Iterators vs Array access (in terms of compiler optimizations?)

[lonelyjoey1905]

Dear Rustaceans,

https://rust-embedded.github.io/book/c-tips/index.html#iterators-vs-array-access

In the above link, it is mentioned that

indexed access can be slower (as it needs to be bounds checked) and may prevent various compiler optimisations.

I did check how two different versions of array accesses(indexing vs iterators) are compiled down to assembly with [https://godbolt.org/](Compiler Explorer). I would love to learn more about how array accesses can actually prevent various compiler optimisations, but currently I can't think of any optimizations that could be prevented by array accesses. Any advice would be greatly appreciated! Thank you

[RustyYato]

Because arrays accesses are bounds checked in Rust, putting a panic edge that interferes with optimization. Iterators don't suffer from this problem.

C and C++ don't have these checks on array accesses. So they don't have this problem.

[lonelyjoey1905]

Thanks for the response! May I ask how an extra panic edge in the control flow graph would interfere with optimization?

[RustyYato]

Because LLVM would have to prove that the panic will never happen in order to auto-vectorize the loop. Auto-vectorizing loops is a large source of performance gains, so losing it is bad.