roc/BUILDING_FROM_SOURCE.md

Building the Roc compiler from source

Installing LLVM

To build the compiler, you need a particular version of LLVM installed on your system.

To see which version of LLVM you need, take a look at Cargo.toml, in particular the branch section of the inkwell dependency. It should have something like llvmX-Y where X and Y are the major and minor revisions of LLVM you need.

For Ubuntu, I used the Automatic installation script at apt.llvm.org - but there are plenty of alternative options at http://releases.llvm.org/download.html

You may run into an error like this:

    Updating git repository `https://github.com/TheDan64/inkwell`
error: failed to load source for a dependency on `inkwell`

Caused by:
  Unable to update https://github.com/TheDan64/inkwell?branch=llvm8-0#d0f5c1e1

Caused by:
  revspec 'd0f5c1e198853bc06d8427fbafb7b068032d1d1a' not found; class=Reference (4); code=NotFound (-3)

This seems to be caused by cargo being out of date (even if it's freshly installed), and can be fixed with cargo update.

Use LLD for the linker

Using lld for Rust's linker makes build times a lot faster, and I highly recommend it.

Create ~/.config/cargo and add this to it:

[build]
# Link with lld, per https://github.com/rust-lang/rust/issues/39915#issuecomment-538049306
# Use target-cpu=native, per https://deterministic.space/high-performance-rust.html
rustflags = ["-C", "link-arg=-fuse-ld=lld", "-C", "target-cpu=native"]

Then install lld version 9 (e.g. with $ sudo apt-get install lld-9) and add make sure there's a ld.lld executable on your PATH which is symlinked to lld-9.

That's it! Enjoy the faster builds.