ZJIT is now available in Ruby 4.0 | Hacker News Reader

ZJIT is now available in Ruby 4.0

railsatscale.com

89 pointsibobev27 comments

I'm a little sad to see YJIT go down in favour of a traditional design. (Yes, YJIT isn't "deprecated" but the people working on it are now mainly working on something else. That's hardly a great place to be in.)

I'm personally quite interested in trying out an LBBV JIT for JavaScript, following in Chevalier-Boisvert's Higgs engine's footsteps. The note about a traditional method JIT giving more code for the compiler to work with does ring very true, but I'd just like to see more variety in the compiler and JIT world.

Like: perhaps it would be possible to conjoin (say) an LBBV with a SoN compiler such that LBBV takes care of the quick, basic compilation and leaves behind enough data such that a SoN compiler can be put to use on the whole-method result once the whole method has been deemed hot enough? This is perhaps a totally ridiculous idea, but it's the kind of idea that will never get explored in a world with only traditional method JITs.

In that scenario, what would you hope to get out of the LBBV?

Preface: I am not a compiler engineer at all, so I'm just spitballing silliness here.

Avoidance of type feedback counters and such. Get LBBV to clean out the redundant type checks (Higgs proved this well, avoiding something like >90% of them) and produce a format, perhaps a high-level bytecode or just an HIR, that can be used as an input to start a method-level JIT compilation.

So, LBBV gives the quick and easy basic block compilation and cleans up the very easy stuff but leaves enough information so that a follow-up compiler can still use it as input.

Tier-ups for trace-based JITs have been explored before. You can find an example here https://dl.acm.org/doi/abs/10.1145/2398857.2384630 I know LBBV isn't technically tracing, but it's quite similar, so I think similar concepts apply.

It would be useful to explain why ZJIT exists given that there's already YJIT.

Also, what's the long-term plan for YJIT.

I just upgraded my prod apps to run on YJIT so I'm annoyed at this announcement. Feels like javascript-esque runtime churn

Earnestly: why are you annoyed? I tried to make it clear that you don't have to make any changes. If you want, you can try ZJIT (which should not be anything other than a one character change), but you don't have to.

Because now YJIT is deprecated and at some point in a year or two I will have to have my team go through and switch everything from YJIT to ZJIT. So it's creating random tech debt busywork that will suck up dev resources that could better be used building features.

In isolation, having to switch from YJIT to ZJIT isn't that bad, but this same type of churn happens across so much of the frameworks and technologies that my company uses that in aggregate it becomes quite an annoyance.

YJIT is not deprecated. That word has a specific meaning in Ruby. You can continue to use YJIT.

With any luck, this performance in the next year or two will be enough to make it a happy change. "Damn, free money" etc

Most JIT compiled languages have multiple implications, each pushing each other forward.

I assume you upgrade your Ruby VM? You also upgrade to use YJIT? And you will upgrade to ZJIT?

Not only do you get them for free, you get performance improvement for free. And YJIT is still supported and not depreciated. Not only is Ruby on Rails dont have churn anywhere near the order of magnitude of JS world. It is perhaps one of the stablest and non-moving framework and languages, to the point it is moving slowly and boring compared to even modern PHP and Python.

I dont even work on YJIT or ZJIT but I find this entitlement on OSS, even when you are getting newer upgrade and improvement for free, for all the benefits it provides while having so little downside being called "random tech debt busywork" frankly very rude.

ZJIT exists because it's a more traditional design and there's hope more people will have a easier time contributing[0]. Given that, it seems YJIT will become unnecessary if ZJIT succeeds.

0: https://railsatscale.com/2025-05-14-merge-zjit/

The explanation is in the second paragraph of the article.

Non-ruby dev here. Can someone explain the side exit thing for me?

> This meant that the code we were running had to continue to have the same preconditions (expected types, no method redefinitions, etc) or the JIT would safely abort. Now, we can side-exit and use this feature liberally.

> For example, we gracefully handle the phase transition from integer to string; a guard instruction fails and transfers control to the interpreter.

> (example showing add of two strings omitted)

What is the difference between the JIT safely aborting and the JIT returning control to the interpreter? Or does the JIT abort mean the entire app aborts (i.e. I presumed JIT aborting means continuing on the interpreter anyway?)

(Also, why would you want the code that uses the incorrect types to succeed? Isn’t abort of the whole unit of execution the right answer here, anyway?)

Dynamic languages will allow a range of types through functions. JITs add tracing and attempt to specialize the functions based on the observed types at runtime. It is possible that later on, the function is called with different types than what the JIT observed and compiled code for. To handle this, JITs will have stubs and guards. You check the observed type at runtime before calling the JITted code. If the type does not match, you would call a stub to generate the correct machine code, or you could just call into the interpreter slow path.

An example might be the plus operator. Many languages will allow integers, floats, strings and more on either side of the operator. The JIT likely will see mostly integers and optimize the functions call for integer math. If later you call the plus operator with two Point classes, then you would fall back to the interpreter.

In this case, we used to abort (i.e. abort(); intentionally crash the entire process) but now we jump into the interpreter to handle the dynamic behavior.

If someone writes dynamic ruby code to add two objects, it should succeed in both integer and string cases. The JIT just wants to optimize whatever the common case is.

I guess I’m confused why an actual add instruction is emitted rather than whatever overloaded operation takes place when the + symbol (or overloaded add vtable entry) is called (like it would in other OOP languages).

If all you're doing is summing small integers---frequently the case---it's much preferable to optimize that to be fast and then skip the very dynamic method lookup (the slower, less common case)

Presumably other languages with JIT do exactly the same thing?

I’m assuming that when you talk about crashing processes as the status quo you’re referring to earlier versions of zjit rather than current Ruby on yjit? Because I’ve never seen a Ruby process crash because + was called with different arguments.

Yes; see the first ZJIT blog post. https://railsatscale.com/2025-05-14-merge-zjit/

I recently rewrote one of my rails apps in rust. Used Claude 4.5 Opus heavily and it was very fast.

One thing that's struck me with the new code is that's its so easy to follow compared to rails. It's like two different extremes on the implicit-explicit spectrum. Yet it's not like I have tons more boilerplate code now, I think I have maybe 10 or 20% more SLOC than before.

I'll probably do this with my other rails apps as well.

Was your app converted to use some Rust framework, or just Rust?

I use Axum+SQLx and for html templates I use Maud. The plan is to move to Dioxus as a step 2

Any language with a half decent typing system would have improved the easiness to follow code around. C#, Go, PHP, TypeScript, etc.

Ruby/Rails is awesome but it's a bit too magical sometimes and, lacking types by default, doesn't help either.

You're comparing a language with a framework. Better comparison would be Rust (with your choices of libraries) vs Ruby (with your choices of libraries).

If you want to compare with Rails, you need to compare with battery included Rust frameworks with equivalent batteries and convention.

I literally rewrote the whole app, so I think I have a pretty great basis for comparison

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