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rustc: Tweak the effect of --jobs on frontend parallelism #160697

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rustc: Tweak the effect of --jobs on frontend parallelism #160697

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2 changes: 1 addition & 1 deletion compiler/rustc_session/src/config.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1723,7 +1723,7 @@ fn parse_jobs_all(
check_upper_limit(frontend, opt_name);
frontend
}
None => jobs.flatten(),
None => None, // default to 1 thread irrespectively of `jobs` for now
},
};
let backend = match matches.opt_str("jobs-backend") {
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7 changes: 3 additions & 4 deletions src/doc/rustc/src/command-line-arguments.md
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Expand Up @@ -498,8 +498,8 @@ more specific `jobs-*` options cannot specify larger values.
Parallelism used by compilation stages from lexing to generation of backend IR (e.g. LLVM IR).

- If `jobs-frontend` is passed, then it is used as the limit,
- otherwise if `jobs` is passed, then it is used as the limit,
- otherwise `1` is used as the limit (parallelism is disabled), this default may change.
- otherwise `1` is used as the limit (parallelism is disabled), this default may change,
but if it's changed to a larger value the limit from `jobs` will still be respected.

In any case the parallelism here may be additionally limited dynamically by jobserver
passed from a higher level build system like cargo.
Expand All @@ -510,8 +510,7 @@ Parallelism used by compilation stages converting backend IR to object files.

- If `jobs-backend` is passed, then it is used as the limit,
- otherwise if `jobs` is passed, then it is used as the limit,
- otherwise `32` is used as the limit or there's no limit in case of an inherited jobserver,
this default may change.
- otherwise the number of available logical CPUs is used as the limit, this default may change.

@petrochenkov petrochenkov Aug 7, 2026 •
edited by rustbot
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Forgot to update this in #160387.

View changes since the review


In any case the parallelism here may be additionally limited dynamically by jobserver
passed from a higher level build system like cargo.
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