Bytecode cache: numbered strings with stored hashes, so repeat lookups skip the atom table - #506
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…atom table once; store their hashes Every distinct non-symbol string record carries its StringImpl hash and a sequence number assigned by the encoder. The decoder adds to the atom table through the precomputed-hash translator and remembers the atom for each number in a vector, so the second block that names a string gets it by index instead of hashing and probing; 1- and 2-character strings stored inline in their slot are memoized in two flat tables. Payloads appended by CachedBytecode::addFunctionUpdate continue the numbering (Encoder takes the first number; CachedBytecode records the end). The atom table is grown once per decoded block (HashTable::reserveCapacity) rather than as strings trickle in. 30 MB bundle, decoding every top-level function: median 552 -> 468 ms; everything: 883 -> 768 ms (interleaved runs, pinned cores). Cache 87.7 -> 89.2 MB (+8 bytes per string record).
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WalkthroughChangesThe cached bytecode format now assigns string ordinals, reuses decoded atoms, reserves atom-table capacity, and propagates ordinal boundaries through global and function updates. Cached bytecode atom handling
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…en long) instead of going through the atom table; only identifiers need to be atoms 60k-function decode: 991M -> 874M cycles; decode-everything 1846M -> 1781M.
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In `@Source/JavaScriptCore/runtime/CachedBytecode.h`:
- Around line 78-80: Persist CachedBytecode::m_stringOrdinalEnd in the
serialized cache payload and restore it during
ShellSourceProvider::loadBytecode(), ensuring subsequent updates continue above
the stored ordinal boundary. Bump or otherwise invalidate the existing cache
format so older payloads are not loaded without this field.
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Source/JavaScriptCore/jsc.cppSource/JavaScriptCore/runtime/CachedBytecode.cppSource/JavaScriptCore/runtime/CachedBytecode.hSource/JavaScriptCore/runtime/CachedTypes.cppSource/JavaScriptCore/runtime/CachedTypes.hSource/WTF/wtf/HashSet.hSource/WTF/wtf/HashTable.hSource/WTF/wtf/text/AtomStringImpl.cppSource/WTF/wtf/text/AtomStringImpl.h
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… instead of continuing a count that is not persisted The end-of-numbering lived only in memory, so after the shell mapped a cache file the next function update numbered from 0 and collided with the base payload. Appended payloads are read by the base payload's Decoder; rather than persist a high-water mark in the header (and re-seal it on every commitUpdates), they simply do not number their strings and use the atom table as before. Removes CachedBytecode::stringOrdinalEnd and the encodeFunctionCodeBlock parameter.
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Both earlier findings are addressed in the current diff — encodeFunctionCodeBlock now passes Encoder::NumberStrings::No so appended payloads never collide on ordinals, and HashTable::reserveCapacity calls invalidateIterators before rehashing. This pass found no further issues. Given the scope (new on-disk record fields, per-decoder atom ref-counting, and a new public HashTable/AtomStringImpl API), a human look is still worthwhile.
Checked: ref/deref pairing across m_atomsByOrdinal / the length-1 and length-2 tables and ~Decoder; setAtomForOrdinal growth is bounded by RELEASE_ASSERT(ordinal < payload size); decodePlainString ownership for the symbol / ordinal-hit / create paths; reserveCapacity sizing against shouldExpand/computeBestTableSize.
Extended reasoning...
Overview
The PR speeds up bytecode-cache decode by (1) storing each string record's precomputed StringImpl::hash() plus a sequential ordinal so the decoder atom-table lookup happens once per distinct string and later references are a vector index, (2) memoizing 1- and 2-char inline strings in flat per-decoder tables, (3) decoding CachedJSValue string constants as plain StringImpls (skipping the atom table entirely), and (4) adding HashTable::reserveCapacity → HashSet::reserveCapacity → AtomStringImpl::reserveCapacityForCurrentThread so a code block grows the atom table once up front. Files touched: CachedTypes.{cpp,h}, HashTable.h, HashSet.h, AtomStringImpl.{cpp,h}.
