global_asm

The tracking issue for this feature is: #35119


The global_asm! macro allows the programmer to write arbitrary assembly outside the scope of a function body, passing it through rustc and llvm to the assembler. The macro is a no-frills interface to LLVM's concept of module-level inline assembly. That is, all caveats applicable to LLVM's module-level inline assembly apply to global_asm!.

global_asm! fills a role not currently satisfied by either asm! or #[naked] functions. The programmer has all features of the assembler at their disposal. The linker will expect to resolve any symbols defined in the inline assembly, modulo any symbols marked as external. It also means syntax for directives and assembly follow the conventions of the assembler in your toolchain.

A simple usage looks like this:

#![feature(global_asm)]
// you also need relevant target_arch cfgs
global_asm!(include_str!("something_neato.s"));

And a more complicated usage looks like this:


#![allow(unused)]
#![feature(global_asm)]
fn main() {
#[cfg(any(target_arch="x86", target_arch="x86_64"))]
mod x86 {

pub mod sally {
    global_asm!(r#"
        .global foo
      foo:
        jmp baz
    "#);

    #[no_mangle]
    pub unsafe extern "C" fn baz() {}
}

// the symbols `foo` and `bar` are global, no matter where
// `global_asm!` was used.
extern "C" {
    fn foo();
    fn bar();
}

pub mod harry {
    global_asm!(r#"
        .global bar
      bar:
        jmp quux
    "#);

    #[no_mangle]
    pub unsafe extern "C" fn quux() {}
}
}
}

You may use global_asm! multiple times, anywhere in your crate, in whatever way suits you. The effect is as if you concatenated all usages and placed the larger, single usage in the crate root.


If you don't need quite as much power and flexibility as global_asm! provides, and you don't mind restricting your inline assembly to fn bodies only, you might try the asm feature instead.