JIT Compiling Code in 5μs | Hacker News Reader

JIT Compiling Code in 5μs

malisper.me

167 pointszX41ZdbW114 comments

Reminds me of the 2024 blog post Look ma, I wrote a new JIT compiler for PostgreSQL [0]. Both articles lament that Postgres's LLVM-based JIT [1] takes a while to generate code.

> The rarity of JIT compilers makes me believe that implementing a JIT compiler historically was too difficult for it to be worthwhile.

That's only true of writing a JIT from scratch. There's no rarity of JITs, it's just that LLVM (and other frameworks) are often used. Every major interpreter has a JIT compiler. PCRE2 has a JIT compiler. There are JIT frameworks out there with much faster code-generation than LLVM: Cranelift, GNU Lightning, Mir. I doubt they could do code-generation faster than a custom copy-and-patch JIT, but they'd be much faster than LLVM.

[0] https://www.pinaraf.info/2024/03/look-ma-i-wrote-a-new-jit-c... , discussed: https://news.ycombinator.com/item?id=39742916

[1] https://www.postgresql.org/docs/current/jit-reason.html

A few other small and fast JITs: https://github.com/zherczeg/sljit (used by libpcre), https://github.com/asmjit/asmjit (RPCS3 and FBGEMM) and https://webkit.org/blog/5852/introducing-the-b3-jit-compiler... (only used by JSC in Webkit, I think)

Thanks, sljit looks somewhat similar to GNU lightning.

On reflection I wonder if I overstated the widespread use of JIT and of JIT compiler frameworks. All the 'major' well-resourced high-profile JIT-based interpreters I can think of don't use an off-the-shelf JIT framework for their backend, which makes sense as they want to carefully tune the code-generation. OpenJDK, OpenJ9, .Net, V8, SpiderMonkey, JavaScriptCore. LuaJIT and Python's new JIT don't use one either, nor does the Linux kernel's BPF engine.

The Guile Scheme interpreter uses a fork of the GNU Lightning JIT library. [0] Julia and (as mentioned) Postgres use LLVM for their JITs. I'm trying to think of other projects that use a JIT framework/library.

Similarly, I can't think of many problem domains where it makes sense to use JIT. The ones that spring to mind are interpreters (of course), regex engines, and DBMSs. JIT can also help in high-performance computing, to tailor the code to the particular problem and the particular CPU. [1] I don't think there are many other contexts where it makes sense to use JIT though.

JIT compilation brings its own drawbacks in portability (both between hardware platforms and operating systems), complexity, and perhaps cybersecurity, which might also limit its adoption, even if a good JIT framework could help with all three.

[0] https://doc.guix.gnu.org/guile/latest/en/html_node/Just_002d...

[1] https://www.intel.com/content/www/us/en/developer/articles/t...

The original Dartmouth BASIC had a JIT like approach, the REPL would compile to machine code before execution.

It was the limits of 8 bit home computers hardware that made the interpreter version be more widely known.

Same to Lisp, Smalltalk, and many other languages.

Fully agree with you.

> There's no rarity of JITs, it's just that LLVM (and other frameworks) are often used

Except that using LLVM has high latency limitting it's applicability. Postgres just disabled LLVM by default because of this[0].

[0] https://www.postgresql.org/message-id/E1w8GWU-002bSL-31%40ge...

Interesting, thanks for the link. LLVM isn't the only game in town though, nobody using it (or libgccjit) for JIT should be surprised to see relatively long compile times. I wonder if the postgres project will try a different backend.

There's a strong 'diminishing returns' effect in striking a balance between compile time and the performance of the generated code. I'd expect a more lightweight (less optimising) JIT engine to be able to produce code with pretty respectable performance while taking only a fraction of the time that LLVM takes. There's a follow-up to the blog post I linked above, which bears this out. [0] (I don't know if that JIT solution is production-ready or viable for merging into postgres, mind.)

