@always_inline

You can add the @always_inline decorator on any function to make the Mojo compiler "inline" the body of the function (copy it) directly into the body of the calling function.

This eliminates potential performance costs associated with function calls jumping to a new point in code. Normally, the compiler will do this automatically where it can improve performance, but this decorator forces it to do so. The downside is that it can increase the binary size by duplicating the function at every call site.

For example:

@always_inline
fn add(a: Int, b: Int) -> Int:
    return a + b

print(add(1, 2))

Because add() is decorated with @always_inline, Mojo compiles this program without adding the add() function to the call stack, and it instead performs the addition directly at the print() call site, as if it were written like this:

print(1 + 2)

@always_inline("nodebug")

You can also use the decorator with the "nodebug" argument, which has the same effect to inline the function, but without debug information. This means that you can't step into the function when debugging.

This decorator is intended to be used on the lowest-level functions in a library, which may wrap primitive functions, MLIR operations, or inline assembly. Marking these functions as "nodebug" prevents users from accidentally stepping into low-level non-Mojo code when debugging.