This repository is a Rust-specific fork of LLVM. This is used as a submodule in the https://github.com/rust-lang/rust repository. The Rust repository is always pinned to a branch of this repository, and the master branch of this repository is not used.
Despite this being a “fork” it‘s more of just a place where we can easily tweak the LLVM source for Rust’s own build. All patches should by default go upstream first, and then we can cherry-pick them onto our branches if necessary.
To learn more about updating LLVM in the Rust compiler, consult the documentation for doing so in rustc-guide