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| <h1 id="adding-a-new-target"><a class="header" href="#adding-a-new-target">Adding a new target</a></h1> |
| <p>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.</p> |
| <p>See also the associated documentation in the <a href="https://doc.rust-lang.org/rustc/target-tier-policy.html#adding-a-new-target">target tier policy</a>.</p> |
| <h2 id="specifying-a-new-llvm"><a class="header" href="#specifying-a-new-llvm">Specifying a new LLVM</a></h2> |
| <p>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 <code>src/llvm-project</code> git submodule (you might need to run <code>./x check</code> 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.</p> |
| <p>An example would be:</p> |
| <pre><code>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' |
| </code></pre> |
| <h3 id="using-pre-built-llvm"><a class="header" href="#using-pre-built-llvm">Using pre-built LLVM</a></h3> |
| <p>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.</p> |
| <p>You can tell Rust to use a pre-built version of LLVM using the <code>target</code> section |
| of <code>bootstrap.toml</code>:</p> |
| <pre><code class="language-toml">[target.x86_64-unknown-linux-gnu] |
| llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config" |
| </code></pre> |
| <p>If you are attempting to use a system LLVM, we have observed the following paths |
| before, though they may be different from your system:</p> |
| <ul> |
| <li><code>/usr/bin/llvm-config-8</code></li> |
| <li><code>/usr/lib/llvm-8/bin/llvm-config</code></li> |
| </ul> |
| <p>Note that you need to have the LLVM <code>FileCheck</code> 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 <code>FileCheck</code> in some other way. |
| On Debian-based systems, you can install the <code>llvm-N-tools</code> package (where <code>N</code> |
| is the LLVM version number, e.g. <code>llvm-8-tools</code>). Alternately, you can specify |
| the path to <code>FileCheck</code> with the <code>llvm-filecheck</code> config item in <code>bootstrap.toml</code> |
| or you can disable codegen test with the <code>codegen-tests</code> item in <code>bootstrap.toml</code>.</p> |
| <h2 id="creating-a-target-specification"><a class="header" href="#creating-a-target-specification">Creating a target specification</a></h2> |
| <p>You should start with a target JSON file. You can see the specification |
| for an existing target using <code>--print target-spec-json</code>:</p> |
| <pre><code>rustc -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json |
| </code></pre> |
| <p>Save that JSON to a file and modify it as appropriate for your target.</p> |
| <h3 id="adding-a-target-specification"><a class="header" href="#adding-a-target-specification">Adding a target specification</a></h3> |
| <p>Once you have filled out a JSON specification and been able to compile |
| somewhat successfully, you can copy the specification into the |
| compiler itself.</p> |
| <p>You will need to add a line to the big table inside of the |
| <code>supported_targets</code> macro in the <code>rustc_target::spec</code> module. You |
| will then add a corresponding file for your new target containing a |
| <code>target</code> function.</p> |
| <p>Look for existing targets to use as examples.</p> |
| <p>After adding your target to the <code>rustc_target</code> crate you may want to add |
| <code>core</code>, <code>std</code>, ... with support for your new target. In that case you will |
| probably need access to some <code>target_*</code> 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 |
| we pass <code>--check-cfg</code> in order to tell it to check.</p> |
| <p>To fix the errors you will need to manually add the unexpected value to the |
| different <code>Cargo.toml</code> in <code>library/{std,alloc,core}/Cargo.toml</code>. Here is an |
| example for adding <code>NEW_TARGET_ARCH</code> as <code>target_arch</code>:</p> |
| <p><em><code>library/std/Cargo.toml</code></em>:</p> |
| <pre><code class="language-diff"> [lints.rust.unexpected_cfgs] |
