global_asm - The Rust Unstable Book

# `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:

    #![feature(global_asm)]
    #![cfg(any(target_arch = "x86", target_arch = "x86_64"))]

    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.