In this example, the loop bound is from the lhs, so llvm can prove that it will never get out of bounds and elide the bound checks. But llvm can't be sure that

In this example, the loop bound is from the lhs, so llvm can prove that it will never get out of bounds and elide the bound checks. But llvm can't be sure that the rhs has the same size, so it can't elide the bound checks for the rhs. However, there's a trick. Take a look at https://github.com/rust-lang/rust/commit/6a7bc47a8f1bf4441ca55e42aa50db6dda28181f https://github.com/rust-lang/rust/commit/6a7bc47a8f1bf4441ca... where the Rust developers had the same issue, and managed to avoid the bound checks without having to use unsafe code. You have to somehow make the optimizer see that both sides have the same length, so it'll elide both bounds checks. A slice is actually a pair of a pointer to the first element, and the length. Their trick copies the length from one slice to the other (after a bound check, obviously), so the compiler is sure that the lengths are the same.