Jonesforth: Implementing Forth in Assembly

Re:Jonesforth: Implementing Forth in Assembly by rwmj on Tuesday April 07, 2020 @08:59AM (#22802754)

It's single-threaded because it's a learning tool not a forth you'd ever want to use in an environment where performance would matter. Single-threaded or not, forth interpreters [not compilers] have terrible interaction with branch prediction - they will never perform well on modern CPUs. Virtually every other instruction executed is a jmp that cannot be predicted and thus will collapse the pipeline.

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Re:Jonesforth: Implementing Forth in Assembly by lioeters on Tuesday April 07, 2020 @11:19AM (#22804012)

> forth interpreters..have terrible interaction with branch prediction - they will never perform well on modern CPUs.

Interesting statement. It led me to this paper:

Branch Prediction and the Performance of Interpreters - Don’t Trust Folklore (2015)

https://hal.inria.fr/hal-01100647/document (pdf)

"..Many studies go back to when branch predictors were not very aggressive. Folklore has retained that a highly mispredicted indirect jump is one of the main reasons for the inefficiency of switch-based interpreters."

"The accuracy of branch prediction on interpreters has been dramatically improved over the three last Intel processor generations. This..has reached a level where it cannot be considered as an obstacle for performance anymore."

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Re:Jonesforth: Implementing Forth in Assembly by astrobe_ on Tuesday April 07, 2020 @12:18PM (#22804570)

There is an older paper by Anton Erl that already showing variations in performances for the same implementation technique from one generation of Pentium to another and of course between AMD and Intel.

Personally, I stopped worrying and used the most convenient implementation for my use case (portable interpreter written in C). Your set of primitives and how you code your Forth programs usually have a much larger improvement potential.

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Re:Jonesforth: Implementing Forth in Assembly by rwmj on Tuesday April 07, 2020 @01:05PM (#22805107)

That paper refers to interpreters in the traditional sense. However threaded code is not interpreted in the same way. After every instruction is a computed jump. There are benchmarks on this in the jonesforth code you can actually run and you will observe the exact problem there.

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Re:Jonesforth: Implementing Forth in Assembly by cygx on Tuesday April 07, 2020 @03:14PM (#22806464)

Note that the paper does compare a switch-based dispatcher to a computed-goto version ('jump threading') - cf figure 2. The latter used to have a significant performance advantage over the former, which is apparently no longer true (cf figure 3 (a)). That of course doesn't invalidate your point.

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Re:Jonesforth: Implementing Forth in Assembly by fmakunbound on Tuesday April 07, 2020 @01:24PM (#22805325)

> they will never perform well on modern CPUs

Is this statement for indirect threaded Forths only? What about direct threaded, token threaded and sub-routine threaded Forths?

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Re:Jonesforth: Implementing Forth in Assembly by voldacar on Tuesday April 07, 2020 @09:27AM (#22802991)

So basically the opposite of APL.

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Re:Jonesforth: Implementing Forth in Assembly by rwmj on Tuesday April 07, 2020 @09:46AM (#22803140)

Any language that arranges data in arrays or large structs does well on modern machines, especially with vector and SIMD extensions. To be fair to Forth, there exist machines that are well-suited to running "threaded code"[1], it's just that they are not machines that are commonly available today.

[1] "threaded" has nothing to do with multithreaded: https://en.wikipedia.org/wiki/Threaded_code

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Re:Jonesforth: Implementing Forth in Assembly by throwaway_pdp09 on Tuesday April 07, 2020 @12:37PM (#22804746)

I don't buy this. I understand the use of your term 'threaded', but these are unconditional and therefore can be incorporated into the instruction pipeline with little or no overhead. Here's a very old SO post, CPus won't have got worse since then

https://stackoverflow.com/questions/5127833/meaningful-cost-...

"But in general, on modern processors, there is minimal cost for an unconditional jump. It's basically pretty much free apart from a very small amount of instruction cache overhead. It will probably get executed in parallel with neighbouring instructions so might not even cost you a clock cycle. "

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Re:Jonesforth: Implementing Forth in Assembly by rwmj on Tuesday April 07, 2020 @01:03PM (#22805080)

If they were all unconditional, that would be fine. The problem is they are all computed jumps (see jonesforth for details).

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Re:Jonesforth: Implementing Forth in Assembly by throwaway_pdp09 on Tuesday April 07, 2020 @01:15PM (#22805228)

I see what you're getting at now, thanks

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Re:Jonesforth: Implementing Forth in Assembly by voldacar on Tuesday April 07, 2020 @11:26AM (#22804072)

Thanks for the link, that looks really interesting

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