Emacs native-comp on CentOS 7

tags: emacs

The GNU Emacs feature/native-comp branch has been under development for some time now. The performance enhancements from the natively compiled Emacs Lisp code are exciting. Notably, I’ve been seeing nice speed-ups for Helm completions and a smoother lsp-ui experience.

Andrea Corallo is developing this new feature and updates/descriptions of the work can be tracked/found on his website. I’ve been building the branch on my CentOS 7 machine for a few weeks now, and I thought I’d walk through the process.

Building Emacs using --with-native-compilation

Update March 2021: The configure option was originally --with-nativecomp, but it has changed to --with-native-compilation.

Update June 2021: The feature/native-comp branch was merged into the master branch at the end of April. Originally this post included instructions to checkout the feature branch; those instructions have been removed.

We need to install libgccjit. Unfortunately CentOS 7 shipped with a pretty old GCC release (the 4.8 series). Fortunately Red Hat ships modern GCC builds with a number of devtoolset RPMs. We can install libgccjit (and the necessary development headers) from the GCC 9 series via:

# yum install devtoolset-9-gcc devtoolset-9-libgccjit-devel

We’ll build Emacs from source after checking out the feature branch:

$ git clone https://git.savannah.gnu.org/git/emacs.git
$ cd emacs

Now we’ll build Emacs after enabling devtoolset-9. We ensure that pkg-config will search in /usr/lib64/pkgconfig for installed packages, such as gnutls or libjansson installed with yum (this is necessary because we are installing with GCC 9 from devtoolset-9 and not the default /usr/bin/gcc compiler). (Since we’re playing with an experimental feature, I’m going to assume that you’ve built Emacs from source before and that you can handle all other desirable configure options).

$ source /opt/rh/devtoolset-9/enable
$ ./autogen.sh
$ PKG_CONFIG_PATH=/usr/lib64/pkgconfig:$PKG_CONFIG_PATH ./configure \
   --with-native-compilation # ... other configure options
$ make -j6 NATIVE_FULL_AOT=1

Notice the use of NATIVE_FULL_AOT=1. This will ask Emacs to compile all builtin Emacs Lisp code natively (the AOT stands for ahead-of-time). If this option is not used, only bare minimum will be natively compiled, and lot of the shipped Emacs Lisp packages will be regular byte compiled. If you enable deferred compilation (keep reading), those package will be natively compiled on the fly (the first time they are loaded, that’s the just-in-time compilation). I prefer to natively compile all of Emacs when I build it, that way Emacs will only just-in-time compile my third-party packages.

Another note: the Emacs binary built with the steps above will always require devtoolset-9. Sourcing the enable script at /opt/rh/devtoolset-9 will be necessary.

Once Emacs is compiled we can run it with src/emacs (you can set an install prefix, but this is an experimental feature, so I only run this executable from the development repository and keep a master branch build installed somewhere in my PATH). The remainder of this post is not specifically related to CentOS 7, but it’s still useful.

Deferred and asynchronous compilation

Before we run Emacs we can add a few lines to the top of our init.el file to steer deferred/async compilation. When running Emacs with the native-compile-async symbol defined, we ask if we want to run the deferred async compilation. If yes, set the number of jobs that can run in the background. There are a few other variables that control native compilation. I recommend giving the comp- customization group a quick read.

;; helper boolean I use here and later in my init.el
(defconst dd/using-native-comp (and (fboundp 'native-comp-available-p)
                                    (native-comp-available-p)))
(setq native-comp-deferred-compilation t)
(setq native-comp-async-query-on-exit t)
(setq native-comp-async-jobs-number 4)
(setq native-comp-async-report-warnings-errors nil)

Emacs will asynchronously natively compile all .elc files that it loads. So if your init.el file loads a lot of packages, prepare for Emacs to spend a bit of time compiling. Fortunately you can still use Emacs while that is happening in the background.