alt.hn

12/27/2025 at 2:13:58 PM

Why is calling my asm function from Rust slower than calling it from C?

 https://ohadravid.github.io/posts/2025-12-rav1d-faster-asm/

by gavide

12/30/2025 at 9:23:08 AM

Title reminded me of something completely unrelated:

FFI overhead is not to be neglected sometimes. I've seen cases where replacing a python or js lib with an in isolation much faster native Rust or C lib with bindings had the end result being a decrease in real-world performance due to it being in a hot path or loop and the overhead being more significant than the savings were. There is no substitute for real-world benchmarks.

by baobun

12/29/2025 at 8:37:50 PM

Interesting deep dive. Can't say I understand the nuances really, and some of the Rust syntax looks incredibly arcane to me as an outsider.

by toxik

12/30/2025 at 7:54:33 AM

(Author here) thanks! Even as an experienced Rust developer, this is a lot of syntax which is part of what makes it archaic I think.

As usual with Rust most of it is due to inherent complexity: for example, when you deal with raw pointers, you need to specify if they are const or mut, and you must have a syntax that’s not &, and not too wordy: so * is a good choice (might be changed to &raw in the future!). And if you want to say that something is generic and the generic parameter is a pointer… you just end up with a lot of syntax.

by ohr

12/28/2025 at 7:14:21 AM 

   For example, before we had something like:
     top: *const FFISafe<WithOffset<&'a u8>>
   We can change that to:
     top: WithOffset<*const FFISafe<&'a u8>>
rust, you were meant to replace c++, not join it...

by dmitrygr

12/29/2025 at 8:48:32 PM

FYI to non-Rust devs wondering what's going on with this code: Those two abstractions (FFISafe and WithOffset) are not standard Rust abstractions. They're something the author is building within their own library: https://github.com/memorysafety/rav1d/blob/25e5574/src/ffi_s...

So for all of the complaining about Rust abstractions and syntax in this thread, keep in mind that these particular abstractions that caused this problem are not standard. If you just want to call an extern "C" function you can do so directly in an unsafe block. The article should be read as the author debugging their Rust wrapper types, not as a generic article about calling C functions from Rust.

by Aurornis

12/29/2025 at 8:44:14 PM

> rust, you were meant to replace c++, not join it...

Turns out that not all of the C++ noise people make fun of is due to C++, sometimes the problems you want to solve with Rust or C++ is just hard to express simply to the compiler.

by mpyne

12/30/2025 at 9:03:45 PM

And then zig shows up...

by bnolsen

12/28/2025 at 1:04:59 PM

The point of Rust is ostensibly to provide a safer version of C++-like semantics, not necessarily to avoid the same level of complexity. Especially if you're directly using unsafe code (which is necessary in some cases, like FFI), it's not really clear to me that Rust was "meant" to be doing something wildly different here. The large majority of the code not needing to use unsafe will still be better off even if this type of thing is necessary in some places.

(To preempt any potential explanations about this: yes, I understand the reference being made with that quote. I just don't think it actually applies here at all)

by saghm

12/28/2025 at 8:07:37 PM

This is all well and good, but "zero cost abstractions" implies, well, that you're crawling _up_ the abstraction pyramid, not lost in twisty little side passages.

by zephen

12/28/2025 at 11:59:39 PM

I've never understood that to mean that every possible abstraction would be zero cost, but that the language itself provides abstractions that are zero cost. I'm not sure how you could avoid having external libraries ever implement an abstraction with a cost. I honestly can't tell from this article alone what library the type `FFISafe` is from, but it's not from std.

(As an side someone whose spent a lot of time in the ecosystem might be able to infer this is likely a third-party library just from the fact that the capitalization is not in the typical format that official things use; if it were in std, I'd expect it to be FfiSafe, similar to how IoSlice[1], Arc[2], Shl[3], TypeId[4] etc. don't treat separate parts of an abbreviation/acronym/initialism as word breaks).

[1]: https://doc.rust-lang.org/std/io/struct.IoSlice.html [2]: https://doc.rust-lang.org/std/sync/struct.Arc.html [3]: https://doc.rust-lang.org/std/ops/trait.Shl.html [4]: https://doc.rust-lang.org/std/any/struct.TypeId.html

by saghm

12/29/2025 at 2:21:40 AM

> I've never understood that to mean that every possible abstraction would be zero cost, but that the language itself provides abstractions that are zero cost.

To me, "zero cost abstractions" means that you should use the language, because even the fancy abstractions it provides are free. Which presupposes that the language designers _believe_ that abstractions _and_ performance are both _important_.

Judging by all the puff pieces on the language, I don't think I'm alone in this belief of what the average Rustacean holds to be true.

> I'm not sure how you could avoid having external libraries ever implement an abstraction with a cost.

