undefined | Better HN

0 pointsformerly_proven3y ago0 comments

If we understand "straightforward" in the sense that small changes in the input lead to predictable and small changes in the output, then I can't see how this is reasonably true for pretty much any optimizing compiler.

Just today we had this nice example on the front page where initializing a variable differently completely changed the codegen for large parts of the program: https://jpieper.com/2022/08/05/debugging-bare-metal-stm32-fr...

0 comments

5 comments · 2 top-level

j-pb3y ago· 3 in thread

Rust screams UB much quicker when it comes to memory semantics than any other language that I know. Rusts lower bounds of code mangling are also much higher than in other languages.

I don't expect to have a good mental model of an optimising compiler in "gotta go fast"-mode, but a mental model of what the memory layout looks like is pretty darn relevant when designing or working with ABIs and binary data representations.

Rusky3y ago

> I don't expect to have a good mental model of an optimising compiler in "gotta go fast"-mode, but a mental model of what the memory layout looks like is pretty darn relevant when designing or working with ABIs and binary data representations.

I'm not sure where you got the idea otherwise, but Rust supports the same memory layouts and ABIs as C on a given platform.

You don't get a specific layout by default because that allows for the implementation to improve over time, but that's not really relevant when working with binary data representations, where you simply specify which one to use.

ralfj3y ago

> Rust screams UB much quicker when it comes to memory semantics than any other language that I know.

If you only use raw pointers, Rust has significantly less UB that C does.

But if you use references, then yeah we have those aliasing rules and they can be quite restrictive.

j-pb3y ago

My brain isn't cached into the issue anymore, because I've since solved the same problem with Zig, but about a year ago there was no way to have a reference counted tagged pointer that points to a boxed DST with the minimum number of bytes. Every path lead through UB or "not yet defined" somewhere.

You wrote a PHD on it so you might have a better idea of the issues that past me ran into, than what present me can still recall ;)

1 more reply

SAI_Peregrinus3y ago

It's also worth noting that (at least for C), the way the standard is worded an optimizing compiler and a non-optimizing compiler can generate exactly the same output in every situation. NONE of the unpredictability actually depends on optimization, it's just that optimization can (sometimes, if you're lucky) expose it. If you're not lucky, the code gen changes based on something harder to control than a single compiler option.

j / k navigate · click thread line to collapse