[killercup]

> If the work is done I expect Rust to be faster than C and C++ for the same reason that C++ can sometimes be faster than C: a more advanced type system can allow better optimization in many cases.

Every now and then I check in on whether LLVM can deal with rustc spamming "noalias" on all references. You can find the latest change in [1]. While in theory this unlocks _a ton_ of optimizations, noalias is used very rarely in C/C++ code so these compiler passes are not exercised a lot by existing LLVM tests and/or not realized in full.

[1] https://github.com/rust-lang/rust/pull/82834

[amelius]

A more advanced type system may also drive you into a corner where you make bad decisions wrt performance.

[Blikkentrekker]

Only so long as it be mandatory to obey.

Rust was specifically designed so that one can ignore all the restrictions if one be confined and tell the compiler “I know better, and I know it is safe.”.

Of course, if one be wrong in such confidence, u.b. lurks around the corner.

I suppose one big problem with Rust is that it's less specified than in C what is and isn't safe, so it's harder to be so confident.

[titzer]

Can you give an example?

[Arnavion]

Not amelius, but one case that happened to me is that Rust requires wrapping a `T` in a `RefCell` if two closures use it as `&mut T`. This happens even if you the caller know that the closures are invoked from a single thread and do not invoke each other, and thus only one `&mut T` will be in effect at any time. This is because closures are effectively structs with the captures as fields, so both struct values (and thus both `&mut` borrows) exist at the same time even though their respective fields are not used at the same time.

Not only do you have to use `RefCell`, but you now also have panicking code when the `RefCell` borrow "fails", even though you know it can't. rustc is also not smart enough to notice the exclusivity at compile-time and elid away the RefCell borrow flag test and the panic branch.

    fn foo(mut cb1: impl FnMut(), mut cb2: impl FnMut()) {
        for _ in 0..10 {
            cb1();
            cb2();
        }
    }

    let mut x = String::new();
    foo(|| x.push_str("cb1,"), || x.push_str("cb2,"));

https://play.rust-lang.org/?version=stable&mode=debug&editio...

Fixed using `RefCell`: https://play.rust-lang.org/?version=stable&mode=release&edit... . Inspect the ASM and trace the use of the string constant "already borrowed"; you'll see it being used for the borrow flag test because it wasn't elided.

The equivalent non-Rust program could use String pointers for the two closures. I'm not sure whether they could be noalias or not, but at the very least they wouldn't need to generate any panicking code.

[comex]

FWIW, another option is to use `std::cell::Cell`. That only allows replacing the value rather than borrowing it in place, so you would have to take the value out and put it back after you're done with it, which also results in unnecessary code generation. But there'd be no branch and no panic, so the impact should be less than RefCell. There's also no borrow flag to take up space (not that it really matters when this is a single value on the stack).

[Arnavion]

This is a good point. I always forget Cell can be used with non-Copy types too.

[Diggsey]

The "used from a single thread" aspect is a red herring: RefCell can only be used from a single thread anyway, and the compiler enforces this statically.

The "state" value in a RefCell is overhead, although it's fairly minor given that it doesn't need any synchronization to access. The extra panic branches are probably the largest overhead.

That said, these overheads stem from Rust's safety guarantees rather than its strong type system: you can have a language with a strong type system that does not do these checks.

Furthermore, there are of course ways to avoid this overhead within safe Rust: if you can use the type system to prove that the cell cannot be borrowed at the same time, then you don't need to do the checks, and in that sense a strong type system can actually help avoid overheads that were introduced by being a safe language.

[Arnavion]

>That said, these overheads stem from Rust's safety guarantees rather than its strong type system: you can have a language with a strong type system that does not do these checks.

The difference in semantics between a `&mut T` and a `*mut T` is a type system one. `&mut T` requires that two do not exist at the same time, regardless of whether they are used at the same time or not; this is the contract of the type.

>Furthermore, there are of course ways to avoid this overhead within safe Rust: if you can use the type system to prove that the cell cannot be borrowed at the same time, then you don't need to do the checks, and in that sense a strong type system can actually help avoid overheads that were introduced by being a safe language.

Correct, which is why I made the effort of pointing out that rustc is not smart enough to do it, not that it's impossible to do it.

[codeflo]

This may be splitting hairs a bit, because we all agree that this is a good example where using Rust in this straightforward manner leads to suboptimal performance. But I agree with the grand parent that this is mainly an issue with safety, not with the type system itself.

To show why, consider two alternative languages.