But the point is that the exact same external library code has a _lower_ cost if called from the non-abstract horror of a language C, and the (partial) fix for that is really ugly looking low-level Rust crap that that took a _long_ time to figure out and is the exact opposite of an abstraction, and the full fix is not even known yet.

Yes, we all know that abstractions sometimes obscure what is really going on, but the tradeoff is that the code is shorter and prettier, and easier to understand at a higher level. That's... not what is going on here.

by zephen

12/29/2025 at 8:44:00 PM

> if called from the non-abstract horror of a language C, and the (partial) fix for that is really ugly looking low-level Rust crap that that took a _long_ time to figure out and is the exact opposite of an abstraction,

In case it's not clear, the FFISafe abstraction is not a Rust standard library feature. It's a wrapper type that the author created within their library to bypass some compiler warnings:

https://github.com/memorysafety/rav1d/blob/25e5574/src/ffi_s...

If you want to call C functions from Rust it's as simple as writing an extern block and then calling the function, though you'll have to put the call in an unsafe block because obviously the Rust compiler can't make safety guarantees across that boundary.

The Rust docs for extern are concise: https://doc.rust-lang.org/std/keyword.extern.html

by Aurornis

12/29/2025 at 2:32:04 AM

> But the point is that the exact same external library code has a _lower_ cost if called from the non-abstract horror of a language C, and the (partial) fix for that is really ugly looking low-level Rust crap that that took a _long_ time to figure out and is the exact opposite of an abstraction, and the full fix is not even known yet.

No, it's not the exact same external library. There's an additional Rust library in between that they used, which provides the `FFISafe` type, and that has overhead. This is not a standard Rust library, or even one that I'm able to find within a few minutes of googling despite having used Rust for over a decade. It's not clear to me why you think this is necessarily representative of Rust rather than one specific library; someone could just as easily wrap a C library in another C library in a way that adds overhead, and it would be equally nonsensical to cite that as an argument that C isn't efficient.

Your argument seems to boil down to "Rust claims to be efficient, but it's possible for someone to write inefficient code, and for me to use that code, so therefore those claims are wrong".

by saghm

12/29/2025 at 2:59:54 AM

> No, it's not the exact same external library.

It uses the exact same assembly language.

> There's an additional Rust library in between that they used, which provides the `FFISafe` type, and that has overhead.

Look, I wrote " But the point is that the exact same external library code has a _lower_ cost if called [from C]" and that remains a true statement. It's pretty obvious that I was referring to the shared code, otherwise, it wouldn't have ever been called from C, right?

The profiler showed that the identical assembly language itself was taking more cycles when called from C.

> There's an additional Rust library in between that they used, which provides the `FFISafe` type, and that has overhead. This is not a standard Rust library, or even one that I'm able to find within a few minutes of googling despite having used Rust for over a decade.

The point is that they did the absolute normal expected thing in Rust, and it slowed down the external assembly language library, and after a _lot_ of digging and debugging, they changed the Rust code to be a lot less flexible and more convoluted, and now the assembly language is almost as fast as when it is called from C.

> Your argument seems to boil down to "Rust claims to be efficient, but it's possible for someone to write inefficient code, and for me to use that code, so therefore those claims are wrong".

No, that's not my argument at all. Look, the people doing this _obviously_ know Rust, and it took them a _long_ time, and some _really_ ugly concrete low-level concrete code, to take external code and make it perform almost as well as if it had been called from C++.

To me, that looks like a high-cost non-abstraction.

by zephen

12/29/2025 at 9:29:27 PM

> Look, I wrote " But the point is that the exact same external library code has a _lower_ cost if called [from C]" and that remains a true statement.

I don't think you've understood the article at all.

The overhead came from the author's own abstractions that were designed to accommodate both the assembly version or the Rust fallback. The assumption was that they take the same types and therefore the abstraction would be zero-cost, but in practice the Rust version had different types and therefore there was extra overhead in the way they were called.

If the author had simply done an extern "C" of the function and called it from Rust there wouldn't be overhead.

> To me, that looks like a high-cost non-abstraction.

The abstractions that caused this probably were not standard Rust library abstractions. The author wrote them as part of the library.

You don't need an abstraction at all to just call C functions from Rust. It's build right into the language.

If you think the problems in this article are a common Rust failure mode, you haven't understood the article at all. The unique method of calling C functions was something the author wrote, not a standard Rust practice.

by Aurornis

12/30/2025 at 3:14:26 AM

> It uses the exact same assembly language.

> The profiler showed that the identical assembly language itself was taking more cycles when called from C.

No, again, it showed that using a certain wrapper type around the same assembly had more overhead. My argument is that there's no reason to assume that this single type is representative of the entire ecosystem. I don't understand what you could possibly mean if that's not what you're saying.

