What does it mean to "open code" something in Common Lisp?

Question

In the SBCL user manual there are several references to the term "open code". Common Lisp hackers also use this term when referring to optimizing code.

Could you please explain what it means to "open code" something and give an example of how it works?

Answer 1

What

Open-coding, AKA inlining, means replacing function calls with inline assembly. The idea is that funcall is expensive (it requires saving and restoring the stack &c) and replacing it with the few operations which constitute the function can be beneficial.

E.g., the function 1+ is a single instruction when the argument is a fixnum (which it usually is in practice), so turning the funcall into the two parallel branches (fixnum and otherwise) would be a win.

Control

Declarations

They user can control this optimization explicitly by the inline declaration.

The user can also influence this optimization by the optimize declaration.

Both will affect inlining the code of a function defined just as a function (see below).

Macros

The "old" way is to implement the function as a macro. E.g., instead of

(defun last1f (list)
  (car (last list)))

write

(defmacro last1m (list)
  `(car (last ,list)))

and last1m will be always open-coded. The problem with this approach is that you cannot use last1m as a function - you cannot pass it to, say, mapcar.

Thus Common Lisp has an alternative way - compiler macros, which tell the compiler how to transform the form before compiling it:

(define-compiler-macro last1f (list)
   `(car (last ,list)))

See also the excellent examples in the aforelinked CLHS page.