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| <h1 id="optimized-build-of-the-compiler"><a class="header" href="#optimized-build-of-the-compiler">Optimized build of the compiler</a></h1> |
| <p>There are multiple additional build configuration options and techniques that can be used to compile a |
| build of <code>rustc</code> that is as optimized as possible (for example when building <code>rustc</code> for a Linux |
| distribution). The status of these configuration options for various Rust targets is tracked <a href="https://github.com/rust-lang/rust/issues/103595">here</a>. |
| This page describes how you can use these approaches when building <code>rustc</code> yourself.</p> |
| <h2 id="link-time-optimization"><a class="header" href="#link-time-optimization">Link-time optimization</a></h2> |
| <p>Link-time optimization is a powerful compiler technique that can increase program performance. To |
| enable (Thin-)LTO when building <code>rustc</code>, set the <code>rust.lto</code> config option to <code>"thin"</code> |
| in <code>bootstrap.toml</code>:</p> |
| <pre><code class="language-toml">[rust] |
| lto = "thin" |
| </code></pre> |
| <blockquote> |
| <p>Note that LTO for <code>rustc</code> is currently supported and tested only for |
| the <code>x86_64-unknown-linux-gnu</code> target. Other targets <em>may</em> work, but no guarantees are provided. |
| Notably, LTO-optimized <code>rustc</code> currently produces <a href="https://github.com/rust-lang/rust/issues/109114">miscompilations</a> on Windows.</p> |
| </blockquote> |
| <p>Enabling LTO on Linux has <a href="https://github.com/rust-lang/rust/pull/101403#issuecomment-1288190019">produced</a> speed-ups by up to 10%.</p> |
| <h2 id="memory-allocator"><a class="header" href="#memory-allocator">Memory allocator</a></h2> |
| <p>Using a different memory allocator for <code>rustc</code> can provide significant performance benefits. If you |
| want to enable the <code>jemalloc</code> allocator, you can set the <code>rust.jemalloc</code> option to <code>true</code> |
| in <code>bootstrap.toml</code>:</p> |
| <pre><code class="language-toml">[rust] |
| jemalloc = true |
| </code></pre> |
| <blockquote> |
| <p>Note that this option is currently only supported for Linux and macOS targets.</p> |
| </blockquote> |
| <h2 id="codegen-units"><a class="header" href="#codegen-units">Codegen units</a></h2> |
| <p>Reducing the amount of codegen units per <code>rustc</code> crate can produce a faster build of the compiler. |
| You can modify the number of codegen units for <code>rustc</code> and <code>libstd</code> in <code>bootstrap.toml</code> with the |
| following options:</p> |
| <pre><code class="language-toml">[rust] |
| codegen-units = 1 |
| codegen-units-std = 1 |
| </code></pre> |
| <h2 id="instruction-set"><a class="header" href="#instruction-set">Instruction set</a></h2> |
| <p>By default, <code>rustc</code> is compiled for a generic (and conservative) instruction set architecture |
| (depending on the selected target), to make it support as many CPUs as possible. If you want to |
| compile <code>rustc</code> for a specific instruction set architecture, you can set the <code>target_cpu</code> compiler |
| option in <code>RUSTFLAGS</code>:</p> |
| <pre><code class="language-bash">RUSTFLAGS="-C target_cpu=x86-64-v3" ./x build ... |
| </code></pre> |
| <p>If you also want to compile LLVM for a specific instruction set, you can set <code>llvm</code> flags |
| in <code>bootstrap.toml</code>:</p> |
| <pre><code class="language-toml">[llvm] |
| cxxflags = "-march=x86-64-v3" |
| cflags = "-march=x86-64-v3" |
| </code></pre> |
| <h2 id="profile-guided-optimization"><a class="header" href="#profile-guided-optimization">Profile-guided optimization</a></h2> |
| <p>Applying profile-guided optimizations (or more generally, feedback-directed optimizations) can |
| produce a large increase to <code>rustc</code> performance, by up to 15% (<a href="https://blog.rust-lang.org/inside-rust/2020/11/11/exploring-pgo-for-the-rust-compiler.html#final-numbers-and-a-benchmarking-plot-twist">1</a>, <a href="https://github.com/rust-lang/rust/pull/96978">2</a>). However, these techniques |
| are not simply enabled by a configuration option, but rather they require a complex build workflow |
| that compiles <code>rustc</code> multiple times and profiles it on selected benchmarks.</p> |
| <p>There is a tool called <code>opt-dist</code> that is used to optimize <code>rustc</code> with <a href="https://doc.rust-lang.org/rustc/profile-guided-optimization.html">PGO</a> (profile-guided |
| optimizations) and <a href="https://github.com/llvm/llvm-project/blob/main/bolt/README.md">BOLT</a> (a post-link binary optimizer) for builds distributed to end users. You |
| can examine the tool, which is located in <code>src/tools/opt-dist</code>, and build a custom PGO build |
| workflow based on it, or try to use it directly. Note that the tool is currently quite hardcoded to |
| the way we use it in Rust's continuous integration workflows, and it might require some custom |
| changes to make it work in a different environment.</p> |
| <p>To use the tool, you will need to provide some external dependencies:</p> |
| <ul> |
| <li>A Python3 interpreter (for executing <code>x.py</code>).</li> |
| <li>Compiled LLVM toolchain, with the <code>llvm-profdata</code> binary. Optionally, if you want to use BOLT, |
| the <code>llvm-bolt</code> and |
| <code>merge-fdata</code> binaries have to be available in the toolchain.</li> |
| </ul> |
| <p>These dependencies are provided to <code>opt-dist</code> by an implementation of the <a href="https://github.com/rust-lang/rust/blob/ee451f8faccf3050c76cdcd82543c917b40c7962/src/tools/opt-dist/src/environment.rs#L5"><code>Environment</code></a> struct. |
| It specifies directories where will the PGO/BOLT pipeline take place, and also external dependencies |
| like Python or LLVM.</p> |
| <p>Here is an example of how can <code>opt-dist</code> be used locally (outside of CI):</p> |
| <ol> |
| <li>Enable metrics in your <code>bootstrap.toml</code> file, because <code>opt-dist</code> expects it to be enabled: |
| <pre><code class="language-toml">[build] |
| metrics = true |
| </code></pre> |
| </li> |
| <li>Build the tool with the following command: |
| <pre><code class="language-bash">./x build tools/opt-dist |
| </code></pre> |
| </li> |
| <li>Run the tool with the <code>local</code> mode and provide necessary parameters: |
| <pre><code class="language-bash">./build/host/stage1-tools-bin/opt-dist local \ |
| --target-triple <target> \ # select target, e.g. "x86_64-unknown-linux-gnu" |
| --checkout-dir <path> \ # path to rust checkout, e.g. "." |
| --llvm-dir <path> \ # path to built LLVM toolchain, e.g. "/foo/bar/llvm/install" |
| -- python3 x.py dist # pass the actual build command |
| </code></pre> |
| You can run <code>--help</code> to see further parameters that you can modify.</li> |
| </ol> |
| <blockquote> |
| <p>Note: if you want to run the actual CI pipeline, instead of running <code>opt-dist</code> locally, |
| you can execute <code>cargo run --manifest-path src/ci/citool/Cargo.toml run-local dist-x86_64-linux</code>.</p> |
| </blockquote> |
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