Emacs Byte Compilation

A feature unique to some Lisps is the ability to compile functions individually at any time. This could be to a bytecode or native code, depending on the dialect and implementation. In a Lisp implementations where compilation matters (such as CLISP), there are typically two forms in which code can be evaluated: a slower, unoptimized uncompiled form and a fast, efficient compiled form. The uncompiled form would have some sort of advantage, even if it's merely not having to spend time on compilation.

In Emacs Lisp, the uncompiled form of a function is just a lambda s-expression. The only thing that gives it a name is the symbol it's stored in. The compiled form is a (special) vector, with the actual byte codes stored in a string as the second element. Constants, the docstring, and other things are stored in this function vector as well. The Elisp function to compile functions is byte-compile. It can be given a lambda function or a symbol. In the case of a symbol, the compiled function is installed over top of the s-expression form.

(byte-compile (lambda (x) (* 2 x)))
  => #[(x) "^H\301_\207" [x 2] 2]

The compiler will not only convert the function to bytecode and expand macros, but also perform optimizations such as removing dead code, evaluating safe constant forms, and inline functions. This provides a nice performance boost (testing using my measure-time macro),

(defun fib (n)
  "Fibonacci sequence."
  (if (<= n 2) 1
    (+ (fib (- n 1)) (fib (- n 2)))))
 (fib 30))
  => 1.0508708953857422

(byte-compile 'fib)

 (fib 30))
  => 0.4302399158477783

Most of the installed functions in a typical Emacs instance are already compiled, since they are loaded already compiled. But a number of them aren't compiled. So, I thought, why not spend a few seconds to do this?

In Common Lisp, there is a predicate for testing whether a function has been compiled or not: compiled-function-p. For whatever reason, there is no equivalent predefined in Elisp, so I wrote one,

(defun byte-compiled-p (func)
  "Return t if function is byte compiled."
   ((symbolp   func) (byte-compiled-p (symbol-function func)))
   ((functionp func) (not (sequencep func)))
   (t nil)))

My idea was to iterate over every interned symbol and, if the function slot contains an uncompiled function, using the test above, I would call byte-compile on it. Well, it turns out that byte-compile is very flexible and will ignore symbols with no function and symbols with already compiled functions.

So next, how do we iterate over every interned symbol? There is a mapatoms function for this. Provide it a function and it calls it on every interned symbol. Well, that's simple and anticlimactic.

(mapatoms 'byte-compile)

That's it! It will take only a few seconds and spew a lot of warnings. I haven't found a way to disable those warnings, so this isn't something you'd want to have run automatically, unless you like having an extra window thrown in your face. I've only discovered this recently, so I'm not sure what sort of bad things this may do to your Emacs session. Not every function was written with compilation in mind. There are interactions with macros to consider.

I doubt there will be a noticeable performance difference. Like I said before, most everything is already compiled, and those are the functions that get used the most. There's just something nice about knowing all your functions are compiled and optimized.

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Chris Wellons