# Inline assembly

When you need an instruction the compiler will not emit on its own — a special register, a fence, a custom-ABI trampoline — drop to assembly with `#asm`. The template is a string; the compiler hands it to the backend and wires up the operands you describe.

```cplus
unsafe {
    #asm("nop");
}
```

`#asm` is **unsafe**: the body is opaque to sema and the borrow checker, so it must sit inside an `unsafe` block.

## Operands and clobbers

Operands are named, Rust-style. Each binds a template placeholder `{name}` to a C+ value with a direction and a register constraint:

```cplus
fn add(a: i64, b: i64) -> i64 {
    let mut sum: i64 = 0;
    unsafe {
        #asm("add {s}, {a}, {b}",
            s = out(reg) sum,
            a = in(reg) a,
            b = in(reg) b,
            clobber("cc"));
    }
    return sum;
}
```

- **Direction.** `in` reads a value into the instruction, `out` writes a result back, and `inout` does both through one register.
- **Constraint.** `reg` lets the compiler pick a register — when you use `reg`, the matching `{name}` must appear in the template. Pin a specific register with a string instead, e.g. `"x0"`.
- **Targets.** An `out` or `inout` operand must name a `mut` variable. Operands are register-sized scalars.
- **`clobber("...")`** tells the compiler which registers or flags (`"cc"`, a named register) the block trashes, so it does not assume they survive.

## `#[naked]` functions

A `#[naked]` function gets **no** compiler-generated prologue or epilogue. Its body is inline assembly that handles the ABI itself and returns on its own. This is the tool for entry stubs, trampolines, and custom-ABI shims where even the standard frame setup is in the way.

```cplus
#[naked]
fn trampoline() {
    unsafe {
        #asm("ret");
    }
}
```

The body must be asm-only — any non-asm statement in a `#[naked]` function is **E0909**.

> Module-level global assembly (a top-level `#asm` outside any function) is deferred to a later release. Today inline assembly lives inside function bodies.