Prior findings
Two issues were raised on earlier revisions and both are now fixed in the diff: the ordinal-collision bug across addFunctionUpdate payloads is resolved by having encodeFunctionCodeBlock construct its Encoder with NumberStrings::No (so appended payloads emit m_ordinal = noOrdinal and always go through the atom table), and reserveCapacity now calls invalidateIterators(this) before rehash to match the other public mutators. The author reports JSTests/stress (persistent + non-persistent disk cache) is clean.
Security risks
The payload is developer-controlled bytecode cache, not attacker-controlled input in the threat model here. The one payload-driven allocation (m_atomsByOrdinal.grow) is guarded by RELEASE_ASSERT(ordinal < m_cachedBytecode->size()), so a malformed ordinal cannot force an unbounded allocation beyond the payload's own size. The stored m_hash is only used as the probe seed for HashTranslatorCharBuffer; equality still compares characters, so a wrong hash degrades to a miss-then-insert rather than returning a wrong atom. No auth/crypto/permissions surface.
Level of scrutiny
High. This adds two uint32_t fields to the on-disk string record (a format change), threads manual ref()/deref() of AtomStringImpl through a long-lived Decoder, and introduces a new public API on WTF::HashTable that every WebKit container inherits. The logic is compact and I traced the ownership paths without finding a leak or double-free, but this is exactly the kind of change that benefits from a second pair of eyes.
Other factors
The bug-hunting pass on the current revision found nothing. I additionally spot-checked that setAtomForInlineString cannot double-store (the only caller checks atomForInlineString first), that each string record's encode runs once per distinct pointer so ordinals are unique per record, and that reserveCapacityForCurrentThread's size + additionalCount wrapping would only under-reserve, not misbehave. Deferring rather than approving because of the breadth (WTF core + on-disk format), not because of a specific concern.
…toward GC pacing as if generated Since #494 an UnlinkedCodeBlock whose bytecode aliases a persistent cache payload reported none of it to reportExtraMemoryVisited. The collector sizes the heap limit from visited cells + reported extra memory, so a program loaded from the cache looked ~10x smaller than the same program generated from source and got full collections far earlier: on a 30 MB app, 1.4.0 (bytecode or not) finishes startup with Eden/Eden/Full(24 MB)/Eden, this format did Eden/Full/Eden/Full(71 MB) - an extra 42 ms x 8 marker threads, which is the whole startup regression vs 1.4.0. visitChildren now reports what a generated block of the same shape would own; estimatedSize() (heap snapshots) still reports owned.
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I reviewed the latest revision and didn't find any bugs — both earlier findings (the unpersisted string-ordinal high-water mark and the missing invalidateIterators in HashTable::reserveCapacity) are addressed. Given the scope (bytecode cache format, new HashTable/AtomStringImpl public API, GC-pacing accounting), a human look would still be worthwhile.
Checked this pass:
- Ordinal collisions across appended payloads:
encodeFunctionCodeBlocknow passesNumberStrings::No, so appended blocks emitnoOrdinaland fall through to the atom table;stringOrdinalEndis gone. - Ref-counting:
setAtomForOrdinal/setAtomForInlineStringeachref()and~Decoderwalks all three tables toderef();decode()refs before returning the memoized atom;decodePlainStringreturns owningStrings. reserveCapacity: only grows (guarded onnewTableSize > tableSize()), delegates toreserveInitialCapacityon an empty table, and now invalidates iterators beforerehash.- GC pacing:
byteSizeForGCPacing/sizeInBytesForGCintentionally count borrowed cache-backed storage as if generated — matches the added comment invisitChildrenImpl.
Extended reasoning...
Overview
This PR speeds up bytecode-cache decode by ~18-25% via three mechanisms: (1) each cached string record stores its precomputed StringImpl::hash() and a per-payload ordinal so the Decoder memoizes atoms in a Vector<AtomStringImpl*> and skips repeat atom-table lookups; (2) string constants in CachedJSValue decode as plain StringImpls rather than atoms; (3) a new HashTable::reserveCapacity / AtomStringImpl::reserveCapacityForCurrentThread lets a block pre-grow the atom table once. It also flips GC-pacing accounting so cache-borrowed instruction streams / expression info / metadata count as if generated. Ten files across JSC bytecode/, runtime/CachedTypes, and WTF HashTable/HashSet/AtomStringImpl.
