This package provides two minor modes which automatically recompile Emacs Lisp source files. Together these modes guarantee that Emacs never loads outdated byte code files.

auto-compile-on-save-mode re-compiles source files when they are being saved and auto-compile-on-load-mode does so before they are being loaded (by advising load and require). Both modes only ever re-compile a source file when the respective byte code file already exists but is outdated. Otherwise they do not compile the source file.

Even when using auto-compile-on-save-mode it can happen that some source file is newer than the respective byte code file, which is a problem because by default Emacs loads the byte code file even when the respective source file has been modified more recently.

Starting with Emacs version 24.4, setting load-prefer-newer to t prevents outdated byte code files from being loaded. However this does not cause re-compilation of the source file, to actually do that auto-compile-on-load-mode is still required.

To reduce the risk of loading outdated byte code files, you should set load-prefer-newer and enable auto-compile-on-load-mode as early as possible. Then also enable auto-compile-on-save-mode. You should also consider not byte-compiling your personal init file, or setting load-prefer-newer in a system-wide init file.

If you use package.el then use something like this:

;;; init.el --- user init file
(setq load-prefer-newer t)
(package-initialize)
(require 'auto-compile)
(auto-compile-on-load-mode)
(auto-compile-on-save-mode)

otherwise:

;;; init.el --- user init file
(setq load-prefer-newer t)
(add-to-list 'load-path "/path/to/auto-compile")
(require 'auto-compile)
(auto-compile-on-load-mode)
(auto-compile-on-save-mode)

You might want to set the file-local value of no-byte-compile to t, e.g., by adding -*- no-byte-compile: t -*- at the end of the very first line. That way all user files benefit from the protection offered by load-prefer-newer and the modes that are defined here, otherwise ~~/.emacs.d/init.el~ is the unfortunate exception.

If you are using Emacs 27 or later, then these settings should be placed in early-init.el, which should never be compiled:

;;; early-init.el --- early bird  -*- no-byte-compile: t -*-
(setq load-prefer-newer t)
(add-to-list 'load-path "/path/to/auto-compile")
(require 'auto-compile)
(auto-compile-on-load-mode)
(auto-compile-on-save-mode)
;;; early-init.el ends here

Take note of the compile warnings and fix them.

To permanently or temporarily toggle automatic compilation of some source file use the command toggle-auto-compile. Since the modes only ever update byte code files, toggling automatic compilation is done simply by either creating the byte code file or by removing it. toggle-auto-compile can also toggle automatic compilation of multiple files at once; see its doc-string for more information.

Constantly having the *Compile-Log* buffer pop up when a file is being saved can quickly become annoying. Obviously the first thing you should do to about that is to actually fix outstanding issues.

Once you have done that you might also want to keep that buffer from being automatically displayed and instead only show the number of compile warnings for the current file in the mode-line.

(setq auto-compile-display-buffer nil)
(setq auto-compile-mode-line-counter t)

To display the buffer use M-x auto-compile-display-log or click on the counter in the mode-line.

Using auto-compile-inhibit-compile-hook it is possible to inhibit automatic compilation under certain circumstances; e.g., when HEAD is detached inside a Git repository (useful during rebase sessions).