perf(common): inline declared_symbols with SmallVec#9920
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> [!NOTE] > Base PR #9919 (pack `TaggedSymbolRef` to 8 bytes) is now merged into `main`; this PR contains only the `Vec` → `SmallVec` change on top of it. ### What Switch `StmtInfo::declared_symbols` storage from `Vec<TaggedSymbolRef>` to `SmallVec<[TaggedSymbolRef; 2]>`, keeping up to two declared symbols inline and only spilling to the heap beyond that. ### Why The overwhelming majority of statements declare zero, one, or two top-level symbols, but `Vec` rounds its first allocation up to capacity 4 (an 8-byte element falls in the `MIN_NON_ZERO_CAP = 4` bucket) — a ~3.7× over-allocation that heap-allocates for nearly every statement. Because `TaggedSymbolRef` is packed to 8 bytes (the base PR), `SmallVec<[_; 2]>` is **24 bytes — identical to `Vec`** — so `StmtInfo` does not grow. A `const` assertion pins `size_of::<DeclaredSymbols>() == size_of::<Vec<TaggedSymbolRef>>()`. ### Impact (threejs10x) Measured two independent ways — both agree. **1. Per-site — exact final heap state of every container.** Instrumented `LinkStage::link()` to walk every `StmtInfo.declared_symbols` after a real threejs10x link and read each one's `len()` / `spilled()` / `capacity()`. | storage | heap allocations | live heap bytes | |---|---|---| | `Vec` | 24061 | 793,952 B (~775 KB) | | `SmallVec<[_; 2]>` | 180 | 29,760 B (~29 KB) | **~99% of these allocations elided.** Length histogram (`len: count`): `{0: 7920, 1: 23531, 2: 350, 3: 40, 4: 10, 5: 10, 6: 10, 9: 30, 10: 30, 14: 10, 17: 10, 20: 10, 27: 10, 68: 10}` — non-empty = 24061, `len ≥ 3` (what still spills with inline cap 2) = 180. (Large buckets are multiples of 10: threejs10x = 10× three.js.) **2. Whole-program — real counting global allocator, A/B.** Wrapped the system allocator to count every `alloc`/`realloc` op, toggling *only* the inline capacity: `[_; 2]` (this PR) vs `[_; 0]` (no inline buffer → allocates like `Vec` for every non-empty container; the walk confirms `spilled = 24061` in that build). | build | total alloc/realloc ops | total bytes requested | |---|---|---| | `SmallVec<[_; 2]>` (this PR) | ~1,003,900 | ~1316 MB | | `SmallVec<[_; 0]>` (≈ `Vec`) | ~1,029,000 | ~1316 MB | | **delta** | **≈ −24,600 … −25,000 ops** | **~0** | Run-to-run noise was ±~150–270 ops, so the ~24k delta is ~100× the noise floor. > **Measured vs derived.** The `24061` allocation count is empirical — both the per-site walk and the inline-cap-0 A/B build report it directly (`spilled = 24061`). The `~775 KB` is the one figure *derived* rather than read live: it applies `Vec`'s real `MIN_NON_ZERO_CAP = 4` rounding, whereas the `[_; 0]` A/B build spills exact-fit (~218 KB), so that build confirms the count but not the bytes. > > **What this does and doesn't buy.** The win is **allocation *count*** — ~24k near-per-statement `malloc`s on a hot path turned into inline writes — not memory footprint. Whole-program allocation *volume* is essentially unchanged (~1316 MB either way), and even this struct's live bytes (~746 KB saved) are tiny against that. So the absolute whole-program memory win is modest by design; the value is allocator/CPU pressure, bought with **zero growth of `StmtInfo`** (the `size_of` assertion). ### Verification - `cargo clippy` (both crates, all targets) + `cargo fmt`: clean. - Release `StmtInfo == 80` assertion: holds. - Snapshot tests (`cargo test -p rolldown`, excluding hmr/test262): 1723 passed, 0 failed — zero behavior change. 🤖 Generated with [Claude Code](https://claude.com/claude-code)
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## [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>
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Note
Base PR #9919 (pack
TaggedSymbolRefto 8 bytes) is now merged intomain; this PR contains only theVec→SmallVecchange on top of it.What
Switch
StmtInfo::declared_symbolsstorage fromVec<TaggedSymbolRef>toSmallVec<[TaggedSymbolRef; 2]>, keeping up to two declared symbols inline and only spilling to the heap beyond that.Why
The overwhelming majority of statements declare zero, one, or two top-level symbols, but
Vecrounds its first allocation up to capacity 4 (an 8-byte element falls in theMIN_NON_ZERO_CAP = 4bucket) — a ~3.7× over-allocation that heap-allocates for nearly every statement.Because
TaggedSymbolRefis packed to 8 bytes (the base PR),SmallVec<[_; 2]>is 24 bytes — identical toVec— soStmtInfodoes not grow. Aconstassertion pinssize_of::<DeclaredSymbols>() == size_of::<Vec<TaggedSymbolRef>>().Impact (threejs10x)
Measured two independent ways — both agree.
1. Per-site — exact final heap state of every container. Instrumented
LinkStage::link()to walk everyStmtInfo.declared_symbolsafter a real threejs10x link and read each one'slen()/spilled()/capacity().VecSmallVec<[_; 2]>~99% of these allocations elided. Length histogram (
len: count):{0: 7920, 1: 23531, 2: 350, 3: 40, 4: 10, 5: 10, 6: 10, 9: 30, 10: 30, 14: 10, 17: 10, 20: 10, 27: 10, 68: 10}— non-empty = 24061,len ≥ 3(what still spills with inline cap 2) = 180. (Large buckets are multiples of 10: threejs10x = 10× three.js.)2. Whole-program — real counting global allocator, A/B. Wrapped the system allocator to count every
alloc/reallocop, toggling only the inline capacity:[_; 2](this PR) vs[_; 0](no inline buffer → allocates likeVecfor every non-empty container; the walk confirmsspilled = 24061in that build).SmallVec<[_; 2]>(this PR)SmallVec<[_; 0]>(≈Vec)Run-to-run noise was ±~150–270 ops, so the ~24k delta is ~100× the noise floor.
Verification
cargo clippy(both crates, all targets) +cargo fmt: clean.StmtInfo == 80assertion: holds.cargo test -p rolldown, excluding hmr/test262): 1723 passed, 0 failed — zero behavior change.🤖 Generated with Claude Code