Why doesn't the Godbolt compiler explorer show any output for my function when compiled in release mode?

Question

I want to use https://rust.godbolt.org to see the assembly output of this function:

fn add(a: u8, b: u8) -> u8 {
    a + b
}

Pasting this on the website works fine, but shows a lot of assembly. This is not unsurprising, given that rustc compiles my code in debug mode by default. When I compile in release mode by passing -O to the compiler, there is no output at all!

What am I doing wrong? Why does the Rust compiler remove everything in release mode?

Answer 1

Godbolt compiles your Rust code as a library crate by passing --crate-type=lib to the compiler. And code from a library is only useful if it's public. So in your case, your add() function is private and is removed from the compiler completely. The solution is rather simple:

Make your function public by adding pub to it. Now the compiler won't remove the function, since it is part of the public interface of your library.