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

link-dead-code

Normally, the linker will remove dead code. This flag disables this behavior.

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

lto

This flag instructs LLVM to use link time optimizations to produce better optimized code, using whole-program analysis, at the cost of longer linking time.

This flag may take one of the following values:

  • y, yes, on, fat, or no value: Performs "fat" LTO which attempts to perform optimizations across all crates within the dependency graph.
  • n, no, off: Disables LTO.
  • thin: Performs "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: Disables LTO.
    • opt-level=0: Disables LTO.
  • When -C lto=true:
    • lto=true: 16 codegen units, performs 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

Defers LTO optimizations to the linker. See linkger-plugin-LTO for more details. Takes one of the following values:

  • y, yes, on, or no value: Enabled.
  • n, no, or off: Disabled (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

rustc will generate temporary files during compilation; normally it will delete them after it's done with its work. This option will cause them to be preserved instead of removed.

rpath

This option allows you to enable rpath.

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

The pass manager comes pre-populated with a list of passes; this flag ensures that list is empty.

no-vectorize-loops

By default, rustc will attempt to vectorize loops. This flag will turn that behavior off.

no-vectorize-slp

By default, rustc will attempt to vectorize code using superword-level parallelism. This flag will turn that behavior off.

soft-float

This option will make rustc generate code using "soft floats." By default, a lot of hardware supports floating point instructions, and so the code generated will take advantage of this. "soft floats" emulate floating point instructions in software.

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.

no-integrated-as

rustc normally uses the LLVM internal assembler to create object code. This flag will disable the internal assembler and emit assembly code to be translated using an external assembler, currently the linker such as cc.

no-redzone

This flag allows you to disable the red zone. This flag can be passed one of the following options:

  • y, yes, on, or no value: Disables the red zone.
  • n, no, or off: Enables the red zone.

The default if 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, if not specified, is 16. This flag is ignored if incremental is enabled, in which case an internal heuristic is used to split the crate.

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 lets you control debug information:

  • 0: no debug info at all (default)
  • 1: line tables only
  • 2: full debug info

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

opt-level

This flag lets you control the optimization level.

  • 0: no optimizations, also turns on cfg(debug_assertions).
  • 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-levelThreshold
0N/A, only inlines always-inline functions
1N/A, only inlines always-inline functions and LLVM lifetime intrinsics
2225
3275
s75
z25

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: Frame pointers are forced to be enabled.
  • n, no, or off: Frame pointers are not forced to be enabled. This does not necessarily mean frame pointers will be removed.

The default if not specified 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: Default libraries are included.
  • n, no, or off: Default libraries are not included.

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

The default is yes if not specified.