Replace `rustc_span::Span` with a stripped down version for librustdoc's highlighter #147189

yotamofek · 2025-09-30T14:33:58Z

While profiling rustdoc's syntax highlighter, I noticed a lot of time being spent in the Span interner, due to the highlighter creating a lot of (new) spans.
Since the only data from the Span that we use is the hi and lo byte positions - I replaced the regular Span with a simple one with two fields, and in my benchmarks it seemed to make a big dent in the highlighter's perf, so thought I would see what the perf runner says.

GuillaumeGomez · 2025-09-30T14:37:15Z

@bors try @rust-timer queue

…2, r=<try> [PERF] Use small `Span`s in rustdoc's highlighter

rust-bors · 2025-09-30T16:50:37Z

☀️ Try build successful (CI)
Build commit: 7b69a13 (7b69a134c1e31187359ea130f26dfc5c18c6a8ce, parent: a2db9280539229a3b8a084a09886670a57bc7e9c)

rust-timer · 2025-09-30T18:01:23Z

Finished benchmarking commit (7b69a13): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@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
Improvements ✅ (primary)	-1.3%	[-5.4%, -0.2%]	18
Improvements ✅ (secondary)	-4.1%	[-12.6%, -0.1%]	12
All ❌✅ (primary)	-1.3%	[-5.4%, -0.2%]	18

Max RSS (memory usage)

Results (primary -2.8%, secondary -7.6%)

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)	-2.8%	[-3.9%, -2.1%]	5
Improvements ✅ (secondary)	-7.6%	[-16.7%, -1.8%]	7
All ❌✅ (primary)	-2.8%	[-3.9%, -2.1%]	5

Cycles

Results (primary -3.9%, secondary -11.8%)

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)	-	-	0
Improvements ✅ (primary)	-3.9%	[-4.7%, -3.1%]	2
Improvements ✅ (secondary)	-11.8%	[-23.4%, -3.1%]	6
All ❌✅ (primary)	-3.9%	[-4.7%, -3.1%]	2

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 470.437s -> 471.961s (0.32%)
Artifact size: 387.69 MiB -> 387.72 MiB (0.01%)

yotamofek · 2025-09-30T18:05:46Z

Yay! Haven't had a perf run this positive in a while. I'll finish this up and mark is as non-draft

…c's highlighter

rustbot · 2025-09-30T19:03:59Z

r? @notriddle

rustbot has assigned @notriddle.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

yotamofek · 2025-09-30T19:04:35Z

r? @GuillaumeGomez

yotamofek · 2025-09-30T19:06:43Z

src/librustdoc/html/render/span_map.rs

+/// never actually use the context and parent that are stored in a normal `Span`, we can replace its usages with this
+/// one, which is much cheaper to construct.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub(crate) struct Span {


Wasn't really sure where to put this..

rust-log-analyzer · 2025-09-30T19:11:22Z

The job pr-check-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

yotamofek · 2025-09-30T19:12:32Z

The job pr-check-1 failed! Check out the build log: (web) (plain enhanced) (plain)
Click to see the possible cause of the failure (guessed by this bot)

This error make it seems like a spurious network error:

2025-09-30T19:10:40.4566335Z curl: (7) Failed to connect to ci-artifacts.rust-lang.org port 443 after 8340 ms: Network is unreachable

GuillaumeGomez · 2025-09-30T19:16:48Z

Restarted the CI. Once green, r=me.

@bors delegate+
@bors rollup=maybe

bors · 2025-09-30T19:16:53Z

✌️ @yotamofek, you can now approve this pull request!

If @GuillaumeGomez told you to "r=me" after making some further change, please make that change, then do @bors r=@GuillaumeGomez

yotamofek · 2025-09-30T20:06:02Z

@bors r=@GuillaumeGomez

bors · 2025-09-30T20:06:05Z

📌 Commit 2d03ab1 has been approved by GuillaumeGomez

It is now in the queue for this repository.

…imizations-2, r=GuillaumeGomez Replace `rustc_span::Span` with a stripped down version for librustdoc's highlighter While profiling rustdoc's syntax highlighter, I noticed a lot of time being spent in the `Span` interner, due to the highlighter creating a lot of (new) spans. Since the only data from the `Span` that we use is the `hi` and `lo` byte positions - I replaced the regular `Span` with a simple one with two fields, and in my benchmarks it seemed to make a big dent in the highlighter's perf, so thought I would see what the perf runner says.

Rollup of 11 pull requests Successful merges: - #146918 (add regression test) - #146980 (simplify setup_constraining_predicates, and note it is potentially cubic) - #147170 (compiletest: Pass around `DirectiveLine` instead of bare strings) - #147180 (add tests) - #147188 (Remove usage of `compiletest-use-stage0-libtest` from CI) - #147189 (Replace `rustc_span::Span` with a stripped down version for librustdoc's highlighter) - #147199 (remove outdated comment in (inner) `InferCtxt`) - #147200 (Fix autodiff empty ret regression) - #147209 (Remove `no-remap-src-base` from tests) - #147213 (Fix broken STD build for ESP-IDF) - #147217 (Don't create a top-level `true` directory when running UI tests) r? `@ghost` `@rustbot` modify labels: rollup

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rust-bors bot added a commit that referenced this pull request Sep 30, 2025

Auto merge of #147189 - yotamofek:pr/rustdoc/highlight-optimizations-…

7b69a13

…2, r=<try> [PERF] Use small `Span`s in rustdoc's highlighter

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 30, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 30, 2025

yotamofek added 2 commits September 30, 2025 21:13

Add #[bench] for librustdoc's syntax highlighter

6edf05b

Replace rustc_span::Span with a stripped down version for librustdo…

2d03ab1

…c's highlighter

yotamofek force-pushed the pr/rustdoc/highlight-optimizations-2 branch from a2250d0 to 2d03ab1 Compare September 30, 2025 19:03

yotamofek marked this pull request as ready for review September 30, 2025 19:03

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 30, 2025

rustbot assigned notriddle Sep 30, 2025

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 30, 2025

yotamofek changed the title ~~[PERF] Use small Spans in rustdoc's highlighter~~ Replace rustc_span::Span with a stripped down version for librustdoc's highlighter Sep 30, 2025

rustbot assigned GuillaumeGomez and unassigned notriddle Sep 30, 2025

yotamofek commented Sep 30, 2025

View reviewed changes

GuillaumeGomez approved these changes Sep 30, 2025

View reviewed changes

bors 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 Sep 30, 2025

Zalathar mentioned this pull request Oct 1, 2025

Rollup of 11 pull requests #147220

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Replace `rustc_span::Span` with a stripped down version for librustdoc's highlighter #147189

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Overall result: ✅ improvements - no action needed

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Replace `rustc_span::Span` with a stripped down version for librustdoc's highlighter #147189

Replace `rustc_span::Span` with a stripped down version for librustdoc's highlighter #147189