Optimizing Rust Compiler Builds

# Optimized build of the compiler  There are multiple additional build configuration options and techniques that can be used to compile a build of `rustc` that is as optimized as possible (for example when building `rustc` for a Linux distribution). The status of these configuration options for various Rust targets is tracked [here]. This page describes how you can use these approaches when building `rustc` yourself. ## Link-time optimization Link-time optimization is a powerful compiler technique that can increase program performance. To enable (Thin-)LTO when building `rustc`, set the `rust.lto` config option to `"thin"` in `bootstrap.toml`: ```toml [rust] lto = "thin" ``` > Note that LTO for `rustc` is currently supported and tested only for > the `x86_64-unknown-linux-gnu` target. Other targets *may* work, but no guarantees are provided. > Notably, LTO-optimized `rustc` currently produces [miscompilations] on Windows. Enabling LTO on Linux has [produced] speed-ups by up to 10%. ## Memory allocator Using a different memory allocator for `rustc` can provide significant performance benefits. If you want to enable the `jemalloc` allocator, you can set the `rust.jemalloc` option to `true` in `bootstrap.toml`: ```toml [rust] jemalloc = true ``` > Note that this option is currently only supported for Linux and macOS targets. ## Codegen units Reducing the amount of codegen units per `rustc` crate can produce a faster build of the compiler. You can modify the number of codegen units for `rustc` and `libstd` in `bootstrap.toml` with the following options: ```toml [rust] codegen-units = 1 codegen-units-std = 1 ``` ## Instruction set By default, `rustc` 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 `rustc` for a specific instruction set architecture, you can set the `target_cpu` compiler option in `RUSTFLAGS`: ```bash RUSTFLAGS="-C target_cpu=x86-64-v3" ./x build ... ``` If you also want to compile LLVM for a specific instruction set, you can set `llvm` flags in `bootstrap.toml`: ```toml [llvm] cxxflags = "-march=x86-64-v3" cflags = "-march=x86-64-v3" ``` ## Profile-guided optimization Applying profile-guided optimizations (or more generally, feedback-directed optimizations) can produce a large increase to `rustc` performance, by up to 15% ([1], [2]). However, these techniques

This section details methods to optimize rustc builds, including Link-Time Optimization (LTO), enabling the jemalloc memory allocator, reducing codegen units, targeting specific instruction sets, and Profile-Guided Optimization (PGO). Configuration is primarily done through `bootstrap.toml` and `RUSTFLAGS`.