> The point is that they did the absolute normal expected thing in Rust, and it slowed down the external assembly language library, and after a _lot_ of digging and debugging, they changed the Rust code to be a lot less flexible and more convoluted, and now the assembly language is almost as fast as when it is called from C.

> Look, the people doing this _obviously_ know Rust, and it took them a _long_ time, and some _really_ ugly concrete low-level concrete code, to take external code and make it perform almost as well as if it had been called from C++.

Sure, and you'd probably need a lot of similar ugliness to do the same in reverse to call a random Rust library that wasn't explicitly designed to be called from C/C++. You seem to be inferring that Rust on the whole can't be performant because the boundary between C/C++ isn't as easy as staying on the same side of the boundary, but that works both ways. If your argument is "I only want to use a language that lets me call all of these C/C++ libraries I want", that's totally reasonable. That's not the same as "Rust doesn't have zero cost abstractions" though, unless you think "calling a C/C++ library directly" is what abstraction means.

> Look, the people doing this _obviously_ know Rust, and it took them a _long_ time, and some _really_ ugly concrete low-level concrete code, to take external code and make it perform almost as well as if it had been called from C++.

> To me, that looks like a high-cost non-abstraction.

I don't understand this argument at all. You don't think that a type called `WithOffset` that wraps a pointer to provide an offset is an abstraction? From what I can tell, they were using an abstraction that was implemented inefficiently, and they switched to one written efficiently. I guess we just have completely incompatible definitions of what "abstraction" means, given that from my perspective, you genuinely seem to be arguing that the presence of this one abstraction that someone wrote being inefficient happens to be representative of the entire language and ecosystem.

by saghm

12/29/2025 at 9:44:36 PM

"Zero cost abstractions" refers to some features of the language that provide functionalities with no runtime cost, e.g. (safe) iterators, not to a presumed simplicity of the whole language. Therefore, this is not mutually exclusive with the fact that certain concepts in Rust require more complexity than their counterpart in other languages (after all, the complexities of the borrow checker don't exist in C).

In general, and it applies to the referenced article, programming with a high level of control over the implementation is complex, and there's no way around it. This article explains the concept: https://matklad.github.io/2023/01/26/rusts-ugly-syntax.html.

by pizza234

12/30/2025 at 4:18:26 AM

> "Zero cost abstractions" refers to some features of the language that provide functionalities with no runtime cost...

In that case, all languages provide zero cost abstractions.

> programming with a high level of control over the implementation is complex, > (after all, the complexities of the borrow checker don't exist in C).

The point of an abstraction is to _hide_ complexity.

> This article explains the concept

The article is only half-right. People complain both about the semantics and the syntax, because Rust's implementation of both presents (in some cases) a huge cognitive load that simply doesn't exist in other languages.

by zephen

12/29/2025 at 10:52:47 PM

From this thread it's pretty clear:

1. You've come up with your own unique definition of "zero cost" which isn't what the term means, at least as popularized by C++ where the concept of "zero cost abstractions" comes from. It means zero runtime cost, not zero cognitive cost.

2. Rust generally has more "zero cost abstractions" than C++ in terms of shifting traditional runtime checks into compile-time checks (e.g. while unique_ptr exists in Rust as Box, it's far more rarely used because ownership is easier to transfer).

3. Rust generally has faster defaults in the standard library so the abstractions are lower-cost than equivalents in C++ (e.g. C++ took forever to add a hash table via unordered_map and then proceeded to really fuck up the definition to inhibit high performance designs despite this being raised during standardization & haven't since revisited the issue).

4. Rust offers far more opportunities for "zero cost abstractions" than C++. Notably it's ownership rules prevent aliasing which allows the compiler to do more aggressive optimizations that can't generally be done in C++. It provides abstractions like NonNullPointer and NonZero so that unused bit patterns can be leveraged for enums (e.g. Option<NonNullPointer> is the size of a raw pointer but in C++ it's harder to pull off). Functional style and iterators are implemented to be aggressively inlined and overheads like bounds checking elided by the compiler, whereas C++ can't elide as easily and various bounds checks have to be inserted. Could go on and on.

Anyway, Rust as the base language vs C++ has simpler cognitive abstractions, it has more zero cost abstractions, and it has more efficient abstractions overall, both in the language and the standard library.

by vlovich123

12/29/2025 at 8:30:52 PM

> but "zero cost abstractions" implies, well, that you're crawling _up_ the abstraction pyramid, not lost in twisty little side passages

The zero cost refers to the runtime cost, not the programming complexity.

by Aurornis

12/29/2025 at 8:36:06 PM

What's a language with zero complexity-cost abstractions?

by kelseyfrog

12/29/2025 at 8:43:56 PM

Natural language. It's a language interpreted by LLMs. It is really high level.

by eptcyka

12/30/2025 at 10:57:54 AM

Natural languages are much more complex.