The blog post [0] gives this performance comparison:

> So, on our stupid benchmark, doing 10 times a simple SELECT * FROM demo WHERE a = 42 on a 10 million rows table...

    PostgreSQL             No JIT          LLVM JIT               Copyjit
    ----------             ------          --------               -------
    Average time (ms)      120             106 (-12%)             101 (-15%)
    Compilation time (ms)  0               19                     0.06
    Instructions           13,350,766,209  10,643,820,667 (-21%)  12,769,013,536 (-5%)
    Cycles                 4,660,821,596   4,005,881,863 (-14%)   3,924,602,439 (-16%)
    Branches               2,322,470,659   1,798,221,785 (-23%)   2,031,456,214 (-13%)

[0] https://www.pinaraf.info/2025/12/jit-episode-iii-warp-speed-...

I recommend Russ Cox's articles on implementing a regex engine: https://swtch.com/~rsc/regexp/

It is very relevant

This is perhaps a little meta, but this was a pleasant read. It’s refreshing to read an article about using an LLM that doesn’t read like it was also written by that LLM.

I might use this approach to generate the stencils for a JIT firewall I’ve been experimenting with.

It also occurs to me that this could be used to generate eBPF byte code on the fly as well

Uhm, Common Lisp, where JIT is not only available but is also manageable: the programmer can decide what deserves to be compiled and what does not.

Besides run time, JIT is available also when the code is compiled or loaded for execution (i.e., do you have a compilation or loading speed-up in mind? no problem, you can also compile that speed-up into native machine code, and so ad infinitum...).

is an xtensa lx7 (esp32-s3) target available that does not use llvm?

I am told there is http://www.ulisp.com/show?2AJI Never used it myself; I cannot attest to the completeness of the implementation.

Not in typical builds of SBCL: all code is compiled before evaluation.

It depends on the implementation. CLISP compiles when it is told to.

Last I checked, CLISP compiles to byte code. Did they add a JITter for the byte code?

Compiling to bytecode is still compilation. An interpreter would operate directly on the S-expressions. Obviously, this makes some constructs (such as TAGBODY) very inefficient.

Eval would count as JIT compilation though[0]

[0] https://www.sbcl.org/manual/#compiler-only-implementation

You can turn that off by setting or binding sb-ext:*evaluator-mode* to :interpret.

Also, on my machine, compiling the identity lambda form takes about 200 usec with (optimize (compilation-speed 3) (debug 0)). SBCL could use a faster JIT mode, perhaps at compilation-speed 3/speed 0. Perhaps there are some other internal special variables that could be tweaked to reduce compile time.

Are you sure this is actually makes a difference in your build?

Mine doesn't have the SB-INTERPRETER package, so I doubt binding the variable has an effect.

The problem with the approach is that it's not real JIT-compilation, it's just assembly templates with basic substitutions.

By not using LLVM, you're missing all the optimizations it does.

A non-optimizing compiler is a real compiler.

People do not call assemblers compilers, and yet, it's doing most of what the "JIT compiler" is doing.

This absolutely is real JIT compilation. Copy and patch is a very well known JIT compilation technique.

This is absolutely a JIT-compiler. It compiles code into machine code. This is a surprisingly efficient way to get noticeable speedup relative to interpretation. Also it is much safer than proper optimising compiler. Say ebpf jit-compiler functions very similarly, because it is fast and _secure_ way to jit. (well, there's a bit of cheating because before emitting bpf bytecode it goes through gcc/clang pipeline).

LLVM is a large dependency if you need to JIT. There are plenty of smaller (and much faster) alternatives which are much better fit for smaller projects. Larger projects usually roll out their own jit-pipeline because they can integrate better with the source language/interpreter and apply tricks LLVM is not well suited to (say, LLVM is not great at deoptimisation). I think only Julia is really a heavy user of LLVM JIT, also it is known for extremely slow repl from time to time.

To back up the "surprisingly efficient way to get a speed-up" thing: I once wrote a toy compiler, without an optimizer and without even a register allocator (so all variables lived in stack memory). In my test benchmarks it was roughly 4x slower than Clang at -O3, IIRC.

