codegen-units and link-time optimization (LTO) affect different parts of a Rust build, so neither is a universal substitute for the other. More codegen units can improve compilation parallelism; LTO can give the optimizer a wider view of the program, often at the cost of link time. For release builds, compare the combinations that match your goals—build time, runtime performance, or binary size—on your actual project and deployment target.
What each setting controls
Codegen units control parallel code generation
The rustc -C codegen-units option, exposed in Cargo profiles as codegen-units, sets the maximum number of code-generation units into which a crate is split. LLVM can process multiple units in parallel. That can shorten compilation, but may reduce optimization opportunities and produce slower code. As the Rust Project puts it in its Codegen Options documentation, “Increasing parallelism may speed up compile times, but may also produce slower code.” Neither effect is guaranteed for every project or machine.
Setting the value to 1 removes that code-generation parallelism and may improve generated-code performance, while potentially slowing compilation. It does not mean “turn LTO on”; it is a separate control.
LTO broadens optimization at link time
LTO lets LLVM optimize with information available at link time. Fat LTO attempts optimization across crates in the dependency graph and can make linking take longer. Thin LTO is designed to take substantially less time than fat LTO while achieving similar performance gains, according to the Rust Project’s Codegen Options documentation. The same documentation notes that for larger projects such as the Rust compiler, ThinLTO can even produce better performance than fat LTO. These are general documentation observations, not a guarantee for your application.
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Understand what “LTO off” means in Cargo
Rust has an important distinction between cross-crate LTO and thin local LTO. If rustc’s -C lto is not specified, it attempts thin local LTO across codegen units within the local crate; this is not cross-crate LTO. That behavior is disabled when codegen-units = 1 or opt-level = 0.
In Cargo, lto = false means thin local LTO, while lto = "off" disables LTO. The default development profile uses lto = false. So a build described casually as “LTO off” may still use thin local LTO. Check the effective profile and spell out the setting you intend. See the Cargo Profiles reference and the rustc Codegen Options documentation.
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Defaults and profile differences matter
Cargo documents codegen-units = 16 as the default for non-incremental builds and codegen-units = 256 for incremental builds. Its development profile enables incremental compilation by default and uses 256 codegen units. These profile differences can change both compilation behavior and what a benchmark is actually comparing.
Before drawing conclusions from a flag change, inspect and record the profile settings for at least opt-level, incremental, codegen-units, and lto. Cargo’s Profiles reference documents the profile keys and defaults.
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Which setting should you try?
| Your priority | A sensible starting point | What to evaluate |
|---|---|---|
| Fast edit/build iteration | Keep the normal development profile and its parallel code generation unless measurement identifies a different bottleneck. | Incremental rebuild time and the actual stage that limits your workflow. |
| Release runtime performance | Benchmark ThinLTO against your current release baseline. | Runtime on the target workload, plus clean build and link time. |
| Considering fat LTO | Try it only after establishing a baseline, and retain it only if its result justifies the extra link cost. | Whether the measured runtime or size gain matters enough to offset linking time. |
| Considering one codegen unit | Test codegen-units = 1 separately and in combination with LTO. |
Compilation and link time as well as generated-program behavior; this setting also changes whether implicit thin local LTO applies. |
| Rust linked with C or C++ | Check linker-plugin LTO requirements for all participating toolchains and the linker. | LLVM toolchain compatibility, matching thin or fat mode, and linker plugin support. |
This is a benchmark plan, not a promise that any option will win. The official documentation does not establish a universal best setting for ordinary Rust applications.
How to make a useful comparison
- Use the release profile that you deploy. Do not compare a development build with a release build or rely on an assumed default. Record the effective profile values, especially optimization level, incremental compilation, codegen units, and LTO.
- Change one meaningful variable at a time. Compare your baseline with a ThinLTO build; evaluate fat LTO or one codegen unit as distinct follow-up variants. If you test combinations, record each combination rather than attributing the result to one flag.
- Keep the environment stable. Use the same Rust toolchain, target, dependencies, hardware, and workload for each run.
- Record build stages separately. Measure clean compilation and linking separately, and measure incremental rebuild time if that matters to your workflow.
- Measure the outcome you actually need. Run the representative target workload for runtime performance, or compare binary size if that is a project requirement. Do not treat a faster build as proof of a faster program.
Compatibility notes for LTO
Embedded LLVM bitcode
Rust requires LLVM bitcode for LTO. Combining -C embed-bitcode=no with -C lto is invalid and causes rustc to abort. Cargo manages the related rustc options through the profile’s lto setting. See the Rust Project’s Codegen Options documentation.
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Rust and C/C++ in one linked program
Cross-language optimization uses linker-plugin LTO, which defers optimization to the linker. Documented cases include Rust static libraries used from C or C++, and C or C++ dependencies linked into Rust. Participating objects need LLVM-based toolchains using the same LTO mode—thin or fat—and the linker must support the LLVM plugin. Those requirements are additional to choosing a Cargo profile value. The details are in the rustc Linker-plugin-based LTO documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep rustc-specific performance claims in scope
The Rust Compiler Development Guide says that enabling LTO for rustc on Linux has produced speed-ups of up to 10%. That figure concerns building rustc, not an expected gain for an arbitrary Rust application. The guide says this configuration is currently supported and tested only on x86_64-unknown-linux-gnu, offers no guarantees for other targets, and warns that LTO-optimized rustc produces miscompilations on Windows. Read the optimized-build guidance before applying that rustc-specific setup to compiler development.
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