perf(sourcemap): skip newline scan on the no-sourcemap join fast path by Boshen · Pull Request #9936 · rolldown/rolldown

Boshen · 2026-06-23T08:44:51Z

What

SourceJoiner::join advanced line_offset on every iteration via source.lines_count() — a memchr newline scan over the full content. But line_offset is only ever read by the sourcemap builder, so on the common no-sourcemap build (no source carries a map → builder is None) it is a dead value, and every module's content was scanned twice: once by push_str (the copy) and once by lines_count().

This splits join into:

a no-sourcemap fast path — plain concatenation with \n separators, no lines_count() scan;
the existing sourcemap loop, with the per-iteration if let Some(builder) check hoisted out (so the sourcemap path is, if anything, slightly tighter than before).

Numbers

Interleaved A/B (--profile profile, thin-LTO) against a recompiled-original control. The control matters: join_with_sourcemap is bandwidth-bound and swings ±27% between identical-source recompiles from code layout alone, so a saved baseline would falsely report a regression.

bench	before	after
`crates/bench` `sourcemap/join_no_sourcemap`	~62.3 µs	~42.2 µs	−32%
`rolldown_sourcemap/benches/join` (10k × 1KB modules)	~391 µs	~254 µs	−35%
`crates/bench` `sourcemap/join_with_sourcemap`	~417 µs	~411 µs	neutral
`bundle@threejs` (e2e, no-sourcemap)	~23.25 ms	~23.17 ms	no regression

Two independent no-sourcemap join benches agree (~−32% / −35%). End-to-end magnitude is small (join is a thin slice of generate), but it is a clean, isolated win on a path every no-sourcemap build hits.

Correctness

Behavior-preserving by construction — the no-sourcemap path emits the identical concatenation; only the dead line_offset bookkeeping is skipped.

cargo test -p rolldown_sourcemap — 4/4 pass (exact-output assertions on both paths)
cargo test -p rolldown --test integration — 1756 passed, 0 failed (byte-identical snapshots, all output formats)
cargo clippy -p rolldown_sourcemap — clean

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codspeed-hq · 2026-06-23T08:59:18Z

Merging this PR will improve performance by 18.23%

⚠️

Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 6 untouched benchmarks
⏩ 10 skipped benchmarks¹

Performance Changes

	Benchmark	`BASE`	`HEAD`	Efficiency
⚡	`join_no_sourcemap`	1.6 ms	1.3 ms	+18.23%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

_{Comparing perf/sourcemap-join-no-sourcemap-fast-path (544ea96) with main (b311823)}

10 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

IWANABETHATGUY · 2026-06-24T02:55:35Z

LGTM, would you mind having a look @hyf0 ?

hyf0 · 2026-06-24T14:54:07Z

Merge activity

Jun 24, 2:54 PM UTC: The merge label 'graphite: merge-when-ready' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Jun 24, 2:54 PM UTC: hyf0 added this pull request to the Graphite merge queue.
Jun 24, 2:59 PM UTC: Merged by the Graphite merge queue.

…#9936) ## What `SourceJoiner::join` advanced `line_offset` on every iteration via `source.lines_count()` — a `memchr` newline scan over the full content. But `line_offset` is only ever read by the sourcemap builder, so on the common no-sourcemap build (no source carries a map → builder is `None`) it is a dead value, and every module's content was scanned twice: once by `push_str` (the copy) and once by `lines_count()`. This splits `join` into: - a **no-sourcemap fast path** — plain concatenation with `\n` separators, no `lines_count()` scan; - the existing **sourcemap loop**, with the per-iteration `if let Some(builder)` check hoisted out (so the sourcemap path is, if anything, slightly tighter than before). ## Numbers Interleaved A/B (`--profile profile`, thin-LTO) against a *recompiled-original* control. The control matters: `join_with_sourcemap` is bandwidth-bound and swings ±27% between identical-source recompiles from code layout alone, so a saved baseline would falsely report a regression. | bench | before | after | | |---|---|---|---| | `crates/bench` `sourcemap/join_no_sourcemap` | ~62.3 µs | ~42.2 µs | **−32%** | | `rolldown_sourcemap/benches/join` (10k × 1KB modules) | ~391 µs | ~254 µs | **−35%** | | `crates/bench` `sourcemap/join_with_sourcemap` | ~417 µs | ~411 µs | neutral | | `bundle@threejs` (e2e, no-sourcemap) | ~23.25 ms | ~23.17 ms | no regression | Two independent no-sourcemap join benches agree (~−32% / −35%). End-to-end magnitude is small (join is a thin slice of generate), but it is a clean, isolated win on a path every no-sourcemap build hits. ## Correctness Behavior-preserving by construction — the no-sourcemap path emits the identical concatenation; only the dead `line_offset` bookkeeping is skipped. - `cargo test -p rolldown_sourcemap` — 4/4 pass (exact-output assertions on both paths) - `cargo test -p rolldown --test integration` — 1756 passed, 0 failed (byte-identical snapshots, all output formats) - `cargo clippy -p rolldown_sourcemap` — clean

