I too run an Arch and am happy with it, and I would like to know why Cachy. The only reason I can see is having x86-64-v3 packages instead of baseline. That is nice, but on it’s own doesn’t feel worth the effort of switching over.

Defaults don’t matter to me much, as I automate and manage my system config in git (using a tool I wrote myself: https://github.com/VorpalBlade/paketkoll/tree/main/crates/konfigkoll inspired by https://github.com/CyberShadow/aconfmgr). It makes it a breeze to set up a new computer as I want it.

12th verse: I’m sane, I promise

Hmm…

As to LLVM and alloca, it doesn’t optimise or even work well in practise. Some basic cases work, others are less well tested. There are lots of “should” that “doesn’t” in practice in LLVM.

I have not looked at alloca in LLVM myself but from what I have heard from those who are experts on this, it is quite brittle.

Second of all: sub is my favourite allocator

https://docs.rs/bumpalo/latest/bumpalo/ (and bump allocators in general).

Fourth of all: second point is literally a skill issue idk, especially if your compiler is already proving bounds anyway.

In general proving bounds for stack growth is very difficult. With recursion it is undecidable. This follows directly from Rice’s Theorem. (This is my favourite theorem, it is nice to know that something is impossible rather than a skill issue.)

(Of course you could have a static analyser that instead of yes/no returns yes/no/don’t know, and then you assign don’t know to be either of the other classes depending on if you care more about false positives or false negatives. This is how the rust borrow checker works: forbid if it can’t prove it is safe, but there will be safe code that it doesn’t allow.)

Alloca for buffers is generally a bad idea:

• If you have an upper known bound, just use that. If you don’t you risk stack overflow. Especially on a malicious input.
• Alloca optimises poorly (especially with LLVM) since the compiler can’t really see how large the stack frame is.

The only reasonable usage I have seen is in the Swift ABI. See this blog post on the topic by a rust compiler developer: https://faultlore.com/blah/swift-abi/ (and even there it is only for some cases that it can be used).

Not really. Heapless uses compile time sized backing buffers to implement Vec, string etc with a max upper size. You would typically use heapless with a statically allocated variable, but it is possible to use it on the stack too.

Alloca is different and allocates a dynamically sized block on the stack. Rust doesn’t really support alloca, but there is a crate for it that works by calling through a helper function in C: https://lib.rs/crates/alloca

Also worth noting is smallvec and compact_str crates. These are useful when you most of the time have small data that you want inline, but are OK with falling back to heap allocation for the occasional outlier.

I have used both inside structs to optimise cache usage. For these uses i tend to use rather short smallvec.

And smallvec in particular is also useful on the stack in a larger variant in hot functions where you might have a Vec that almost always is less than (say) 32 elements, but the program should gracefully handle the occasional long case. I found this offered a small speed up in some batch data processing code I wrote.

• https://docs.rs/smallvec
• https://docs.rs/compact_str