That's not exactly blazing fast for a low level C-like language, but it's not bad. It's infinitely faster than what I've ever gotten a toy interpreter to be.

By choosing LLVM you're also taking a serious latency hit, and potentially burning a ton of CPU cycles on optimizations that will never apply. JSC, for instance, implemented LLVM for FTLJIT (which is where many of the JS bits jangling around in LLVM originated from), but it was only useful for code that was highly likely to benefit from the optimizations because of the high cost of compilation. The webkit folks have since ripped out LLVM and replaced it with their own specialized JIT, which is essentially just what's demonstrated here (with some optimization passes).

> By not using LLVM, you're missing all the optimizations it does

And yet, a good portion of software that runs today's world is written in scripting languages & executed using interpreters.

Which is okay! Imho: multiply [# of users] with [how often each user sees that software's effect] and [how much that contributes to the overall user experience], then you get a ballpark idea of how much $$/effort is worth spending on optimization.

In other words: for a one-off, don't bother. But as usercount, frequency of use by individual users, poor UX or RAM/CPU consumption goes up, progress from script -> compiled -> optimizing compiler -> (if necessary) hand-optimized assembly as needed. And of course consider high-level design, data structures, algorithms etc in that process. A change there might be more effective than a switch from interpreted -> optimizing compiler.

"Developer time" should not factor into that much (again: imho) unless users=developers.

Thoughtlessly putting every change through a (slow?) pipeline that does 'random' toolbox-of-optimizations without need, is wasteful. Apply that toolbox as needed while keeping the above in mind.

Sounds like a "no true Scotsman" statement. How are you defining "real"?

Some human, somewhere, has to describe how to turn high-level language constructs into machine code. "When you see this pattern, emit this sequence of bytes." That's just templates and stencils. There's no magic for turning source code into machine code by divining the ISA at compile time.

Anything that's taking source code and, at the time of execution, is compiling it to machine code on-the-fly is JIT compilation. Regardless how long it takes, lack of optimization, or which machine is the target (x86, ARM32/64, RISC-V, JVM, WebAssembly), it's JIT.

I thought the difference was that a JIT is used alongside an interpreter? It wouldn't need to do optimization, just compile while interpreting?

How is “alongside an interpreter” meaningfully different than “at the time of execution”?

"at the time of execution" could mean that your program never runs an interpreter and instead compiles the input code before running any of it, basically like the --run option in the tiny c compiler

There’s been a meme circulating about how AI doesn’t help because “code was never the hard part.” I think that’s true in some domains, but in others, writing the code absolutely was the hard part. JIT compilers are a great example of that.

Anyone who thinks AI is good with writing code that is hard to write for the operator, not due to lack of basic software engineering know how but complexity of the domain, either has access to models beyond what is available to the public or is completely lost.

I believe this because every time I use AI for domains that I consider myself above competent, if it is anything beyond UI components or a simple CRUD endpoints, I cringe at the quality of what it generates.

This has made me to be extremely cautious of starting working in a new domain with AI if I want anything beyond throw away quick hacks or junk, shy of quick bug fixes perhaps.

Three quarters of my CS class at university could barely code and/or understand code.

That’s not a joke. A lot went on to be programmers professionally. And judging by the quality of closed & open source code I witness daily those figures from university accurately depict people’s capabilities.

Now that said, if you can’t really code then using AI will be a godsend to said individuals.

Fair, but I fear that now even more people who can't code will code, and code that is not any better than what people who could barely code write. Growing cabbages starting to look more and more interesting.

I'd say that is the problem we're observing. A lot of decent code is being written with weird inconsistencies, because it's actually written by AI driven by people who don't really understand what they're doing.

the tell-tale mark of AI code is highly over-engineered local solutions to trivial problems that don't matter, or that were already solved better elsewhere and that no sane human would ever duplicate.

