Add a single-entry parent `SpanData` cache by nnethercote · Pull Request #163628 · rust-lang/rust

nnethercote · 2026-10-02T03:23:10Z

For use in encoding, decoding, and stable hashing. It's a decent performance win on a number of benchmarks.

r? @oli-obk

For use in encoding, decoding, and stable hashing. It's a decent performance win on a number of benchmarks.

nnethercote · 2026-10-02T03:23:33Z

LLM disclosure: the idea for this came from an LLM. I wrote all the code and text myself.

nnethercote · 2026-10-02T03:23:41Z

@bors try @rust-timer queue

Add a single-entry parent `SpanData` cache

rust-bors · 2026-10-02T04:27:33Z

☀️ Try build successful (CI)
Build commit: c1ea28c (c1ea28c6c87385ed20846dbdebf16f06813f7d34)
Base parent: c36f145 (c36f1457196e315bc204b9564a6a5a7fe7f5a51f)

rust-timer · 2026-10-02T05:11:07Z

Finished benchmarking commit (c1ea28c): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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)	-	-	0
Improvements ✅ (primary)	-0.4%	[-2.4%, -0.1%]	139
Improvements ✅ (secondary)	-0.8%	[-4.4%, -0.1%]	109
All ❌✅ (primary)	-0.4%	[-2.4%, -0.1%]	139

Max RSS (memory usage)

Results (primary 3.0%, secondary -4.3%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-4.3%, -4.3%]	1
All ❌✅ (primary)	3.0%	[3.0%, 3.0%]	1

Cycles

Results (secondary 1.8%)

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)	3.7%	[2.2%, 6.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.9%	[-2.1%, -1.6%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 489.203s -> 491.876s (0.55%)
Artifact size: 406.46 MiB -> 406.49 MiB (0.01%)

nnethercote · 2026-10-02T06:43:22Z

Lots of green for incremental builds, as expected.

oli-obk

@bors r+

neat! makes sense

View changes since this review

rust-bors · 2026-10-02T11:02:29Z

📌 Commit 5e04ccd has been approved by oli-obk

It is now in the queue for this repository.

rust-bors · 2026-10-03T03:14:39Z

☀️ Test successful - CI
Approved by: oli-obk
Duration: 3h 4m 54s
Pushing e0452e2 to main...

github-actions · 2026-10-03T03:17:57Z

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 0abfedb (parent) -> e0452e2 (this PR)

Test differences

Show 4 test diffs

4 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 e0452e266c9a4d6522507318f8365741f0b12d29 --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

test-x86_64-rust-for-linux: 38m 49s -> 1h 1m (+57.2%)
dist-arm-linux-musl: 1h 56m -> 1h 7m (-42.0%)
test-i686-msvc: 2h 30m -> 1h 32m (-38.4%)
test-x86_64-gnu-parallel-frontend: 1h 32m -> 2h 6m (+37.5%)
test-x86_64-gnu-debug: 2h 13m -> 1h 24m (-37.0%)
dist-x86_64-msvc: 3h 5m -> 1h 59m (-35.6%)
test-x86_64-gnu-tools: 1h 11m -> 45m 46s (-35.5%)
test-armhf-gnu: 1h 33m -> 1h 2m (-33.7%)
dist-x86_64-msvc-alt: 2h 52m -> 1h 55m (-33.1%)
dist-s390x-linux: 1h 33m -> 1h 4m (-31.6%)

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 · 2026-10-03T03:58:59Z

Finished benchmarking commit (e0452e2): 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)	-0.4%	[-2.4%, -0.1%]	152
Improvements ✅ (secondary)	-0.7%	[-4.4%, -0.1%]	111
All ❌✅ (primary)	-0.4%	[-2.4%, -0.1%]	152

Max RSS (memory usage)

Results (secondary -1.3%)

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)	3.2%	[2.6%, 3.9%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-6.4%, -1.9%]	5
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -1.0%)

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)	3.2%	[2.3%, 3.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.2%	[-10.0%, -2.1%]	4
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 492.59s -> 491.518s (-0.22%)
Artifact size: 409.38 MiB -> 408.68 MiB (-0.17%)

Add a single-entry parent SpanData cache

5e04ccd

For use in encoding, decoding, and stable hashing. It's a decent performance win on a number of benchmarks.

rustbot assigned oli-obk Oct 2, 2026

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 2, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 2, 2026

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rust-bors Bot pushed a commit that referenced this pull request Oct 2, 2026

Auto merge of #163628 - nnethercote:SourceSpanCache, r=<try>

c1ea28c

Add a single-entry parent `SpanData` cache

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 2, 2026

nnethercote marked this pull request as ready for review October 2, 2026 06:42

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 Oct 2, 2026

oli-obk approved these changes Oct 2, 2026

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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 Oct 2, 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 Oct 3, 2026

rust-bors Bot merged commit e0452e2 into rust-lang:main Oct 3, 2026
16 checks passed

rustbot added this to the 1.101.0 milestone Oct 3, 2026

github-actions Bot mentioned this pull request Oct 4, 2026

Toolchain upgrade to nightly-2026-10-11 flux-rs/flux#1811

Merged

nnethercote deleted the SourceSpanCache branch October 5, 2026 03:23

nnethercote mentioned this pull request Oct 30, 2026

Compiler performance optimizations rust-lang/goals#789

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Add a single-entry parent `SpanData` cache - #163628

Add a single-entry parent `SpanData` cache#163628
rust-bors[bot] merged 1 commit into
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nnethercote:SourceSpanCache

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Overall result: ✅ improvements - no action needed

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Test differences

Job duration changes

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Overall result: ✅ improvements - no action needed

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