3 ms·

I find this decision frustrating. They made it clear by making it trap in debug that this is a "bug" but denied the optimization benefits of calling it undefine

• eddyb 6y ago
  > but I have not seen any evidence that the average program is actually more likely to behave correctly in the face of wrapping. Rust takes a simple stance of "safe code should be UB-free". Yes, you may have logic bugs with wrapping, but you can't cause memory unsafety with it in, because things like array/slice bound checks aren't ever disabled. Also, C relies on undefined signed overflow for things like "`for` loops incorrectly using `int` instead of `size_t` because it's easier to type", which doesn't really apply to Rust (which require `usize`, the `size_t` equivalent, for indexing, and has pointer-range-based iterators for slices), so I doubt UB overflow in Rust would help performance much. It would be trivial to change `rustc_codegen_llvm` to set LLVM's `nsw`/`nuw` (in order to make signed/unsigned overflow UB), if you want to prove it improves performance somewhere.