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Attempt to clarify LTO and codegen-units.

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Eric Huss 2019-11-02 09:47:14 -07:00
parent c6bfe28b95
commit 9b9d651288
1 changed files with 21 additions and 10 deletions
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src/doc/rustc/src/codegen-options/index.md
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@ -74,10 +74,18 @@ This flag may take one of the following values:
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](#codegen-units). In this case, LTO is disabled if codegen units is 1 or
optimizations are disabled ([`-C opt-level=0`](#opt-level)).
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](#codegen-units). When `-C lto` is not specified, LTO is
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.
* 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](#linker-plugin-lto) for cross-language LTO.
@ -237,14 +245,17 @@ flag][option-emit] for more information.
## codegen-units
This flag lets you control how many threads are used when doing code
generation. It takes an integer greater than 0.
This flag controls how many code generation units the crate is split into. It
takes an integer greater than 0.
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.
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.
The default, if not specified, is 16. This flag is ignored if
[incremental](#incremental) is enabled, in which case an internal heuristic is
used to split the crate.
## remark