3 ms·

Gee, if only someone would question whether this whole “UB” thing is a good idea.

• pickdenis 8y ago
  My understanding is that there's no way around it if you want the best possible optimization. Otherwise you'd have to insert extra runtime checks, right?
  • FartyMcFarter 8y ago
    Indeed.
  • dodobirdlord 8y ago
    Not necessarily. Rust has taken an alternative approach to runtime checks by designing the language in such a way that scenarios that could cause undefined behavior at runtime are instead compile-time errors. It's probably too late to carry out an initiative like that in older compilers, but its certainly an option.
    • leod 8y ago
      This is true for some scenarios, like invalidating iterators through deletion (detected at compile-time by the borrow checker), but other scenarios still require runtime checks, right? Consider e.g. array out of bound accesses -- are you aware of approaches that move bounds checks to compile-time? It seems to me that this would be a painstaking process that would require programmers to annotate their code in many places to enable compile-time verification.
      • steveklabnik 8y ago
        It just depends. Yes, Rust has no UB in safe Rust (modulo bugs). Sometimes, that means compile-time checks. Sometimes, that means runtime checks. It depends on the specific thing. The compiler _will_ attempt to prove that bounds checks aren't needed and eliminate them; see https://godbolt.org/z/7QPfhR https://godbolt.org/z/7QPfhR vs https://godbolt.org/z/Vx39fv https://godbolt.org/z/Vx39fv for example. In the first, there's an array and the compiler knows that it has a length of 3, so an index of zero needs no checks. In the second, we don't know how long the slice is, so we have to do the check.