{"id":3175,"date":"2018-04-30T15:13:45","date_gmt":"2018-04-30T04:13:45","guid":{"rendered":"http:\/\/blog.mozilla.org\/nnethercote\/?p=3175"},"modified":"2020-09-08T16:21:29","modified_gmt":"2020-09-08T05:21:29","slug":"how-to-speed-up-the-rust-compiler-in-2018","status":"publish","type":"post","link":"https:\/\/blog.mozilla.org\/nnethercote\/2018\/04\/30\/how-to-speed-up-the-rust-compiler-in-2018\/","title":{"rendered":"How to speed up the Rust compiler in 2018"},"content":{"rendered":"

18 months ago I wrote about<\/a> some work<\/a> I did to speed up the Rust compiler (rustc). I’ve recently taken this work up again. Also, in the meantime rustc’s build system has been replaced and its benchmark suite has been overhauled. So it’s a good time for an update.<\/p>\n

Getting the code<\/h3>\n

The steps for getting the rustc code haven’t changed. First, I fork the main Rust repository<\/a> on GitHub. Then I make two local clones: a base clone that I won\u2019t 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  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
The compiler’s build system is complex<\/a> with many possible invocations and configurations. I’ll cover the absolute minimum information required to understand how I’ve been using it.<\/p>\n
(UPDATE, December 2019:<\/strong> I have updated this to account for config.toml<\/code> \u00a0 changes since this post was written.)<\/p>\n
First, you need a config.toml<\/code> file, which sits at the root of the repository and dictates the compiler’s configuration. I used the provided config.toml.example<\/code> as a starting point, cut it down a lot, and ended up with the following.<\/p>\n
# Last updated in December 2019\r\n[llvm]\r\nrelease-debuginfo = true\r\n[rust]\r\ndebuginfo-level = 1\r\n<\/pre>\n
This configuration matches the compiler that ships (in particular, in terms of optimization levels), with the exception that it has maximal debug info present, which ensures that profiles are as detailed as possible.<\/p>\n
Building rustc can be confusing, particularly because of the multiple compiler stages and the terminology around them. Here is the command I use.<\/p>\n
(UPDATE, September 2020:<\/strong> I have updated this to account for the src<\/code> directory being renamed compiler<\/code>.)<\/p>\n
.\/x.py build --stage 2 compiler\/rustc<\/pre>\n
This produces a stage 2 compiler that can handle procedural macros and so is suitable for profiling the benchmark suite.<\/p>\n
The benchmark suite<\/h3>\n
rustc’s benchmark suite is called rustc-perf<\/a>. It consists of two parts:<\/p>\n