Codegen options - The rustc book

# Codegen options

All of these options are passed to `rustc` via the `-C` flag, short for "codegen." You can see a version of this list for your exact compiler by running `rustc -C help`.

## ar

This option is deprecated and does nothing.

## linker

This flag controls which linker `rustc` invokes to link your code. It takes a path to the linker executable. If this flag is not specified, the linker will be inferred based on the target. See also the linker-flavor flag for another way to specify the linker.

## link-arg

This flag lets you append a single extra argument to the linker invocation.

"Append" is significant; you can pass this flag multiple times to add multiple arguments.

## link-args

This flag lets you append multiple extra arguments to the linker invocation. The options should be separated by spaces.

## linker-flavor

This flag controls the linker flavor used by `rustc`. If a linker is given with the `-C linker` flag, then the linker flavor is inferred from the value provided. If no linker is given then the linker flavor is used to determine the linker to use. Every `rustc` target defaults to some linker flavor. Valid options are:

- `em`: use Emscripten `emcc`.
- `gcc`: use the `cc` executable, which is typically gcc or clang on many systems.
- `ld`: use the `ld` executable.
- `msvc`: use the `link.exe` executable from Microsoft Visual Studio MSVC.
- `ptx-linker`: use `rust-ptx-linker` for Nvidia NVPTX GPGPU support.
- `wasm-ld`: use the `wasm-ld` executable, a port of LLVM `lld` for WebAssembly.
- `ld64.lld`: use the LLVM `lld` executable with the `-flavor darwin` flag for Apple's `ld`.
- `ld.lld`: use the LLVM `lld` executable with the `-flavor gnu` flag for GNU binutils' `ld`.
- `lld-link`: use the LLVM `lld` executable with the `-flavor link` flag for Microsoft's `link.exe`.

## link-dead-code

This flag controls whether the linker will keep dead code. It takes one of the following values:

- `y`, `yes`, `on`, or no value: keep dead code.
- `n`, `no`, or `off`: remove dead code (the default).

An example of when this flag might be useful is when trying to construct code coverage metrics.

## lto

This flag controls whether LLVM uses link time optimizations to produce better optimized code, using whole-program analysis, at the cost of longer linking time. It takes one of the following values:

- `y`, `yes`, `on`, `fat`, or no value: perform "fat" LTO which attempts to perform optimizations across all crates within the dependency graph.
- `n`, `no`, `off`: disables LTO.
- `thin`: perform "thin" LTO. This is similar to "fat", but takes substantially less time to run while still achieving performance gains similar to "fat".

If `-C lto` is not specified, then the compiler will attempt to perform "thin local LTO" which performs "thin" LTO on the local crate only across its codegen units. When `-C lto` is not specified, LTO is disabled if codegen units is 1 or optimizations are disabled (`-C opt-level=0`). That is:

- When `-C lto` is not specified:
  - `codegen-units=1`: disable LTO.
  - `opt-level=0`: disable LTO.
- When `-C lto=true`:
  - `lto=true`: 16 codegen units, perform fat LTO across crates.
  - `codegen-units=1` + `lto=true`: 1 codegen unit, fat LTO across crates.

See also linker-plugin-lto for cross-language LTO.

## linker-plugin-lto

This flag defers LTO optimizations to the linker. See linker-plugin-LTO for more details. It takes one of the following values:

- `y`, `yes`, `on`, or no value: enable linker plugin LTO.
- `n`, `no`, or `off`: disable linker plugin LTO (the default).
- A path to the linker plugin.

## target-cpu

This instructs `rustc` to generate code specifically for a particular processor.

You can run `rustc --print target-cpus` to see the valid options to pass here. Additionally, `native` can be passed to use the processor of the host machine. Each target has a default base CPU.

## target-feature

Individual targets will support different features; this flag lets you control enabling or disabling a feature. Each feature should be prefixed with a `+` to enable it or `-` to disable it. Separate multiple features with commas.

To see the valid options and an example of use, run `rustc --print target-features`.

Using this flag is unsafe and might result in undefined runtime behavior.

See also the `target_feature` attribute for controlling features per-function.

This also supports the feature `+crt-static` and `-crt-static` to control static C runtime linkage.

Each target and `target-cpu` has a default set of enabled features.

## passes

This flag can be used to add extra LLVM passes to the compilation.

The list must be separated by spaces.

See also the `no-prepopulate-passes` flag.

## llvm-args

This flag can be used to pass a list of arguments directly to LLVM.

The list must be separated by spaces.

Pass `--help` to see a list of options.

## save-temps

This flag controls whether temporary files generated during compilation are deleted once compilation finishes. It takes one of the following values:

- `y`, `yes`, `on`, or no value: save temporary files.
- `n`, `no`, or `off`: delete temporary files (the default).

## rpath

This flag controls whether `rpath` is enabled. It takes one of the following values:

- `y`, `yes`, `on`, or no value: enable rpath.
- `n`, `no`, or `off`: disable rpath (the default).

## overflow-checks

This flag allows you to control the behavior of runtime integer overflow. When overflow-checks are enabled, a panic will occur on overflow. This flag takes one of the following values:

- `y`, `yes`, `on`, or no value: enable overflow checks.
- `n`, `no`, or `off`: disable overflow checks.

If not specified, overflow checks are enabled if debug-assertions are enabled, disabled otherwise.