@shulaoda

## [1.1.4] - 2026-07-01 ### 🚀 Features - disable `experimental.lazyBarrel` by default (#10071) by @shulaoda ### 🐛 Bug Fixes - dev: disable lazy barrel in dev mode (#10060) by @shulaoda - generate: keep full JSON interface under preserveModules namespa… (#10056) by @IWANABETHATGUY - check finalize_other_specifiers in its own Debug attribute (#10032) by @shulaoda - serialize the KeepAssign unused minify option as "keep_assign" (#10031) by @shulaoda - keep fragments after the newline fragment in MagicString::last_line (#10023) by @shulaoda - generate: undeclared JSON named exports under preserveModules (#10020) (#10027) by @IWANABETHATGUY - deconflict: rename CJS-wrapped locals that shadow chunk-root bindings (#9921) by @IWANABETHATGUY - rolldown: keep entry facade when a shared chunk holds another entry's module (#9997) by @hyf0 - treeshake: also bail JSON default split when the object escapes (#9996) by @IWANABETHATGUY - don't classify await in a strict-mode function as top-level await (#9987) by @shulaoda - avoid spurious leading newline in addon hooks (banner/footer/intro/outro) (#9989) by @shulaoda - handle JSON default mutation bailouts (#9972) by @TheAlexLichter - plugin: make lazy hook metadata enumerable (#9991) by @TheAlexLichter - dev: make init errors in lazy-compiled modules catchable (#9981) by @h-a-n-a - treeshake: keep computed-key side effects on namespace member access (#9986) by @shulaoda - binding: validate replace plugin delimiters length instead of panicking (#9984) by @shulaoda - reconstruct nested rest patterns in into_expression (#9980) by @IWANABETHATGUY - reconstruct rest patterns as spread in into_expression (#9976) by @shulaoda - preserve export keyword on multi-declarator exports under keepNames (#9974) by @shulaoda - deterministically keep the shortest name for deduplicated assets (#9948) by @x1024 - treeshake: apply @__NO_SIDE_EFFECTS__ to cross-chunk namespace calls (#9960) by @IWANABETHATGUY ### 🚜 Refactor - drop redundant program scope enter/leave in finalizer (#10049) by @shulaoda - deconflict: extract collect_chunk_scope_captured_names (#10006) by @IWANABETHATGUY - unify pre-scan multi-declarator split into one decision site (#9982) by @IWANABETHATGUY - common: return bool from SymbolRef::is_not_reassigned (#9962) by @IWANABETHATGUY ### 📚 Documentation - rolldown: remove outdated comment for removing parenthesized expression (#10062) by @Dunqing - use GitHub-flavored alert for Etiquette note in contribution guide (#10012) by @IWANABETHATGUY - replace: explain the delimiters left and right boundaries (#9985) by @shulaoda - ast-mutation: remove stale Address Use section after pre-scan refactor (#9983) by @IWANABETHATGUY - remove fathom (#9968) by @mdong1909 - contribution-guide: code-format main branch references (#9966) by @IWANABETHATGUY - contribution-guide: fix stale REPL note and tidy wording (#9957) by @hyf0 - contribution-guide: clarify when to discuss before opening a PR (#9955) by @hyf0 ### ⚡ Performance - disable preserve_parens across all parse paths (#10057) by @Dunqing - common: inline declared_symbols with SmallVec (#9920) by @IWANABETHATGUY - common: pack TaggedSymbolRef into 8 bytes (#9919) by @IWANABETHATGUY - sourcemap: skip newline scan on the no-sourcemap join fast path (#9936) by @Boshen ### 🧪 Testing - dev: error in lazy module should be catchable (#9975) by @sapphi-red - dev: reject unknown lazy compile modules (#9969) by @sapphi-red ### ⚙️ Miscellaneous Tasks - deps: update actions/cache action to v6 (#10001) by @renovate[bot] - trigger vite ecosystem-ci from PR comments (#10058) by @shulaoda - deps: update napi to v3.10.0 (#10063) by @renovate[bot] - remove unused From impl for RolldownLabelSpan (#10055) by @shulaoda - remove dead Diagnostic::with_kind method (#10054) by @shulaoda - remove unused StatementExt methods (#10053) by @shulaoda - remove unused ExpressionExt methods (#10052) by @shulaoda - remove commented-out re_export_all_names field (#10051) by @shulaoda - deps: update pnpm to v11.9.0 (#10047) by @renovate[bot] - remove the unused BindingGenerateHmrPatchReturn napi type (#10034) by @shulaoda - remove the dead inline_entry_chunk_wrapping scaffolding (#10037) by @shulaoda - deps: bump oxc_resolver to 11.22.0 (#10045) by @Boshen - remove never-constructed MatchImportKind::_Ignore variant (#10041) by @shulaoda - remove the unused ScheduledBuild napi struct (#10033) by @shulaoda - remove dead compute_hmr_update_single method (#10040) by @shulaoda - drop the redundant visited.insert in manual code splitting (#10038) by @shulaoda - remove the dead output_assets vector in render_chunk_to_assets (#10036) by @shulaoda - remove the unused From<String>/Display impls for BindingLogLevel (#10035) by @shulaoda - deps: upgrade oxc to 0.138.0 and migrate to per-type AST construction (#10018) by @shulaoda - deps: update rust crates (#9911) by @renovate[bot] - deps: update test262 submodule for tests (#10016) by @rolldown-guard[bot] - deps: update github actions (#9999) by @renovate[bot] - deps: update npm packages (#10000) by @renovate[bot] ### ◀️ Revert - "fix(plugin): make lazy hook metadata enumerable (#9991)" (#10005) by @shulaoda ### ❤️ New Contributors * @x1024 made their first contribution in [#9948](#9948) Co-authored-by: shulaoda <165626830+shulaoda@users.noreply.github.com>

Boshen assigned IWANABETHATGUY Jun 23, 2026

Boshen marked this pull request as ready for review June 23, 2026 08:52

Boshen requested review from IWANABETHATGUY, hyf0 and shulaoda as code owners June 23, 2026 08:52

IWANABETHATGUY assigned hyf0 and unassigned IWANABETHATGUY Jun 24, 2026

hyf0 approved these changes Jun 24, 2026

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hyf0 added the graphite: merge-when-ready label Jun 24, 2026

graphite-app Bot force-pushed the perf/sourcemap-join-no-sourcemap-fast-path branch from 544ea96 to 12d9647 Compare June 24, 2026 14:54

graphite-app Bot merged commit 12d9647 into main Jun 24, 2026
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graphite-app Bot removed the graphite: merge-when-ready label Jun 24, 2026

graphite-app Bot deleted the perf/sourcemap-join-no-sourcemap-fast-path branch June 24, 2026 14:59

rolldown-guard Bot mentioned this pull request Jul 1, 2026

release: v1.1.4 #10072

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perf(sourcemap): skip newline scan on the no-sourcemap join fast path#9936

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codspeed-hq Bot commented Jun 23, 2026

Merging this PR will improve performance by 18.23%

Performance Changes

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