Emacs: Byte Compile Elisp Files

What is Byte Compiled File

Emacs lisp files can be byte compiled.

• Any emacs lisp file can be byte compiled into a file.
• Any emacs lisp expression can be byte compiled.

• Byte compiled elisp file has file name extension .elc
• Normal elisp file is .el

Advantage of Byte Compiled Elisp File

• Byte compiled elisp files load faster.
• Byte compiled elisp functions also run faster. (by a simple test of a loop 50 million times, it is about 8 times faster.)
• Byte compiling will often tell you errors or warning in your elisp code that you normally wouldn't know.

As of today 2025-09-19, for init files and light-weight packages, byte compile don't make any noticeable speed difference.

Disadvantage of Byte Compiled Elisp File

Byte compile creates a extra source of the code. If you edit elisp code and forget to byte compile it, it create a sync problem. Emacs usually check file date when loading a file, and if byte compiled file is older, emacs warns you.

Byte compile may not be compatible across different version of emacs or operating system. Technically it is, but sometimes not. When you are using a incompatible byte compiled file, it creates strange bugs.

How to Byte Compile Emacs Lisp File

There are several ways to byte compile elisp files. The simplest and most useful are:

Alt+x byte-compile-file

Compile a single file. It'll ask for a file name.

Alt+x dired-do-byte-compile 【B】

Compile all marked files in dired.

Alt+x byte-recompile-directory

Batch compile all elisp files in current dir and subdirectory.

Loading Byte Compiled File

• Elisp: load, load-file, autoload

Recompile When Upgrade

you should recompile your personal elisp files when:

• Upgrade to a new emacs version
• Upgrade packages
• Moving byte compiled elisp files from one machine to another

because, often emacs has some incompatible elisp changes, and big packages may fail without recompile.

Reference

• Byte Compilation (ELISP Manual)