Fable (and even Opus if kept tightly under reigns) does generate high-quality code even for highly complex tasks.

It generally performs better if the tasks are broken done into small manageable pieces, and the person is actually reviewing and calling out problems, which usually requires the person to be a competent engineer in the problem domain to begin with.

But yes, I have personally used it to build what the OP calls a JIT. I would usually write that by hand and it would take me one week. The AI does it in an hour.

You can't make assertions about "quality" of code that was generated under an hour while it would have taken a human 40 hours, unless you put substantial amount of work into reviewing it.

I used Fable on a Zephyr project with time sensitive code for LR-WPAN and it broke everything. Literally made the code worst to the point that the devices stopped connecting.

I do put substantial amount of work into reviewing code: that's essentially what my job is.

And even if everyone is using the same LLM model, quality varies on lot on who's driving it.

Your second paragraph invalidates your first.

If I need to be a domain expert anyway, the value of the tool goes down by orders of magnitude. Same if I need to first break the task down into pieces and keep reviewing all the output. That sounds to me like >80% of the work I'd need to do anyway.

If I need to design and understand all of the code anyway, I might as well skip the whole process of repeatedly fixing the subpar-at-every-level LLM output and write it all myself.

Personally, I've found the greatest value in asking for simple tasks, like wiring up APIs, generating boilerplate, bug finding etc. Anything that requires effort to do but results in either very little or very simple output, so that I can easily verify its correctness.

But give the LLM anything remotely complex to generate and it cakes its pants.

That is my experience as well. I get a (subjective) speedup between 1 and 3 for parts of the code I'd consider critical and where I check the output tightly, and 5-20 for menial work OR for important code that's well isolated into its own module such that its quality doesn't matter because I can have it rewritten easily if it doesn't work as expected.

> If I need to be a domain expert anyway, the value of the tool goes down by orders of magnitude. Same if I need to first break the task down into pieces and keep reviewing all the output. That sounds to me like >80% of the work I'd need to do anyway.

You absolutely don't. You only need to be roughly aware of what the code needs to be doing. Similar to how a software architect historically didn't personally oversee every line of code in an org, only it's overall structure. The implementation specific details can be left to the AI.

Only getting the code right "roughly" is not enough in complex code. An example would be an (multi-symbol) arithmetic coder. They all look very similar at a first glance, but choices on the order of operations influence performance and compression characteristics, off-by-one errors are extremely easy to make.

When did you last try? They've improved a lot.

Also often the difficulty with writing code is simply knowing where to start - getting past the blank page. AI can help a lot with that. Often there's a task where I've got kind of writers block, but you can ask AI to do it and suddenly it's like "ah yeah, sort of but actually that's not quite right we should do it this way".

> I cringe at the quality of what it generates.

Besides all the other mentioned points, I think a good remaining less-discussed point is that quality in software has always been in the eye of the beholder. You may very well see the AI output as low quality, I may not, and its not necessarily clear who is right or wrong, because there was always little precedent in objectively evaluating code quality.

This is a long standing issue, and was never resolved before AI happened, and the coming of AI has not really changed things, except that AI is under a magnifying glass obviously. How do we objectively measure the quality of code? There's some general consensus on things, but surprisingly little is true professional agreed upon consensus.

If I can make up a figure, I would guess 95% of software engineering quality rhetoric, and craftmanship advice, is just strongly held opinions.

This is not something I can prove, but if I look at the (still ongoing.....) debates on very basic ideas like clean code, and the reactions from also-great programmers like Carmack, Blow & Muratori, it is clear to me that there is little consensus on even the fundamentals of software design.

If all these people can produce excellent working software while disagreeing on these fundamentals (of quality), it means we do not yet understand what the fundamentals are.

https://en.wikipedia.org/wiki/Relativist_fallacy

He’s the obviously and exactly saying code quality is not objective fact. And replying with a fallacy link is just embarrassing.

