Resolve: more preperation work for parallelizing the import resolution loop by LorrensP-2158466 · Pull Request #159440 · rust-lang/rust

LorrensP-2158466 · 2026-07-17T12:42:34Z

This is basically #158845 but we do not:

actually use par_slice because:
we do not migrate CmRefCell to use RwLocks (yet) because of perf reasons.

The resolution loop is now in place instead of recollecting indeterminate imports.

LorrensP-2158466 · 2026-07-17T15:09:46Z

I double-checked with git diff f3d5c561f4c compiler/rustc_resolve and I only see comments or the unsafe impls as + diff.

@rustbot ready

petrochenkov · 2026-07-17T20:16:01Z

+            self.write_import_resolutions(&to_resolve_imports);
+
+            self.indeterminate_imports = to_resolve_imports
+                .extract_if(.., |(_, _, count)| {


It would be nicer to extract_if into determined_imports instead and then do self.indeterminate_imports = imports_to_resolve;.
But it may be slower due to majority of the elements in the vector being determinate.
I'll benchmark it separately later after this PR lands.

View changes since the review

I would expect it to be faster, now that I thought about more clearly.

Currently we extract all indeterminate imports, which will leave small holes in the vector and thus requiring a lot of chunks to be moved. And then we still have to iterate over the vector such that we only get the import itself.

Extracting into determined imports is the same kind of map as we have now, with the occasional skip of an undetermined import. And then we just have a simple assignment. This looks like less work.

Done in #159681.

Done in #159681.

It's a minor regression.

…par_slice`. Currently it is all still single threaded, but almost everything needed to make the loop use multithreaded code is done in this commit. For performance reasons we do the resolution now in place instead of recollecting the indeterminate imports ([see benchmark pr](rust-lang#159387) and pr of this commit). Populating external resolution tables now use `Once` to ensure only 1 thread builds the table and all others wait if they need it, this has no impact on single threaded behaviour. `Cache(Ref)Cells` are not tackled yet because they are not needed here (and require extra work).

LorrensP-2158466 · 2026-07-18T08:58:30Z

Force pushed to keep it at 1 commit, @rustbot ready.

petrochenkov · 2026-07-20T14:34:09Z

Perf-sensitive change, benchmarked in #159387 (comment).
@bors r+ rollup=never

rust-bors · 2026-07-20T14:34:12Z

📌 Commit 2f3022a has been approved by petrochenkov

It is now in the queue for this repository.

rust-bors · 2026-07-21T13:36:04Z

☀️ Test successful - CI
Approved by: petrochenkov
Duration: 3h 31m 11s
Pushing f65b272 to main...

github-actions · 2026-07-21T13:39:07Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing cde3f8a (parent) -> f65b272 (this PR)

Test differences

Show 14 test diffs

14 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard f65b272fc92b1e7527dea4a224430a249ab25c2d --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-gnu-debug: 1h 36m -> 2h 31m (+56.8%)
x86_64-gnu-llvm-21-2: 1h 41m -> 1h 4m (-36.8%)
dist-armv7-linux: 1h 32m -> 58m 33s (-36.8%)
dist-various-1: 56m 14s -> 1h 16m (+35.4%)
optional-x86_64-gnu-parallel-frontend: 2h 11m -> 1h 25m (-35.3%)
x86_64-gnu-nopt: 1h 53m -> 2h 32m (+34.4%)
i686-gnu-nopt-2: 1h 45m -> 2h 20m (+33.5%)
x86_64-gnu-llvm-21-3: 1h 23m -> 1h 50m (+32.5%)
armhf-gnu: 1h 10m -> 1h 33m (+32.3%)
dist-x86_64-llvm-mingw: 1h 35m -> 2h (+26.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-07-21T18:09:12Z

Finished benchmarking commit (f65b272): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.2%]	18
Improvements ✅ (secondary)	-0.6%	[-1.5%, -0.1%]	29
All ❌✅ (primary)	-0.2%	[-0.3%, -0.2%]	18

Max RSS (memory usage)

Results (primary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.6%, 1.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.6%	[1.6%, 1.6%]	1

Cycles

This perf run didn't have relevant results for this metric.

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	15
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	4

Bootstrap: 486.164s -> 487.415s (0.26%)
Artifact size: 387.87 MiB -> 387.80 MiB (-0.02%)

rustbot assigned petrochenkov Jul 17, 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 Jul 17, 2026

LorrensP-2158466 mentioned this pull request Jul 17, 2026

Resolver: Parallelize the import resolution loop #158845

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LorrensP-2158466 force-pushed the inplace-import-res branch from 0d9f4a2 to aa051d9 Compare July 17, 2026 15:09

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LorrensP-2158466 force-pushed the inplace-import-res branch from aa051d9 to 2e9c9d2 Compare July 17, 2026 15:23

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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 Jul 17, 2026

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LorrensP-2158466 force-pushed the inplace-import-res branch from 2e9c9d2 to 2f3022a Compare July 18, 2026 08:59

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 Jul 20, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 21, 2026

rust-bors Bot merged commit f65b272 into rust-lang:main Jul 21, 2026
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petrochenkov mentioned this pull request Jul 21, 2026

resolve: Cleanup collection of resolved imports #159681

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Test differences

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rust-timer commented Jul 21, 2026

Overall result: ✅ improvements - no action needed

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LorrensP-2158466 commented Jul 17, 2026 •

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