Inline Assembly

Vex supports asm!(...) expressions for low-level and freestanding code. The current parser accepts the builtin-style operand helpers shown on this page.

Current Syntax

unsafe {
    asm!("nop")
}

Operands and options are passed as builtin-style helpers inside the asm!(...) call.

Operand Helpers

The parser currently recognizes these forms:

• $asmIn("constraint", value)
• $asmOut("constraint", place)
• $asmLateOut("constraint", place)
• $asmInOut("constraint", place)
• $asmInLateOut("constraint", place)
• $asmOpt("pure", "nomem", ...)
• $asmClobber("memory", "cc", ...)

Basic Example

fn main() {
    let x: i64 = 20
    let! result: i64 = 0

    unsafe {
        asm!("nop", $asmIn("r", x))
        asm!("mov $0, 0", $asmOut("=r", result))
        asm!("nop", $asmOpt("pure", "nomem"))
    }

    $print(result)
}

This is the same builtin-style surface used by current example code.

Outputs and Wildcards

Output helpers accept either a destination or _ when you want to discard the result:

unsafe {
    asm!("nop", $asmLateOut("=r", _))
}

Clobbers and Options

unsafe {
    asm!(
        "nop",
        $asmOpt("pure", "nomem", "nostack"),
        $asmClobber("memory", "cc")
    )
}

The exact accepted constraint strings and option combinations still depend on backend support, target, and codegen path. Keep inline asm minimal and target-specific.

Legacy Syntax Note

Older in(...), out(...), and similar Rust-style operand forms are no longer the documented surface here. Use the $asmIn and $asmOut family instead.

Guidance

1. Keep asm! inside unsafe blocks.
2. Prefer higher-level intrinsics or stdlib APIs when they exist.
3. Use the narrowest possible constraints and the fewest clobbers needed.
4. Treat inline assembly as target-specific code, not portable application logic.

Related

• Freestanding
• Unsafe
• Builtins