“Weak Rust”: an equally safe Rust with a weaker type system. It might not distinguish & and &mut, but it would still need those checks, because you might use those shared references to break a data structure invariant. It would have to detect such unsafe usage at runtime and raise the equivalent of Java’s ConcurrentModificationException.

“Unsafe Rust”: a less safe Rust with an equally strong type system. It wouldn’t need to do those checks. In fact, that’s basically C++.

[adwn]

Just use `UnsafeCell` instead of `RefCell` [1]: It has zero overhead, but you have to be sure that there's really no simultaneous write/write or read/write access – just like using raw pointers in C or C++.

[1] https://doc.rust-lang.org/beta/std/cell/struct.UnsafeCell.ht...

[Arnavion]

Yes, I'm not averse to using `unsafe`, but one has to justify it on a case-by-case basis. Eg if you're doing this in a library, then keep in mind that some users are very adamant about using unsafe-free crates, so you may prefer to take the hit.

[colejohnson66]

> then keep in mind that some users are very adamant about using unsafe-free crates

Couldn’t you just put the use of unsafe as a default and add a feature flag to force the safe (but slower) behavior. Then you get the best of both worlds: those who don’t care get performance for “free”, while those who care can force it when they want.

[gpm]

Apart from rust not being smart enough to see what you're doing in this sort of situation, the access pattern you're trying to use would actually result in undefined behavior with &mut pointers (or I assume C restrict pointers) because of the aliasing guarantees. For example one optimization you could imagine the compiler actually doing would result in the following

   let mut x = String::new();
   
   let str1 = x; (store x in a local pointer, no one else is touching it because we have aliasing guarantees)
   let str2 = x; (store x in a local pointer, no one else is touching it because we have aliasing guarantees)
   
   for _ in 0.. 10 {
       str1.push_str("cb1,");
       str2.push_str("cb2,");
   }

   x = str1; (restore x to the original variable before our aliasing guarantee goes away)
   x = str2; (restore x to the original variable before our aliasing guarantee goes away)

And you just:

- Leaked str1

- Created an x that just says cb2 repeatedly instead of alternating between cb1 and cb2.

Obviously it's possible to fix this problem by having different guarantees on pointers (C's pointers, rust raw pointers), but it's not clear that the occasional overhead of some metadata tracking (refcell) isn't actually going to be more performant than the constant overhead of not having aliasing guarantees everywhere else. The most performant would be obviously having both, but as we've seen with C asking programmers to go around marking pointers as restrict is too much work for too little benefit.

[Arnavion]

>the access pattern you're trying to use would actually result in undefined behavior with &mut pointers (or I assume C restrict pointers) [...] Obviously it's possible to fix this problem by having different guarantees on pointers (C's pointers, rust raw pointers)

Yes, I said as much in the last paragraph.

>but it's not clear that the occasional overhead of some metadata tracking (refcell) isn't actually going to be more performant than the constant overhead of not having aliasing guarantees everywhere else.

Generating panicking code where it's not needed is bad in general. It adds unwinding (unless disabled), collects backtraces (unavoidable), and often pulls in the std::fmt machinery.

Yes, in general it's almost certainly true that non-aliasing pointers produce more benefits regardless. My comment was in the context of the very specific example it gave.

[nicoburns]

You could rewrite this as an iterator, which would avoid this problem and make the code much nicer.

[Arnavion]

The hypothetical `foo` is a third-party library function.

(The real code which I reduced to this example is https://github.com/Arnavion/k8s-openapi/blob/1fcfe4b34a1f4f1... , and the callee does happen to be another crate in my control. While this can't be reduced to something like an Iterator, it can be resolved by making the calle take a trait with two `&mut self` methods instead of taking two closures. That still requires changing the callee, of course.)

[Too]

In c/c++, Marking every argument as const throughout a deep call chain to later find some edge case where you need to mutate one member far down the call stack, and where this would not have broken the top level contract of the function. Forcing you to do expensive copies instead.

[api]

This is why I think Rust could eventually be faster than C and C++ for a lot of things. The work has to be done though. You're right that noalias enabled optimizations are neglected because you can rarely use them in C code.

On the Rust side I think the language needs some way to annotate if's as likely/unlikely. This doesn't matter in most cases but can occasionally matter a lot in tight high performance code. It can allow the compiler to emit code that is structured so as to cause the branch predictor to usually be right, which can have a large impact.

[steveklabnik]

That's tracked by https://github.com/rust-lang/rust/issues/26179 by the way.

[drran]

Why not just use PGO (Profile Guided Optimizations)?

Sadly, PGO does not work with cross-language LTO, because of conflict of LLVM module name ('Profile').