Adding Support for a New Target in Rust

# Adding a new target These are a set of steps to add support for a new target. There are numerous end states and paths to get there, so not all sections may be relevant to your desired goal. See also the associated documentation in the [target tier policy].  ## Specifying a new LLVM For very new targets, you may need to use a different fork of LLVM than what is currently shipped with Rust. In that case, navigate to the `src/llvm-project` git submodule (you might need to run `./x check` at least once so the submodule is updated), check out the appropriate commit for your fork, then commit that new submodule reference in the main Rust repository. An example would be: ``` cd src/llvm-project git remote add my-target-llvm some-llvm-repository git checkout my-target-llvm/my-branch cd .. git add llvm-project git commit -m 'Use my custom LLVM' ``` ### Using pre-built LLVM If you have a local LLVM checkout that is already built, you may be able to configure Rust to treat your build as the system LLVM to avoid redundant builds. You can tell Rust to use a pre-built version of LLVM using the `target` section of `bootstrap.toml`: ```toml [target.x86_64-unknown-linux-gnu] llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config" ``` If you are attempting to use a system LLVM, we have observed the following paths before, though they may be different from your system: - `/usr/bin/llvm-config-8` - `/usr/lib/llvm-8/bin/llvm-config` Note that you need to have the LLVM `FileCheck` tool installed, which is used for codegen tests. This tool is normally built with LLVM, but if you use your own preinstalled LLVM, you will need to provide `FileCheck` in some other way. On Debian-based systems, you can install the `llvm-N-tools` package (where `N` is the LLVM version number, e.g. `llvm-8-tools`). Alternately, you can specify the path to `FileCheck` with the `llvm-filecheck` config item in `bootstrap.toml` or you can disable codegen test with the `codegen-tests` item in `bootstrap.toml`. ## Creating a target specification You should start with a target JSON file. You can see the specification for an existing target using `--print target-spec-json`: ``` rustc -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json ``` Save that JSON to a file and modify it as appropriate for your target. ### Adding a target specification Once you have filled out a JSON specification and been able to compile somewhat successfully, you can copy the specification into the compiler itself. You will need to add a line to the big table inside of the `supported_targets` macro in the `rustc_target::spec` module. You will then add a corresponding file for your new target containing a `target` function. Look for existing targets to use as examples. After adding your target to the `rustc_target` crate you may want to add `core`, `std`, ... with support for your new target. In that case you will probably need access to some `target_*` cfg. Unfortunately when building with stage0 (a precompiled compiler), you'll get an error that the target cfg is unexpected because stage0 doesn't know about the new target specification and

This section outlines the steps to add support for a new target architecture in Rust. It covers specifying a custom LLVM fork or using a pre-built LLVM installation, creating and integrating a target specification JSON file into the compiler, and adding support for core libraries. The process involves modifying the `rustc_target` crate and handling potential issues with stage0 builds.