The entire argument is one of Relativism, there is a whole lot of disciplines where there is a huge subjective element, consider journalism and literature in general, but that doesn't mean we can not have a conversation about quality of work in these fields.

Software is no different. So when someone claims that "you can't question the quality of AI output because quality of software is [partly] subjective", they're appealing to subjectivism. This position is generally held by the incompetent to entirely dismiss any question of craftsmanship or quality.

No, you're missing the point. I'm not saying software quality is relative, I'm saying that most current debate on what people think is quality software, is subjective nonsense.

I've also never claimed that "you can't question the quality of AI output because quality of software is [partly] subjective". I'm not sure how you got there. I am simply saying that "quality" is an ill defined term and this is why we might see so much divergence when discussing AI code "quality".

You can absolutely evaluate code quality objectively, e.g performance, bugs, crashes.

Okay, I misunderstood your point. What you're saying seems pretty reasonable to me.

You and vouwfietsman can't know what kind of code asdfsa32 is rejecting.

What if it was rejected on an objective basis? You can't know, so you have to err on the side of caution, but you didn't.

So it's not embarrassing at all. In fact, it is more embarrassing to double down without knowing more.

It's a fallacy because you can always say that the choice of an objective metric was subjective, so everything is subjective.

You're talking about different measures / types of quality.

> Anyone who thinks AI is good with writing code that is hard to write for the operator, not due to lack of basic software engineering know how but complexity of the domain, either has access to models beyond what is available to the public or is completely lost.

I see what you mean.

You could read it as quality in the operational correctness sense, but just as well in the software architectural design sense. My comment indeed applies to only one of those.

However, why judge correctness as a "cringe on quality", rather than just objectively saying its producing errors. This is why my response is in the software direction.

> If I can make up a figure, I would guess 95% of software engineering quality rhetoric, and craftmanship advice, is just strongly held opinions.

Absolutely, at the end of the day the only metrics that matter are performance and code validity. A lot of the "this code is awful" arguments I hear just boil down to "this code is stylistically awful" and never talk about it's performance.

> I believe this because every time I use AI for domains that I consider myself above competent, if it is anything beyond UI components or a simple CRUD endpoints, I cringe at the quality of what it generates.

In a long run session Fable 5 generated a Disney principled (physically based) shading/lighting engine from scratch, both with a CPU (SIMD accelerated) backend _and_ a full GPU Vulkan backend. Exceptional performance too; the CPU backend runs almost realtime and literally looks better than some AAA games outright. Took it about ~8 hours wall time total time to achieve this.

> anything beyond UI components

In fairness, UI components are probably one of the hardest things to do completely correctly, even with just HTML. As soon as you start thinking about i18n, screen reader support, color contrast, keyboard controls, and all of the layout and positioning you're trying to achieve at different viewport sizes, it's extremely hard for a "just competent" engineer to do an S-tier job. Even with the most vanilla default built in components it's not easy to get this correct, and I think we all cringe at what competent engineers create by hand in this domain.

Author here. Let me know if you have any questions about the post or about pgrust.

pgrust sounds very interesting, but with the deep changes there’s no viable path to upstream it - is the end goal to be robust enough that it’ll get wide adoption?

Is it really interesting though? It's essentially just vibe-coded by people who are unqualified for this kind of work. One of the authors claimed that what qualified them was having worked on a large-scale postgres cluster; they never actually worked on databases or compilers.

They are going to lose their coding driver's license.

It uses copy-and-patch compilation to archive that

Didn't know that

Tcc be like

Nice

JIT compilation is unsecure.

It is the core of ClickHouse for example, for many years. Is it secure enough in your opinion?

Maybe, but surely there are users who are willing to trust all users of their db instance.

The issue is that it restricts from locking-down and securing the system with Write xor Execute memory. So it has system wide implication.

https://en.wikipedia.org/wiki/W%5EX

W^X is typically per mapping, not per memory page and does not interfere with JIT compilation.

