rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism by petrochenkov · Pull Request #160387 · rust-lang/rust

petrochenkov · 2026-08-02T14:15:37Z

Follow up to #159675.

petrochenkov · 2026-08-03T09:24:38Z

This PR will become simpler if #160424 is merged first.

@bjorn3

Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. rust-lang#160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

@bjorn3

Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. rust-lang#160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

@bjorn3

Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. rust-lang#160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

@bjorn3

Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. rust-lang#160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

@bjorn3

Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. rust-lang#160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

@bjorn3

Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. rust-lang#160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

@bjorn3

Rollup merge of #160424 - petrochenkov:jobdflt, r=bjorn3 Use `thread::available_parallelism` as the default limit for backend parallelism Instead of the old scheme with 32-or-unlimited, depending on inherited-ness of the jobserver. Not sure if this needs some wider discussion or not. In the most common case (rustc is called from cargo) nothing changes, because cargo already limits the parallelism to `thread::available_parallelism` by default, and passes this limit to rustc through jobserver. #160387 will become simpler if this PR is merged first. r? @bjorn3 @Zoxc

…to backend parallelism

rustbot · 2026-08-04T20:51:00Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

petrochenkov · 2026-08-04T20:52:34Z

Rebased, updated, and added one more fix.
@rustbot ready

bjorn3 · 2026-08-05T09:43:22Z

@bors r+

rust-bors · 2026-08-05T09:43:25Z

📌 Commit 9956e2f has been approved by bjorn3

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 10. This pull request will be tested once the tree is reopened.

Reason for tree closure: manually handling queue due to backlog

@bjorn3

rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism Follow up to rust-lang#159675. r? @bjorn3

…uwer Rollup of 22 pull requests Successful merges: - #158147 (std: fix stack buffer overflow in Windows junction_point) - #159784 (Hint that memchr returns an in-bounds index) - #160130 (Select cache values to verify by key fingerprint, not value fingerprint) - #160343 (Rename `OutlivesPredicate` to `OutlivesClause`) - #160360 (Remove rustc_middle dependency on rustc_hir_pretty) - #160387 (rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism) - #160422 (move mir-opt miri tests to CI logic) - #160444 (Avoid resolving path keywords outside `TypeNS`) - #160510 (Resolver: (un)tracked borrows for `CmRefCell` made safe my unsafe speculative flag) - #155424 ([blocked] Link to proposed LLM policy in CONTRIBUTING and pull request template) - #158726 (std: move futex implementations into sys::sync::futex) - #159225 (Split IncrCompSession out of Session) - #159820 (Make the `rustc_unsafe_specialization_marker` attribute actually `unsafe`) - #160198 (Rework `smallest_range_containing` to handle duplicates) - #160362 (Split `SpannedTypeVisitor` into its own crate, `rustc_ty_walk`) - #160390 (autodiff: Handle slice-tailed DSTs in type trees) - #160420 (Suggest `cast_signed()` for overflowing signed integer literals) - #160520 (Add some tests for specialization) - #160522 (fix(bootstrap): Normalize the names of proc macro dependency crates) - #160531 (docs: fix typo in AllowExprMetavar comment) - #160538 (Update expect messages in tcp.rs doc examples to follow the style guide) - #160548 (bootstrap: Register `coverage-map` and `coverage-run` aliases via a separate step)

@bjorn3

rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism Follow up to rust-lang#159675. r? @bjorn3

…uwer Rollup of 25 pull requests Successful merges: - #154585 (treat no_mangle_generic_items as hard error instead of lint warning) - #158147 (std: fix stack buffer overflow in Windows junction_point) - #160130 (Select cache values to verify by key fingerprint, not value fingerprint) - #160343 (Rename `OutlivesPredicate` to `OutlivesClause`) - #160360 (Remove rustc_middle dependency on rustc_hir_pretty) - #160387 (rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism) - #160422 (move mir-opt miri tests to CI logic) - #160444 (Avoid resolving path keywords outside `TypeNS`) - #160510 (Resolver: (un)tracked borrows for `CmRefCell` made safe my unsafe speculative flag) - #155424 ([blocked] Link to proposed LLM policy in CONTRIBUTING and pull request template) - #158726 (std: move futex implementations into sys::sync::futex) - #158762 (Emit thumb code on VEX V5) - #159225 (Split IncrCompSession out of Session) - #159820 (Make the `rustc_unsafe_specialization_marker` attribute actually `unsafe`) - #160198 (Rework `smallest_range_containing` to handle duplicates) - #160362 (Split `SpannedTypeVisitor` into its own crate, `rustc_ty_walk`) - #160390 (autodiff: Handle slice-tailed DSTs in type trees) - #160420 (Suggest `cast_signed()` for overflowing signed integer literals) - #160516 (Add regression test for HRTB projection in closure) - #160520 (Add some tests for specialization) - #160522 (fix(bootstrap): Normalize the names of proc macro dependency crates) - #160523 (Add regression test for opaque type) - #160531 (docs: fix typo in AllowExprMetavar comment) - #160538 (Update expect messages in tcp.rs doc examples to follow the style guide) - #160548 (bootstrap: Register `coverage-map` and `coverage-run` aliases via a separate step)

