Unless I misread the current arm docs, I don't think this is still present in the ISA as of 2020? The whole RISC/CISC thing is long dead anyway, so I don't really mind having something like this on my CPU. Bring on the mill (I don't think it'll set the world on fire if they ever make it to real silicon but it's truly different)

To understand RISC, ignore the acronym it stands for, instead just think fixed-instruction, load-store architecture. That's what RISC really means today. No variable-length instructions. No arithmetic instructions that can take memory operands, shift them, and update their address at the same time.

...and even ARM breaks instructions into uops, just like x86.

Re: ARM chips have an instruction with JavaScript in the name

#46

Post by est31 »

regularfry wrote: post #32
est31 wrote: post #13

Earlier quoted context omitted.

Not surprising to me to see CPU instructions being added for widely used use cases. Basically, a CPU instruction set maker's job is to perform clustering of the tasks that the CPUs do and accelerate commonly used tasks. If people do a lot of AES, the instruction set maker adds AES extensions. If people do a lot of CRC, they add CRC instructions. If people convert doubles to integers all day in a very JS specific way,…

With reduced instruction sets, though, the idea is to provide common subparts and make them fast rather than dedicated instructions. While it's not odd to see dedicated instructions in CPUs in general, it's jarring to see if you're familiar with ARM chips from back when they had 25 instructions total.

Yeah that's how RISC vs CISC is taught in class, I've heard that same thing. I think it's an outdated paradigm though, if it's not been wrong all along. A CISC chip can still officially support instructions, but deprecate them and implement them very slowly in microcode, as is done with some x86 instructions. And a RISC chip manufacturer might just have used this phrase as a marketing line because designing RISC chips is easier than starting with a monster that has tons of instructions, and they needed some catchy phrase to market their alternative. They then get into the gradual process of making their customers happier one by one by adding special, mostly cold, silicon targeted for special workflows, like in this instance.

Ultimately, the instruction set isn't that relevant anyways, what's more relevant is how the microcode can handle speculative execution for common workflows. There's a great podcast/interview from the x86 specification author: https://www.youtube.com/watch?v=Nb2tebYAaOA

Re: ARM chips have an instruction with JavaScript in the name

#47

Post by aardvark179 »

orf wrote: post #23
est31 wrote: post #13

Earlier quoted context omitted.

Not surprising to me to see CPU instructions being added for widely used use cases. Basically, a CPU instruction set maker's job is to perform clustering of the tasks that the CPUs do and accelerate commonly used tasks. If people do a lot of AES, the instruction set maker adds AES extensions. If people do a lot of CRC, they add CRC instructions. If people convert doubles to integers all day in a very JS specific way,…

Can you name any other instructions with the name of a programming language in the actual instruction name? No? Then it seems way more specific than the other examples you listed. So specific that it’s only applicable to a single language and that language is in the instruction name. That’s surprising, like finding an instruction called “python GIL release”.

Off the top of my head, and ignoring the Java bytecode instructions added at one point, no. I can however think of quite a lot of crypto related instructions, and many CPU and MMU features aimed at very specific language features such as garbage collection.

Re: ARM chips have an instruction with JavaScript in the name

#48

Post by est31 »

orf wrote: post #23
est31 wrote: post #13

Earlier quoted context omitted.

Not surprising to me to see CPU instructions being added for widely used use cases. Basically, a CPU instruction set maker's job is to perform clustering of the tasks that the CPUs do and accelerate commonly used tasks. If people do a lot of AES, the instruction set maker adds AES extensions. If people do a lot of CRC, they add CRC instructions. If people convert doubles to integers all day in a very JS specific way,…

Can you name any other instructions with the name of a programming language in the actual instruction name? No? Then it seems way more specific than the other examples you listed. So specific that it’s only applicable to a single language and that language is in the instruction name. That’s surprising, like finding an instruction called “python GIL release”.

In addition to what the sibling comments said, note that "Python GIL release" is a very high level concept and how the GIL is implemented can change over time. It interacts with how the OS implements locking. FJCVTZS on the other hand is a (mostly) pure function with well defined inputs and outputs, and will likely be useful for Javascript implementations for a long time. Will JS be around in 10 years? Very likely. So ARM adds an instruction for it.

The people at ARM have likely put a lot of thought behind this, trying to find the places where they, as instruction set vendor, can help making JS workflows easier.

And btw, I'm pretty sure that if python has some equally low hanging fruit to make execution faster, and python enjoys equally large use, ARM will likely add an instruction for it as well.

Re: ARM chips have an instruction with JavaScript in the name

#49

Post by nichch »

marcusarmstrong wrote: post #14

If anybody else was curious, it appears that the performance win of use of this instruction looks to be about 1-2% in general javascript workloads: https://bugs.webkit.org/show_bug.cgi?id=184023#c24

Is that significant enough to justify the instruction?

Re: ARM chips have an instruction with JavaScript in the name

#50

Post by lstamour »

nichch wrote: post #49
marcusarmstrong wrote: post #14

If anybody else was curious, it appears that the performance win of use of this instruction looks to be about 1-2% in general javascript workloads: https://bugs.webkit.org/show_bug.cgi?id=184023#c24

Is that significant enough to justify the instruction?

It is if it also increases battery life by a similar amount ;-)
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