## no-prepopulate-passes

This flag tells the pass manager to use an empty list of passes, instead of the usual pre-populated list of passes.

## no-vectorize-loops

This flag disables loop vectorization.

## no-vectorize-slp

This flag disables vectorization using superword-level parallelism.

## soft-float

This option controls whether `rustc` generates code that emulates floating point instructions in software. It takes one of the following values:

- `y`, `yes`, `on`, or no value: use soft floats.
- `n`, `no`, or `off`: use hardware floats (the default).

## prefer-dynamic

By default, `rustc` prefers to statically link dependencies. This option will indicate that dynamic linking should be used if possible if both a static and dynamic versions of a library are available. There is an internal algorithm for determining whether or not it is possible to statically or dynamically link with a dependency. For example, `cdylib` crate types may only use static linkage. This flag takes one of the following values:

- `y`, `yes`, `on`, or no value: use dynamic linking.
- `n`, `no`, or `off`: use static linking (the default).

## no-redzone

This flag allows you to disable the red zone. It takes one of the following values:

- `y`, `yes`, `on`, or no value: disable the red zone.
- `n`, `no`, or `off`: enable the red zone.

The default behaviour, if the flag is not specified, depends on the target.

## relocation-model

This option lets you choose which relocation model to use.

To find the valid options for this flag, run `rustc --print relocation-models`.

## code-model

This option lets you choose which code model to use.

To find the valid options for this flag, run `rustc --print code-models`.

## metadata

This option allows you to control the metadata used for symbol mangling. This takes a space-separated list of strings. Mangled symbols will incorporate a hash of the metadata. This may be used, for example, to differentiate symbols between two different versions of the same crate being linked.

## extra-filename

This option allows you to put extra data in each output filename. It takes a string to add as a suffix to the filename. See the `--emit` flag for more information.

## codegen-units

This flag controls how many code generation units the crate is split into. It takes an integer greater than 0.

When a crate is split into multiple codegen units, LLVM is able to process them in parallel. Increasing parallelism may speed up compile times, but may also produce slower code. Setting this to 1 may improve the performance of generated code, but may be slower to compile.

The default value, if not specified, is 16 for non-incremental builds. For incremental builds the default is 256 which allows caching to be more granular.

## remark

This flag lets you print remarks for optimization passes.

The list of passes should be separated by spaces.

`all` will remark on every pass.

## no-stack-check

This option is deprecated and does nothing.

## debuginfo

This flag controls the generation of debug information. It takes one of the following values:

- `0`: no debug info at all (the default).
- `1`: line tables only.
- `2`: full debug info.

Note: The `-g` flag is an alias for `-C debuginfo=2`.

## opt-level

This flag controls the optimization level.

- `0`: no optimizations, also turns on `cfg(debug_assertions)` (the default).
- `1`: basic optimizations.
- `2`: some optimizations.
- `3`: all optimizations.
- `s`: optimize for binary size.
- `z`: optimize for binary size, but also turn off loop vectorization.

Note: The `-O` flag is an alias for `-C opt-level=2`.

The default is `0`.

## debug-assertions

This flag lets you turn `cfg(debug_assertions)` conditional compilation on or off. It takes one of the following values:

- `y`, `yes`, `on`, or no value: enable debug-assertions.
- `n`, `no`, or `off`: disable debug-assertions.

If not specified, debug assertions are automatically enabled only if the opt-level is 0.

## inline-threshold

This option lets you set the default threshold for inlining a function. It takes an unsigned integer as a value. Inlining is based on a cost model, where a higher threshold will allow more inlining.

The default depends on the opt-level:

opt-level | Threshold
--- | ---
0 | N/A, only inlines always-inline functions
1 | N/A, only inlines always-inline functions and LLVM lifetime intrinsics
2 | 225
3 | 275
s | 75
z | 25

## panic

This option lets you control what happens when the code panics.

- `abort`: terminate the process upon panic
- `unwind`: unwind the stack upon panic

If not specified, the default depends on the target.

## incremental

This flag allows you to enable incremental compilation, which allows `rustc` to save information after compiling a crate to be reused when recompiling the crate, improving re-compile times. This takes a path to a directory where incremental files will be stored.

## profile-generate

This flag allows for creating instrumented binaries that will collect profiling data for use with profile-guided optimization (PGO). The flag takes an optional argument which is the path to a directory into which the instrumented binary will emit the collected data. See the chapter on profile-guided optimization for more information.

## profile-use

This flag specifies the profiling data file to be used for profile-guided optimization (PGO). The flag takes a mandatory argument which is the path to a valid `.profdata` file. See the chapter on profile-guided optimization for more information.

## force-frame-pointers

This flag forces the use of frame pointers. It takes one of the following values:

- `y`, `yes`, `on`, or no value: force-enable frame pointers.
- `n`, `no`, or `off`: do not force-enable frame pointers. This does not necessarily mean frame pointers will be removed.

The default behaviour, if frame pointers are not force-enabled, depends on the target.

## default-linker-libraries

This flag controls whether or not the linker includes its default libraries. It takes one of the following values:

- `y`, `yes`, `on`, or no value: include default libraries (the default).
- `n`, `no`, or `off`: exclude default libraries.

For example, for gcc flavor linkers, this issues the `-nodefaultlibs` flag to the linker.