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Improve codegen option docs.

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This commit:
- Adds "following values" indicators for all the options that are
  missing them.
- Tweaks some wording and punctuation for consistency.
- Rewords some things for clarity.
- Removes the `no-integrated-as` entry, because that option was removed
  in #70345.
This commit is contained in:
Nicholas Nethercote 2020-04-03 14:53:03 +11:00
parent cc93e51ed3
commit dc06539872
1 changed files with 100 additions and 83 deletions
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183
src/doc/rustc/src/codegen-options/index.md
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@ -9,10 +9,10 @@ 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](#linker-flavor) flag for another way to specify the 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](#linker-flavor)
flag for another way to specify the linker.
## link-arg
@ -27,49 +27,52 @@ 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](#linker), 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:
This flag controls the linker flavor used by `rustc`. If a linker is given with
the [`-C linker` flag](#linker), 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`](https://emscripten.org/docs/tools_reference/emcc.html).
* `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
* `em`: use [Emscripten `emcc`](https://emscripten.org/docs/tools_reference/emcc.html).
* `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`](https://github.com/denzp/rust-ptx-linker) for Nvidia
NVPTX GPGPU support.
* `wasm-ld`: Uses the [`wasm-ld`](https://lld.llvm.org/WebAssembly.html)
* `wasm-ld`: use the [`wasm-ld`](https://lld.llvm.org/WebAssembly.html)
executable, a port of LLVM `lld` for WebAssembly.
* `ld64.lld`: Uses the LLVM `lld` executable with the [`-flavor darwin`
* `ld64.lld`: use the LLVM `lld` executable with the [`-flavor darwin`
flag][lld-flavor] for Apple's `ld`.
* `ld.lld`: Uses the LLVM `lld` executable with the [`-flavor gnu`
* `ld.lld`: use the LLVM `lld` executable with the [`-flavor gnu`
flag][lld-flavor] for GNU binutils' `ld`.
* `lld-link`: Uses the LLVM `lld` executable with the [`-flavor link`
* `lld-link`: use the LLVM `lld` executable with the [`-flavor link`
flag][lld-flavor] for Microsoft's `link.exe`.
[lld-flavor]: https://lld.llvm.org/Driver.html
## link-dead-code
Normally, the linker will remove dead code. This flag disables this behavior.
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 instructs LLVM to use [link time
This flag controls whether LLVM uses [link time
optimizations](https://llvm.org/docs/LinkTimeOptimization.html) to produce
better optimized code, using whole-program analysis, at the cost of longer
linking time.
linking time. It takes one of the following values:
This flag may take one of the following values:
* `y`, `yes`, `on`, `fat`, or no value: Performs "fat" LTO which attempts to
* `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`: Performs ["thin"
* `n`, `no`, `off`: disables LTO.
* `thin`: perform ["thin"
LTO](http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html).
This is similar to "fat", but takes substantially less time to run while
still achieving performance gains similar to "fat".
@ -81,22 +84,22 @@ disabled if codegen units is 1 or optimizations are disabled ([`-C
opt-level=0`](#opt-level)). That is:
* When `-C lto` is not specified:
* `codegen-units=1`: Disables LTO.
* `opt-level=0`: Disables LTO.
* `codegen-units=1`: disable LTO.
* `opt-level=0`: disable LTO.
* When `-C lto=true`:
* `lto=true`: 16 codegen units, performs fat LTO across crates.
* `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](#linker-plugin-lto) for cross-language LTO.
## linker-plugin-lto
Defers LTO optimizations to the linker. See
[linkger-plugin-LTO](../linker-plugin-lto.md) for more details. Takes one of
This flag defers LTO optimizations to the linker. See
[linker-plugin-LTO](../linker-plugin-lto.md) for more details. It takes one of
the following values:
* `y`, `yes`, `on`, or no value: Enabled.
* `n`, `no`, or `off`: Disabled (default).
* `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
@ -148,14 +151,19 @@ 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.
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 option allows you to enable
[`rpath`](https://en.wikipedia.org/wiki/Rpath).
This flag controls whether [`rpath`](https://en.wikipedia.org/wiki/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
@ -164,35 +172,46 @@ overflow](../../reference/expressions/operator-expr.md#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.
* `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](#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.
This flag controls whether the pass manager uses a pre-populated list of
passes. It takes one of the following values:
* `y`, `yes`, `on`, or no value: use an empty list of passes.
* `n`, `no`, or `off`: use a pre-populated list of passes (the default).
## 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.
This flag controls whether `rustc` will attempt to [vectorize
loops](https://llvm.org/docs/Vectorizers.html#the-loop-vectorizer). It takes
one of the following values:
* `y`, `yes`, `on`, or no value: disable loop vectorization.
* `n`, `no`, or `off`: enable loop vectorization (the default).
## 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.
This flag controls whether `rustc` will attempt to vectorize code using
[superword-level
parallelism](https://llvm.org/docs/Vectorizers.html#the-slp-vectorizer).
It takes one of the following values:
* `y`, `yes`, `on`, or no value: disable SLP vectorization.
* `n`, `no`, or `off`: enable SLP vectorization (the default).
## 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.
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
@ -201,24 +220,21 @@ 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.
linkage. This flag takes one of the following values:
## 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`.
* `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](https://en.wikipedia.org/wiki/Red_zone_\(computing\)). This flag can
be passed one of the following options:
red zone](https://en.wikipedia.org/wiki/Red_zone_\(computing\)). It takes one
of the following values:
* `y`, `yes`, `on`, or no value: Disables the red zone.
* `n`, `no`, or `off`: Enables the red zone.
* `y`, `yes`, `on`, or no value: disable the red zone.
* `n`, `no`, or `off`: enable the red zone.
The default if not specified depends on the target.
The default behaviour, if the flag is not specified, depends on the target.
## relocation-model
@ -257,7 +273,7 @@ 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 for non-incremental builds. For
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
@ -274,24 +290,26 @@ This option is deprecated and does nothing.
## debuginfo
This flag lets you control debug information:
This flag controls the generation of debug information. It takes one of the
following values:
* `0`: no debug info at all (default)
* `1`: line tables only
* `2`: full debug info
* `0`: no debug info at all (the default).
* `1`: line tables only.
* `2`: full debug info.
Note: The [`-g` flag][option-g-debug] is an alias for `-C debuginfo=2`.
## opt-level
This flag lets you control the optimization level.
This flag controls the optimization level.
* `0`: no optimizations, also turns on [`cfg(debug_assertions)`](#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.
* `0`: no optimizations, also turns on
[`cfg(debug_assertions)`](#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][option-o-optimize] is an alias for `-C opt-level=2`.
@ -303,8 +321,8 @@ This flag lets you turn `cfg(debug_assertions)` [conditional
compilation](../../reference/conditional-compilation.md#debug_assertions) 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.
* `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](#opt-level) is 0.
@ -362,25 +380,24 @@ to a valid `.profdata` file. See the chapter on
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
* `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 if not specified depends on the target.
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: Default libraries are included.
* `n`, `no`, or `off`: Default libraries are **not** included.
* `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.
The default is `yes` if not specified.
[option-emit]: ../command-line-arguments.md#option-emit
[option-o-optimize]: ../command-line-arguments.md#option-o-optimize
[profile-guided optimization]: ../profile-guided-optimization.md