src/doc/rustc/src/codegen-options/index.md - third_party/rust - Git at Google

 # 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.

 ## linker-flavor

 This flag lets you control the linker flavor used by `rustc`. If a linker is given with the
 `-C linker` flag described above 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.

 ## 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](https://llvm.org/docs/LinkTimeOptimization.html).

 It takes one of two values, `thin` and `fat`. 'thin' LTO [is a new feature of
 LLVM](http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html),
 '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`](https://en.wikipedia.org/wiki/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](https://llvm.org/docs/Vectorizers.html#the-loop-vectorizer). This
 flag will turn that behavior off.

 ## no-vectorize-slp

 By default, `rustc` will attempt to vectorize code using [superword-level
 parallelism](https://llvm.org/docs/Vectorizers.html#the-slp-vectorizer). 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](https://en.wikipedia.org/wiki/Red_zone_\(computing\)). 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, also turn 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.

 ## 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.

 ## 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.

 [profile-guided optimization]: ../profile-guided-optimization.md
	# 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.

	## linker-flavor

	This flag lets you control the linker flavor used by `rustc`. If a linker is given with the
	`-C linker` flag described above 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.

	## 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](https://llvm.org/docs/LinkTimeOptimization.html).

	It takes one of two values, `thin` and `fat`. 'thin' LTO [is a new feature of
	LLVM](http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html),
	'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`](https://en.wikipedia.org/wiki/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](https://llvm.org/docs/Vectorizers.html#the-loop-vectorizer). This
	flag will turn that behavior off.

	## no-vectorize-slp

	By default, `rustc` will attempt to vectorize code using [superword-level
	parallelism](https://llvm.org/docs/Vectorizers.html#the-slp-vectorizer). 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](https://en.wikipedia.org/wiki/Red_zone_\(computing\)). 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, also turn 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.

	## 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.

	## 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.

	[profile-guided optimization]: ../profile-guided-optimization.md