Comment by ralfj
3 days ago
I would agree that C's strict aliasing rules are terrible. The rules we are proposing for Rust are very different. They are both more useful for compilers and, in my opinion, less onerous for programmers. We also have an actual in-language opt-out: use raw pointers. And finally, we have a tool you can use to check your code.
But in the end, it's a trade-off, like everything in language design. (In life, really. ;) We think that in Rust we may have found a new sweet spot for this kind of optimizations. Time will tell whether we are right.
As someone who has been writing a lot of unsafe Rust (mostly in an embedded context), I'm thrilled about and thankful for the work that you, your students, and the opsem team are doing.
When you're working with anything below the application level, C's confusing and underspecified rules about UB are almost impossible to keep track of, especially when it comes to aliasing and volatile/MMIO. The spec is so difficult to read and full of complicated cross-references that to actually get a practical answer you have to look for a random Stack Overflow post that may or may not have a correct interpretation of the spec, and may or may not address your specific problem.
Rust right now feels a lot harder to work with, because the spec isn't done. When you have a concrete question about a piece of code, like "is this conversion from an &mut to a *mut and back sound", and you try to look for documentation on it, you get either "Nobody knows, Rust aliasing model isn't defined"; a hand-wavy explanation that is not rigorous or specific; or a model like Stack Borrows or Tree Borrows that's defined a little too formally for easy digestion :)
But when I really started digging, I realized just how much cleaner Rust's semantics are. References aren't actually hard, Tree Borrows basically boils down to "while an &mut reference is live, you can only access the value through pointers or references derived from that reference". Pointer operations have straightforward semantics, there's no confusing notions of typed memory, and no UB "just because" for random things like integer overflow. It's just so much less complicated to understand than C's abstract machine.
I'm really looking forward to things like MiniRust, and to an aliasing model making it into the Reference / other documentation, because at that point I feel like unsafe Rust will be way easier to write confidently and correctly than C.
Congrats on the publication, and thanks again for the work you all have put into this.
In C, you can alias pointers if they have compatible types. Not the case in Rust for mutable references. And the rules of Rust have tripped up even senior Rust developers.
https://github.com/rust-lang/rust/commit/71f5cfb21f3fd2f1740...
Without MIRI, a lot of Rust developers would be lost, as they do not even attempt to understand unsafe. And MIRI cannot and does not cover everything, no matter how good and beloved it is.
It should have been possible for senior Rust developers to write UB-free code without having to hope that MIRI saves them.
The situation is not that bad. The rules of unsafe code were pretty badly defined back then, but they are in process of becoming a lot clearer, and like the grandparent argues, with a well-defined aliasing model like Tree Borrows, they are easier to understand than C's.
If you look into the code you linked, the problem was about accessing undefined bytes though an aliased, differently-typed pointer – something you would have hard time doing in C to begin with. MaybeUninit was a new thing back then. I think that nowadays, a senior Rust developer would clear the hurdles better.
8 replies →
> And the rules of Rust have tripped up even senior Rust developers.
Yeah, even senior Rust devs make mistakes. Thanks to Miri, we can catch such mistakes. No reasonable person would expect even senior Rust devs to be magic superheroes that can write tricky unsafe code without making any mistake.
How confident are you that glibc has zero Undefined Behavior? I rather doubt it. The Rust standard library has its entire test suite (well, almost everything, except for some parts in std::fs and std::net) run through Miri. That's not a proof there's no UB in corner cases not covered by the tests, but it means we are much, much more likely to find such bugs and fix them than comparable C code.
C has an opt-out that works sometimes, if a compatible type exists. Rust has an opt-out that works always: use raw pointers (or interior mutable shared references) for all accesses, and you can stop worrying about aliasing altogether.
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Thank you for your kind words. :)
One example of MIRI not being guaranteed to handle all cases.
https://github.com/rust-lang/rust/pull/139553#issuecomment-2...
The above issue was part of diagnosing UB in Rust stdlib.
Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool. However, we have some ongoing work that should make Miri a lot better at this... we are just not sure yet whether we can get it to have usable performance. ;)
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Agreed about C's aliasing rules. Fortran had a better set of defaults.
Thanks for replying Ralf. I'm barely qualified to have an opinion about these things, if I am at all :)
> The rules we are proposing for Rust are very different. They are both more useful for compilers and, in my opinion, less onerous for programmers.
My question was too vague, what I meant to ask was: what aliasing optimizations will be possible in Rust that aren't possible in C?
Example 18 in the paper is one, if I'm understanding it. But that specific example with a pointer passed to a function seems analogous to what is possible with 'restrict' in C, I'm struggling to come up with more general examples that don't involve gratuitous globals.
It seems to me like having to allow for the possibility of unsafe constrains the ability to do "heroic" optimizations such as what jcranmer described elsewhere in the thread. If that is true to some extent, is there a future where Rust might optimize more aggressively if the programmer promises not to use unsafe anywhere in the program? That's something I've always been curious about.