…uwer Rollup of 25 pull requests Successful merges: - #158147 (std: fix stack buffer overflow in Windows junction_point) - #160130 (Select cache values to verify by key fingerprint, not value fingerprint) - #160343 (Rename `OutlivesPredicate` to `OutlivesClause`) - #160360 (Remove rustc_middle dependency on rustc_hir_pretty) - #160387 (rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism) - #160422 (move mir-opt miri tests to CI logic) - #160444 (Avoid resolving path keywords outside `TypeNS`) - #160510 (Resolver: (un)tracked borrows for `CmRefCell` made safe by unsafe speculative flag) - #155424 ([blocked] Link to proposed LLM policy in CONTRIBUTING and pull request template) - #158726 (std: move futex implementations into sys::sync::futex) - #158762 (Emit thumb code on VEX V5) - #159225 (Split IncrCompSession out of Session) - #159820 (Make the `rustc_unsafe_specialization_marker` attribute actually `unsafe`) - #160198 (Rework `smallest_range_containing` to handle duplicates) - #160362 (Split `SpannedTypeVisitor` into its own crate, `rustc_ty_walk`) - #160390 (autodiff: Handle slice-tailed DSTs in type trees) - #160420 (Suggest `cast_signed()` for overflowing signed integer literals) - #160501 (Add bootstrap CLI snapshot test for testing miri) - #160516 (Add regression test for HRTB projection in closure) - #160520 (Add some tests for specialization) - #160522 (fix(bootstrap): Normalize the names of proc macro dependency crates) - #160523 (Add regression test for opaque type) - #160531 (docs: fix typo in AllowExprMetavar comment) - #160538 (Update expect messages in tcp.rs doc examples to follow the style guide) - #160548 (bootstrap: Register `coverage-map` and `coverage-run` aliases via a separate step)

@bjorn3

Rollup merge of #160387 - petrochenkov:backlimit, r=bjorn3 rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism Follow up to #159675. r? @bjorn3

JonathanBrouwer · 2026-08-06T20:14:29Z

@rust-timer build 08c17aa

rust-timer · 2026-08-07T06:20:16Z

Finished benchmarking commit (08c17aa): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.7%	[0.1%, 2.5%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.1%]	4
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 3.8%, secondary 25.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.8%	[2.0%, 5.6%]	2
Regressions ❌ (secondary)	25.0%	[1.2%, 30.5%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.8%	[2.0%, 5.6%]	2

Cycles

Results (primary 2.8%, secondary 11.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	17.5%	[4.9%, 30.6%]	14
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-7.7%	[-14.4%, -2.2%]	4
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

Binary size

Results (primary 0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	25
Regressions ❌ (secondary)	0.0%	[0.0%, 0.1%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	25

Bootstrap: 454.551s -> 456.307s (0.39%)
Artifact size: 399.93 MiB -> 399.06 MiB (-0.22%)

JonathanBrouwer · 2026-08-07T08:00:16Z

All PRs in the rollup are getting this identical regression, this is most likely a false positive

rustbot assigned bjorn3 Aug 2, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 2, 2026

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petrochenkov mentioned this pull request Aug 3, 2026

Use thread::available_parallelism as the default limit for backend parallelism #160424

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petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 4, 2026

petrochenkov added 2 commits August 4, 2026 23:35

rustc_codegen_ssa: Correctly apply the static --jobs-backend limit …

54877b4

…to backend parallelism

Do not forget to initialize jobserver if only linker is parallel

9956e2f

petrochenkov force-pushed the backlimit branch from 2c3f670 to 9956e2f Compare August 4, 2026 20:50

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 4, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 5, 2026

JonathanBrouwer mentioned this pull request Aug 5, 2026

Rollup of 22 pull requests #160554

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JonathanBrouwer mentioned this pull request Aug 5, 2026

Rollup of 25 pull requests #160557

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JonathanBrouwer mentioned this pull request Aug 5, 2026

Rollup of 25 pull requests #160559

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rust-bors Bot merged commit d734ccb into rust-lang:main Aug 5, 2026
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rustbot added this to the 1.99.0 milestone Aug 5, 2026

rust-timer added a commit that referenced this pull request Aug 5, 2026

Unrolled build for #160387

08c17aa

Rollup merge of #160387 - petrochenkov:backlimit, r=bjorn3 rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism Follow up to #159675. r? @bjorn3

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rustbot added the perf-regression Performance regression. label Aug 7, 2026

petrochenkov mentioned this pull request Aug 7, 2026

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

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rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism - #160387

rustc_codegen_ssa: Correctly apply the static `--jobs-backend` limit to backend parallelism#160387
rust-bors[bot] merged 2 commits into
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Overall result: ❌✅ regressions and improvements - please read:

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