[roca]

Rust has some extra costs, but it also has advantages over C and C++.

For example, the Rust compiler can and does reorder fields in structs to improve packing. In C and C++ you have do it manually, and for C++ templated types you sometimes can't pick an order that's optimal for all type parameters. (Rust can pick different field orders for different monomorphized types.)

The Rust compiler does other nice representation optimizations, e.g. Option<bool> is represented as a single byte with three possible values. Not just a hack for Options, but general.

Maybe even more importantly, Rust is really strict about aliasing and this can improve optimization. E.g. a variable that's a mutable reference to T ("&mut T") cannot alias any other reference to T in scope. An immutable reference to T ("&T") can alias other immutable references to T, but the data is truly immutable (unlike a C or C++ const reference). This completely subsumes "type-based alias analysis" and also C/C++ "restrict", and is stronger than both. This information is potentially really useful for optimizers, but unfortunately, since LLVM is mainly for C/C++, the Rust compiler can't take full advantage of this aliasing information yet :-(.

[vardump]

> For example, the Rust compiler can and does reorder fields in structs to improve packing.

Yikes! That can be disabled, right?

[dthul]

Yeah, if you need "C" style struct representation (which guarantees order), you can enable that on a per-struct basis.

[roca]

Use #[repr(C)] to force C-compatible layout.