{"id":3148,"date":"2016-10-14T16:35:02","date_gmt":"2016-10-14T05:35:02","guid":{"rendered":"http:\/\/blog.mozilla.org\/nnethercote\/?p=3148"},"modified":"2020-11-02T10:49:10","modified_gmt":"2020-11-01T23:49:10","slug":"how-to-speed-up-the-rust-compiler","status":"publish","type":"post","link":"https:\/\/blog.mozilla.org\/nnethercote\/2016\/10\/14\/how-to-speed-up-the-rust-compiler\/","title":{"rendered":"How to speed up the Rust compiler"},"content":{"rendered":"

Rust<\/a> is a great language, and Mozilla plans to use it extensively<\/a> in Firefox. However, the Rust compiler (rustc) is quite slow<\/a> and compile times are a pain point for many Rust users. Recently I’ve been working on improving that. This post covers how I’ve done this, and should be of interest to anybody else who wants to help speed up the Rust compiler. Although I’ve done all this work on Linux it should be mostly applicable to other platforms as well.<\/p>\n

Getting the code<\/h3>\n

The first step is to get the rustc code. First, I fork the main Rust repository<\/a> on GitHub. Then I make two local clones: a base clone that I won’t modify, which serves as a stable comparison point (rust0), and a second clone where I make my modifications (rust1). I use commands something like this:<\/p>\n

user=nnethercote\r\nfor r in rust0 rust1 ; do\r\n  cd ~\/moz\r\n  git clone https:\/\/github.com\/$user\/rust $r\r\n  cd $r\r\n  git remote add upstream https:\/\/github.com\/rust-lang\/rust\r\n  git remote set-url origin git@github.com:$user\/rust\r\ndone<\/pre>\n
Building the Rust compiler<\/h3>\n
Within the two repositories, I first configure:<\/p>\n
.\/configure --enable-optimize --enable-debuginfo<\/pre>\n
I configure with optimizations enabled because that matches release versions of rustc. And I configure with debug info enabled so that I get good information from profilers.<\/p>\n
[Update:<\/strong> I now add –enable-llvm-release-debuginfo which builds the LLVM back-end with debug info too.]<\/p>\n
Then I build:<\/p>\n
RUSTFLAGS='' make -j8<\/pre>\n
[Update:<\/strong> I previously had -Ccodegen-units=8<\/code> in RUSTFLAGS<\/code> because it speeds up compile times. But Lars Bergstrom informed me that it can slow down the resulting program significantly. I measured and he was right — the resulting rustc was about 5–10% slower. So I’ve stopped using it now.]<\/p>\n
That does a full build, which does the following:<\/p>\n