New inline assembly syntax available in nightly

June 8, 2020 · Josh Triplett on behalf of the language team

In the course of optimization, OS or embedded development, or other kinds of low-level programming, you may sometimes need to write native assembly code for the processor you're running on. "Inline assembly" provides a simple way to integrate some assembly instructions into a Rust program, feeding Rust expressions in as input registers, and getting output directly into Rust variables. We've introduced a new syntax for inline assembly in nightly Rust, and we're seeking feedback on it; we believe this new syntax has a path to stabilization in the future.

Nightly Rust has had a syntax for "inline assembly" (asm!) for a long time; however, this syntax just exposed a very raw version of LLVM's assembly construct, with no safeguards to help developers use it. Getting any detail of this syntax even slightly wrong tended to produce an Internal Compiler Error (ICE) rather than the kind of friendly error message you've come to expect from rustc. This syntax was also error-prone for another reason: it looks similar to GCC's inline assembly syntax, but has subtle differences (such as the names in register constraints). This syntax also had little to no hope of being supported on any non-LLVM backend. As a result of all these limitations, the asm! syntax was highly unlikely to ever graduate from nightly to stable Rust, despite being one of the most requested features.

In an effort to improve asm! and bring it to more users, Amanieu d'Antras designed and implemented a new, friendlier syntax for asm!. This syntax has had a long road from concept to compiler implementation:

  • The proposal first started as a pre-RFC on internals.
  • Inline assembly became one of the language team's first project groups, and iteratively designed RFCs in the project group repository.
  • RFC 2873 (still under discussion) provides a specification for the syntax and its interaction with the Rust language.
  • We renamed the existing asm! to llvm_asm!, so that people currently using inline assembly on nightly can continue to use the existing syntax for now. (We plan to remove this syntax eventually, given its fragile ICE-happy nature, but while evaluating the new syntax we want the old syntax available for comparison and alternatives.)
  • PR 69171 (also by Amanieu) implemented the new asm! syntax in nightly.

Here's an example of using the new inline assembly syntax, to print a message to standard output using a direct write syscall on x86-64 Linux:


fn main() {
    let buf = "Hello from asm!\n";
    let ret: i32;
    unsafe {
            in("rax") 1, // syscall number
            in("rdi") 1, // fd (stdout)
            in("rsi") buf.as_ptr(),
            in("rdx") buf.len(),
            out("rcx") _, // clobbered by syscalls
            out("r11") _, // clobbered by syscalls
            lateout("rax") ret,
    println!("write returned: {}", ret);

(You can try this example on the playground.)

The example above specifies the exact inputs, outputs, and clobbers required by the Linux syscall calling convention. You can also provide inputs and outputs via arbitrary registers, and the compiler will select appropriate registers for you. The following example uses bit manipulation instructions to compute the bit numbers of all set bits in a value, and stores them in a slice of memory:


fn main() {
    let mut bits = [0u8; 64];
    for value in 0..=1024u64 {
        let popcnt;
        unsafe {
                "popcnt {popcnt}, {v}",
                "blsi rax, {v}",
                "jz 1f",
                "xor {v}, rax",
                "tzcnt rax, rax",
                "jmp 2b",
                v = inout(reg) value => _,
                popcnt = out(reg) popcnt,
                out("rax") _, // scratch
                inout("rdi") bits.as_mut_ptr() => _,
        println!("bits of {}: {:?}", value, &bits[0..popcnt]);

(You can try this example on the playground. Note that this code serves to demonstrate inline assembly, not to demonstrate an efficient implementation of any particular algorithm.)

Notice that value and popcnt have registers selected for them, while bits.as_mut_ptr() must go in the rdi register for use with the stosb instruction.

Also, note that on x86 platforms, asm! uses Intel syntax by default; however, you can use AT&T syntax with option(att_syntax). You may find this useful when translating existing inline assembly code to the new asm! syntax.

For full details on the new asm! syntax, see RFC 2873. Please try it out (including translating existing inline assembly to the new syntax), and report any bugs via the rust issue tracker with the tag F-asm. You can also discuss inline assembly by creating a topic on the project-inline-asm stream in Zulip.