Prior feedback resolved
Two earlier findings from this bot are now addressed in the current diff: the m_stringOrdinalEnd persistence bug is fixed by dropping the counter entirely and having encodeFunctionCodeBlock pass Encoder::NumberStrings::No (appended payloads leave m_ordinal = noOrdinal and go through the atom table as before); and HashTable::reserveCapacity now calls invalidateIterators(this) before rehash. Both threads are marked resolved by the author.
Security risks
The decoder reads m_ordinal and m_hash from an on-disk payload. setAtomForOrdinal guards against a hostile ordinal with RELEASE_ASSERT(ordinal < m_cachedBytecode->size()) before growing the vector, and the inline-string tables are indexed by masked byte/short values, so there is no obvious out-of-bounds write. A malformed m_hash would just miss in the atom table's hash-translator lookup and create a fresh atom (the translator's equal compares characters, not hashes). No auth/crypto/permission surfaces are touched.
Level of scrutiny
High. This changes the on-disk cache record layout (+8 bytes per string), adds public API to a foundational WTF container (HashTable::reserveCapacity, exposed on HashSet and the per-thread atom table), alters how UnlinkedCodeBlock::visitChildrenImpl reports extra memory to the GC, and threads new ref-counted state through the Decoder lifetime. These are exactly the areas where subtle mistakes surface as heap corruption or GC-pacing regressions far from the change site.
Other factors
The author reports JSTests/stress (persistent + non-persistent disk cache) is clean, which exercises the load → update → reload path that the earlier bug would have broken. The bug-hunting pass on this revision found nothing new. Still, the combination of cache-format change, WTF container API addition, and GC accounting tweak is well past the "simple/mechanical" bar for auto-approval, so deferring to a human reviewer.
…smaller, page-friendly bytecode (WebKit#494) (#40201) ### What does this PR do? `bun build --compile --bytecode` executables get a smaller, faster bytecode section that JSC reads in place instead of copying. Bun side: - `ZigSourceProvider` marks standalone-executable bytecode persistent so decoded instruction streams and expression info alias the mmapped section instead of heap copies. - One `EncoderStringTable` shared by every chunk and every embedded internal module: each ≥4-char non-symbol string is a 4-byte ordinal in every payload; the characters are written once as a graph section (`Flags::HAS_BYTECODE_STRING_TABLE`). `DecoderStringTable` on `JSVMClientData` reads it with a demand-zero atom slot per ordinal. - `Flags::HAS_SOURCE_HASHES`: each module's `SourceCodeKey` hash is baked in so a bytecode launch never pages in source text just to hash it. - The executable also carries ahead-of-time bytecode for the internal modules (`node:*`, `bun:*`, `internal:*`) the bundle imports and everything they eagerly `require()`; `InternalModuleRegistry` decodes instead of parsing. - Chunk names inside an executable are numbered (`./_12.js`) instead of `chunk-<hash>.js`. WebKit side (oven-sh/WebKit `c148a12dd82b`): - Decoded instruction streams and expression info borrow a persistent payload (oven-sh/WebKit#494). - Bytecode format: 32-bit offsets, natural alignment, sparse metadata offset table, per-record stored hashes, arrays-first records, string dedup, ≤3-char strings inline in their slot (oven-sh/WebKit#497, #499, #502). - Numbered strings with stored hashes so repeat lookups skip re-hashing (oven-sh/WebKit#506); the 2-char atom table lives on `VM`, not per `Decoder`. - `EncoderStringTable` / `DecoderStringTable` and `externalStringTag`; `encodeBuiltinFunction` takes the same table. - Per-region checksums skipped for persistent payloads (they guard on-disk cache files, not the executable's own section). ### Measured 57 npm packages (typescript, three, echarts, mathjs, tfjs, aws-sdk, firebase, antd, mui, prettier, babel, …) bundled with `--minify` to 36 MB, `--compile --bytecode`, importing 38 of them and running every module initializer. Medians; footprint = `Bun.unsafe.memoryFootprint()`. macOS arm64, 1.4.0 vs this PR: | `--bytecode` | build | payload | startup | footprint | |---|---|---|---|---| | **cjs** 1.4.0 | 16.7 s | 295 MB | 224 ms | 223 MB | | **cjs** this PR | 2.4 s | 168 MB | 184 ms | 203 MB | | **esm** 1.4.0 | 17.0 s | 295 MB | 215 ms | 236 MB | | **esm** this PR | 2.4 s | 168 MB | 181 ms | 216 MB | | **esm --splitting** 1.4.0 | 2.7 s | 283 MB | 176 ms | 197 MB | | **esm --splitting** this PR | 2.3 s | 164 MB | 149 ms | 177 MB | Linux x64, 1.4.0 vs this PR: | `--bytecode` | build | payload | startup | footprint | |---|---|---|---|---| | **cjs** 1.4.0 | 157 s | 366 MB | 620 ms | 528 MB | | **cjs** this PR | 7.5 s | 211 MB | 489 ms | 304 MB | | **esm** 1.4.0 | 158 s | 366 MB | 604 ms | 535 MB | | **esm** this PR | 7.7 s | 211 MB | 478 ms | 332 MB | | **esm --splitting** 1.4.0 | 13.5 s | 351 MB | 458 ms | 383 MB | | **esm --splitting** this PR | 6.6 s | 207 MB | 382 ms | 260 MB | macOS `phys_footprint` counts anonymous memory only; the file-backed win is separate — `vmmap` shows the mmapped `__BUN` section resident at 129 MB on main vs 41 MB on this PR (0 dirty in both). ### Where the startup time went samply main-thread profile of the esm+split executable, main vs this PR (absolute samples at 8 kHz): | | main | this PR | |---|---|---| | `unlinkedCodeBlockFor` (decode + fallback parse) | 394 | 208 | | identifier atomization (hash + compare + add) | 103 | 33 | | `CachedVector<CachedFunctionExecutable>` | 55 | 0 | | `Lexer::lex` (parse fallback) | 45 | 0 | | `CachedPtr<CachedInstructionStream>` (copy) | 18 | 0 | | `regionChecksumMatches` | 0 | 0¹ | | `VarintReader::u32` | 0 | 25 | ¹ skipped for persistent payloads as of this PR; before the skip it was 21. ### How did you verify your code works? - `test/bundler/bundler_compile.test.ts`, `test/bundler/bundler_compile_splitting.test.ts` — every `--compile --bytecode` matrix cell. - New: a `--compile --bytecode --splitting` executable with 4,000 functions reports its `Anonymous:` from `smaps_rollup`; the same binary with `BUN_JSC_useBorrowedBytecodeFromCache=0` is the control and must be several MB higher. - WebKit: `JSTests/stress` through the disk cache with the persistent-payload switch on and off, unchanged vs main. --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
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Atomizing identifiers and string constants was ~23% of decoding a code block (hash, probe, compare on every reference; the table regrowing as they trickle in). Now:
StringImpl::hash()and a sequence number assigned in encode order. Decode adds viaAtomStringImpl::add(HashTranslatorCharBuffer{chars, hash})(no rehash) and remembers the atom per number in aVectoron theDecoder; later references are an index. 1- and 2-character strings that live inline in their slot (Bytecode cache: inline 1–3 character strings, alias long strings from the payload; follow-ups from #497 review #499) are memoized in two flat tables keyed by their characters.CachedJSValue) decode as plainStringImpls (aliasing the payload when long) — only identifiers need to be atoms — so half the string references never touch the table.HashTable::reserveCapacity/AtomStringImpl::reserveCapacityForCurrentThread: a block reserves for its identifiers+constants before adding them.CachedBytecode::addFunctionUpdate(jsc shell) continue the base payload's numbering:Encodertakes a first number,CachedBytecoderecords the end,encodeFunctionCodeBlocktakes it.30 MB / 58-library bundle, jsc shell, interleaved runs on pinned cores (box under load, so medians):
JSTests/stress through the disk cache (persistent + non-persistent): unchanged vs main.
For scale: on the same bundle compiled with
bun build --compile --bytecode, samply puts bytecode decode at ~21% of main-thread startup (150 ms of 0.72 s), and the atom table is over a third of that.