Rewrite `rustc_span::symbol::Interner` to avoid double hashing by heinwol · Pull Request #157252 · rust-lang/rust

heinwol · 2026-06-01T14:14:18Z

Involves resorting to raw HashTable and writing an ad-hoc IndexMap-like structure, as we cannot get access to raw hashes otherwise.

My local cachegrind profile shows ~ -20_000_000 Ir

r? @petrochenkov

rustbot · 2026-06-01T14:14:25Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

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@rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

petrochenkov · 2026-06-01T14:21:54Z

@bors try @rust-timer queue

Rewrite `rustc_span::symbol::Interner` to avoid double hashing

rust-bors · 2026-06-01T16:36:41Z

☀️ Try build successful (CI)
Build commit: e66b004 (e66b0048988e203053f928d27053451bfb56886a, parent: 4804ad7e93e1b31f4605b7083871d0d3d85a2afe)

rust-timer · 2026-06-01T17:17:26Z

Finished benchmarking commit (e66b004): 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
@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)	0.8%	[0.2%, 1.5%]	5
Improvements ✅ (primary)	-0.6%	[-0.8%, -0.2%]	6
Improvements ✅ (secondary)	-0.4%	[-2.1%, -0.0%]	36
All ❌✅ (primary)	-0.6%	[-0.8%, -0.2%]	6

Max RSS (memory usage)

Results (primary 1.6%, secondary 1.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.2%, 2.2%]	4
Regressions ❌ (secondary)	4.0%	[1.1%, 10.0%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-8.1%, -0.6%]	7
All ❌✅ (primary)	1.6%	[1.2%, 2.2%]	4

Cycles

Results (secondary -2.7%)

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)	1.9%	[1.9%, 1.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-6.3%, -1.6%]	7
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 509.892s -> 512.954s (0.60%)
Artifact size: 400.78 MiB -> 400.82 MiB (0.01%)

petrochenkov · 2026-06-01T17:22:00Z

The red numbers in include-blob are a noise from LLVM codegen.

heinwol · 2026-06-02T09:36:18Z

I've added locking enhancements to reduce contention, maybe this will cause some improvement (or maybe the contrary). Of course, this is only relevant in multithreaded benches, i'm not sure if we can reliably test this.

Also, due to my lack of experience, I'd like a more professional look at whether I have some concurrency bugs. I think I've mitigated the TOCTOU and locking order is deterministic. My benches and tests run fine.

petrochenkov · 2026-06-03T14:44:36Z

I've added locking enhancements to reduce contention, maybe this will cause some improvement (or maybe the contrary). Of course, this is only relevant in multithreaded benches, i'm not sure if we can reliably test this.

Could you remove this change from this PR?
It can be submitted later after the double hashing optimizations merges.

Otherwise LGTM, left some style nits.
@rustbot author

rustbot · 2026-06-03T14:44:41Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Involves resorting to raw `HashTable` and writing an ad-hoc `IndexMap`-like structure, as we cannot get access to raw hashes otherwise. My local cachegrind profile shows ~ -20_000_000 Ir

heinwol · 2026-06-08T14:19:20Z

@rustbot ready

petrochenkov · 2026-06-08T15:24:02Z

@bors r+

rust-bors · 2026-06-08T15:24:05Z

📌 Commit 00d08cb has been approved by petrochenkov

It is now in the queue for this repository.

rust-bors · 2026-06-09T00:04:10Z

☀️ Test successful - CI
Approved by: petrochenkov
Duration: 4h 2m 11s
Pushing 83b3bfc to main...

github-actions · 2026-06-09T00:07:01Z

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 cb46fbb (parent) -> 83b3bfc (this PR)

Test differences

Show 5 test diffs

Stage 0

symbol::tests::interner_get: [missing] -> pass (J0)

Stage 1

symbol::tests::interner_get: [missing] -> pass (J1)

Additionally, 3 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 83b3bfc40a9d2fb07dd08a275d80a6ba8a903f18 --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-llvm-22-1: 45m 45s -> 1h 13m (+61.1%)
i686-gnu-2: 1h 41m -> 1h 3m (-37.8%)
x86_64-msvc-ext3: 1h 27m -> 1h 54m (+29.8%)
i686-gnu-nopt-2: 1h 48m -> 2h 19m (+28.7%)
dist-powerpc64-linux-musl: 1h 15m -> 1h 33m (+25.1%)
i686-gnu-nopt-1: 2h 27m -> 1h 50m (-24.6%)
aarch64-apple: 3h 10m -> 3h 54m (+23.1%)
x86_64-msvc-1: 2h 39m -> 2h 2m (-22.9%)
x86_64-gnu-miri: 1h 35m -> 1h 14m (-22.8%)
dist-i686-msvc: 2h 20m -> 1h 50m (-21.7%)

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 · 2026-06-09T00:47:01Z

Finished benchmarking commit (83b3bfc): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.7%	[0.2%, 1.7%]	6
Improvements ✅ (primary)	-0.5%	[-0.7%, -0.2%]	6
Improvements ✅ (secondary)	-0.4%	[-1.8%, -0.2%]	21
All ❌✅ (primary)	-0.5%	[-0.7%, -0.2%]	6

Max RSS (memory usage)

Results (primary 0.9%, secondary 2.8%)

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

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.8%, 1.1%]	2
Regressions ❌ (secondary)	5.2%	[2.2%, 10.1%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.6%	[-6.9%, -2.3%]	2
All ❌✅ (primary)	0.9%	[0.8%, 1.1%]	2

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%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	4

Bootstrap: 517.707s -> 516.634s (-0.21%)
Artifact size: 400.82 MiB -> 400.78 MiB (-0.01%)

rustbot assigned petrochenkov Jun 1, 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 Jun 1, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jun 1, 2026

Auto merge of #157252 - heinwol:symbol-Interner-double-hashing, r=<try>

e66b004

Rewrite `rustc_span::symbol::Interner` to avoid double hashing

petrochenkov removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 1, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 1, 2026

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petrochenkov reviewed Jun 3, 2026

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rustbot 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 Jun 3, 2026

heinwol force-pushed the symbol-Interner-double-hashing branch 2 times, most recently from 5c67961 to ec77a49 Compare June 8, 2026 14:13

Rewrite rustc_span::symbol::Interner to avoid double hashing

00d08cb

Involves resorting to raw `HashTable` and writing an ad-hoc `IndexMap`-like structure, as we cannot get access to raw hashes otherwise. My local cachegrind profile shows ~ -20_000_000 Ir

heinwol force-pushed the symbol-Interner-double-hashing branch from ec77a49 to 00d08cb Compare June 8, 2026 14:16

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jun 8, 2026

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 8, 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 Jun 8, 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 Jun 9, 2026

rust-bors Bot merged commit 83b3bfc into rust-lang:main Jun 9, 2026
13 checks passed

rustbot added this to the 1.98.0 milestone Jun 9, 2026

heinwol mentioned this pull request Jun 10, 2026

Rewrite rustc_span::symbol::Interner to reduce lock contention #157701

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rust-timer commented Jun 1, 2026

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

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Test differences

Stage 0

Stage 1

Job duration changes

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rust-timer commented Jun 9, 2026

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

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