Complex for humans: I can learn a new programming language in an afternoon and be reasonably productive in it within a week or two. I wish I could say the same for natural languages.

Complex for computers: We’ve had good compilers since the 50s. Satisfactory language models are less than five years old.

by cyco130

12/29/2025 at 9:13:34 PM

Even that has lots and lots of abstractions which add complexity. For example, figures of speech such as "shooting fish in a barrel".

by bigstrat2003

12/30/2025 at 3:16:26 AM

If anything, the fact that it's hard to come up with an example of a language like this is even more evidence that this probably isn't what anyone has ever meant by that term.

by saghm

12/28/2025 at 8:39:15 PM

To me it implies abstraction such as a constructor, but with no overhead compared to the same steps done in some other way.

by cardiffspaceman

12/28/2025 at 8:44:52 PM

Sure, but languages and the problems we solve with them are both multifaceted, so simply pointing to one tool and saying "this is better than the one you have in your toolbox" is fine, but the plural in "zero cost abstractions" kind of implies that most or all the tools are at parity or better.

by zephen

12/29/2025 at 12:02:55 AM

It sounds like you're saying that you consider seeing this single instance of someone writing a library with a costly abstraction to be indicative of the entire language ecosystem not fitting the paradigm. This is kind of hard to take seriously; it's not like C++ doesn't have some costly abstractions as well way more embedded into the language itself (e.g. exceptions).

by saghm

12/29/2025 at 2:26:18 AM

> someone writing a library with a costly abstraction

That's not what happened here.

> it's not like C++ doesn't have some costly abstractions

This is simultaneously both completely orthogonal to my observation that the Rust FFI is borked, and a great example of a problem that wouldn't happen in C++, because in C++ you could completely ignore the costly abstractions if necessary.

by zephen

12/29/2025 at 9:33:18 PM

> That's not what happened here.

Yes, it's what happened here. The article is about debugging the author's own abstraction types that had some incorrect assumptions.

You do not need these wrapper types to call C functions. If the author had removed the complex extractions and called the extern "C" function directly (as is standard in Rust) there would not be the additional overhead.

by Aurornis

12/29/2025 at 2:34:01 AM

> > someone writing a library with a costly abstraction

> That's not what happened here.

Yes it is. Where do you think the `FFISafe` type that they used came from? It's not anything inherent to how Rust does FFI; it's a type someone wrote in an attempt to try to provide an abstraction, and that abstraction happened to have a cost. There's absolutely no reason anyone has to use it in order to do FFI in Rust.

by saghm

12/29/2025 at 2:45:49 AM

The extra time was being taken up in the identical assembly language function.

Fixing it took wizardry.

by zephen

12/30/2025 at 3:17:43 AM

The time was taken by the code around the assembly language. It was an abstraction someone wrote in a way that happened not to be efficient. The ability of someone to write a costly abstraction doesn't take away from Rust any more than the possibility of doing that in C++ would from whether C++ has them.

by saghm

12/30/2025 at 4:04:20 AM

It was caused by the code around it, but the actual CPU cycles stalled out in the assembly language, making it much harder to find the problem.

Is how I understood the post.

Anyway there's noting wrong with rust FFI. The overhead was because this function wanted to support two options and didn't implement that in the best way.

by Dylan16807

12/29/2025 at 8:53:24 PM

So rust isn't just a clever name.

by neuroelectron

12/29/2025 at 10:24:56 PM

Are you proposing a new language named WD40 ?

by belter

12/29/2025 at 10:40:58 PM

I would call it it POR-15

by patrick451

12/30/2025 at 12:05:17 AM

about 40 years

by effnorwood

12/28/2025 at 12:59:05 PM

Not related to the topic, but seeing this tidbit in the article took me by surprise;

> PSA: if you don’t see syntax highlighting, disable the 1Password extension.

This linked to the following discussion that's been ongoing for over a week (with acknowledgement from 1password that they're looking into it but as far as can tell no ETA on the fix or explanation for why it's happening in the first place):

https://www.1password.community/discussions/developers/1pass...

I know that browser extensions are in general a pretty terrible ecosystem in terms of security, but am I off-base for thinking this is not a great look for 1password? Maybe I'm just underestimating how hard this is to solve generally, but my expectation would be that it shouldn't be that hard to keep track of which circumstances you're loading entire external scripts to manipulate the DOM and not do it accidentally in places you have no need for it.

by saghm

12/29/2025 at 9:04:14 PM

Yea i think this discussion was quite interesting as well. And i can also verify that this issue exists in Zen (firefox based) as well.

by Chilinot

12/30/2025 at 3:16:18 AM

From link:

  It looks [like the 1password extension] is using prism.js to try to add syntax highlighting to <code> blocks on the entire page.
Jesus that comment contains so many red flags for a extension managing passwords!

by robocat