| level = "warn" |
| check-cfg = [ |
| 'cfg(bootstrap)', |
| - 'cfg(target_arch, values("xtensa"))', |
| + # #[cfg(bootstrap)] NEW_TARGET_ARCH |
| + 'cfg(target_arch, values("xtensa", "NEW_TARGET_ARCH"))', |
| </code></pre> |
| <p>To use this target in bootstrap, we need to explicitly add the target triple to the <code>STAGE0_MISSING_TARGETS</code> |
| list in <code>src/bootstrap/src/core/sanity.rs</code>. This is necessary because the default compiler bootstrap uses does |
| not recognize the new target we just added. Therefore, it should be added to <code>STAGE0_MISSING_TARGETS</code> so that the |
| bootstrap is aware that this target is not yet supported by the stage0 compiler.</p> |
| <pre><code class="language-diff">const STAGE0_MISSING_TARGETS: &[&str] = &[ |
| + "NEW_TARGET_TRIPLE" |
| ]; |
| </code></pre> |
| <h2 id="patching-crates"><a class="header" href="#patching-crates">Patching crates</a></h2> |
| <p>You may need to make changes to crates that the compiler depends on, |
| such as <a href="https://crates.io/crates/libc"><code>libc</code></a> or <a href="https://crates.io/crates/cc"><code>cc</code></a>. If so, you can use Cargo's |
| <a href="https://doc.rust-lang.org/stable/cargo/reference/overriding-dependencies.html#the-patch-section"><code>[patch]</code></a> ability. For example, if you want to use an |
| unreleased version of <code>libc</code>, you can add it to the top-level |
| <code>Cargo.toml</code> file:</p> |
| <pre><code class="language-diff">diff --git a/Cargo.toml b/Cargo.toml |
| index 1e83f05e0ca..4d0172071c1 100644 |
| --- a/Cargo.toml |
| +++ b/Cargo.toml |
| @@ -113,6 +113,8 @@ cargo-util = { path = "src/tools/cargo/crates/cargo-util" } |
| [patch.crates-io] |
| +libc = { git = "https://github.com/rust-lang/libc", rev = "0bf7ce340699dcbacabdf5f16a242d2219a49ee0" } |
| |
| # See comments in `src/tools/rustc-workspace-hack/README.md` for what's going on |
| # here |
| rustc-workspace-hack = { path = 'src/tools/rustc-workspace-hack' } |
| </code></pre> |
| <p>After this, run <code>cargo update -p libc</code> to update the lockfiles.</p> |
| <p>Beware that if you patch to a local <code>path</code> dependency, this will enable |
| warnings for that dependency. Some dependencies are not warning-free, and due |
| to the <code>deny-warnings</code> setting in <code>bootstrap.toml</code>, the build may suddenly start |
| to fail. |
| To work around warnings, you may want to:</p> |
| <ul> |
| <li>Modify the dependency to remove the warnings</li> |
| <li>Or for local development purposes, suppress the warnings by setting deny-warnings = false in bootstrap.toml.</li> |
| </ul> |
| <pre><code class="language-toml"># bootstrap.toml |
| [rust] |
| deny-warnings = false |
| </code></pre> |
| <h2 id="cross-compiling"><a class="header" href="#cross-compiling">Cross-compiling</a></h2> |
| <p>Once you have a target specification in JSON and in the code, you can |
| cross-compile <code>rustc</code>:</p> |
| <pre><code>DESTDIR=/path/to/install/in \ |
| ./x install -i --stage 1 --host aarch64-apple-darwin.json --target aarch64-apple-darwin \ |
| compiler/rustc library/std |
| </code></pre> |
| <p>If your target specification is already available in the bootstrap |
| compiler, you can use it instead of the JSON file for both arguments.</p> |
| <h2 id="promoting-a-target-from-tier-2-target-to-tier-2-host"><a class="header" href="#promoting-a-target-from-tier-2-target-to-tier-2-host">Promoting a target from tier 2 (target) to tier 2 (host)</a></h2> |
| <p>There are two levels of tier 2 targets:</p> |
| <ul> |
| <li>Targets that are only cross-compiled (<code>rustup target add</code>)</li> |
| <li>Targets that <a href="https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html#tier-2-with-host-tools">have a native toolchain</a> (<code>rustup toolchain install</code>)</li> |
| </ul> |
| <p>For an example of promoting a target from cross-compiled to native, |
| see <a href="https://github.com/rust-lang/rust/pull/75914">#75914</a>.</p> |
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