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 lets you control which linker `rustc` invokes to link your code.

## link-arg=val

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.

## 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.

It takes one of two values, `thin` and `fat`. 'thin' LTO is a new feature of LLVM, 'fat' referring to the classic version of LTO.

## 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.

## target-feature

Individual targets will support different features; this flag lets you control enabling or disabling a feature.

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

## passes

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

The list must be separated by spaces.

## llvm-args

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

The list must be separated by spaces.

## 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 set the value of `rpath`.

## overflow-checks

This flag allows you to control the behavior of integer overflow. This flag can be passed many options:

- To turn overflow checks on: `y`, `yes`, or `on`.
- To turn overflow checks off: `n`, `no`, or `off`.

## 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 loops 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 make it use dynamic linking instead.

## no-integrated-as

LLVM comes with an internal assembler; this option will let you use an external assembler instead.

## no-redzone

This flag allows you to disable the red zone. This flag can be passed many options:

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

## 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=val

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.

## extra-filename

This option allows you to put extra data in each output filename.

## codegen-units

This flag lets you control how many threads are used when doing code generation.

Increasing parallelism may speed up compile times, but may also produce slower code.

## remark

This flag lets you print remarks for these 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
- `1`: line tables only
- `2`: full debug info

## opt-level

This flag lets you control the optimization level.

- `0`: no optimizations
- `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.

## debug-assertions

This flag lets you turn `cfg(debug_assertions)` on or off.

## inline-threshold

This option lets you set the threshold for inlining a function.

The default is 225.

## panic

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

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

## incremental

This flag allows you to enable incremental compilation.