Sure, but it still means that the OS has to decide who is allowed to do it and to what extent. Sophisticated worms like Stuxnet would be much harder with strict W^X for example, since CVE-2010-2568 and the like would be much harder to execute.

> Sure, but it still means that the OS has to decide who is allowed to do it and to what extent

It has to do that anyway?

Only if it wants to allow Writable Memory to become Executable, or basically, allow JIT.

How do you propose preventing that without a check?

Signed binaries with the proper assigned OS capabilities.

Yes, but with JIT, you can't really verify what the application does upfront. That is the entire point.

Capabilities are a way to control that, and the point being that only responsible proven applications get the certificate, hence how it all goes on iOS.

You're making the assumption that "responsible" is something provable, but that is not the case, it is specially not easy to prove software is secure from tampering its behaviour.

For that there is bytecode verification as intermediate step, and if you want to go crazy with security, hardware memory tagging with capabilities.

Which at this point most companies would rather save money and forbid JIT altogether.

Note that mainframes and micros have JIT environments that aren't at the same safety level as regular desktop PCs.

For example,

https://medium.com/@dhemanthc/ibm-i-architecture-how-timi-an...

Nonsense, there's no "system wide implications". Mappings are per process, and W^X is just a strategy to help harden individual processes, not the entire system. There's no herd immunity here.

JITs do not grant the ability to bypass any OS/system sandboxes. The lack of W^X doesn't do that, either. If a process opts out of W^X, such as to enable a JIT, it's voluntarily making itself less hardened, but at the end of the day this isn't any more meaningful than the program being allowed to be written in, say, C, which also voluntarily reduces the processes security hardening.

You don’t understand OP’s point because you’re assuming vulnerabilities don’t exist. That is utter nonsense.

You're entirely correct because JIT requires violating Write xor Execute security policy. This is the reason on iOS, it is limited to Apple shipped software.

https://en.wikipedia.org/wiki/W%5EX

The wiki page mentions this is only a minor problem. Because everyone just writes, then switches and executes.

But this means that you have to decide who is allowed to switch.

GrapheneOS heavily restricts JIT usage, too:

> - Android Runtime Just-In-Time (JIT) compilation/profiling is fully disabled and replaced with full ahead-of-time (AOT) compilation. The only JIT compilation in the base OS is the V8 JavaScript JIT which is disabled by default for the Vanadium browser with per-site exception support.

> - Dynamic code loading for both native code or Java/Kotlin classes is blocked for nearly the entire base OS. […]

> - Dynamic code loading for both native code or Java/Kotlin classes can be disabled for user installed apps via 3 exploit protection toggles: […]

https://grapheneos.org/features

In rust, is jit equivalent to an "unsafe" block ?

Read the code in the post. Everything is written in unsafe Rust. The assembly itself knows no memory safety at all and is completely up to the programmer skill whether it can be trusted to not mess up.

Machine code is insecure, we should all run interpreted code in a formally verified interpreter.

Alternatively, only allow for the execution of cryptographly signed static linked binaries, this naturally includes the interpreter above.

If we are sprinkling formal verification on things, we can sprinkle it on a JIT.

But then we'd have to admit that it's not the JIT that's a problem, it's the lack of guardrails and analysis features in the machine code interfaces that higher level languages expose!

So what, are you willing to go away from von-neumann architecture where instructions are data and data are instructions, i.e. the instruction-data hominocity that underpins JIT compilation? Are you willing to go to a pseudo-Harvard architecture where the ability of JIT compiling is soft locked by other means like VM or strong code authentication or policy protection, which is what Apple is doing.

Fun fact: even Apple themselves have JIT. JavaScriptCore on iOS has JIT, it's just that the App Store policies forbid any application submissions with JIT or trying to mmap/mprotect an executable region. There used to be apps on TrollStore that runs JIT

Typo: it is Homoiconicity, not hominocity. I typed too fast

No, it isn't. JITs don't grant capabilities abilities an equivalent interpreter doesn't already have.

Allowing code execution allows code execution, that's it, that's the entirety of it.

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