Bytecode cache: smaller payloads that page in only what is decoded (alias mapped bytes, region layout, 32-bit offsets, sparse metadata table) - #494
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…persistent payload instead of copying it When an embedder marks a CachedBytecode payload persistent (CachePayload::setIsPersistent: the bytes outlive every use, e.g. a section of the running executable), the instruction streams and expression info decoded from it point into the payload instead of being copied out (Options::useBorrowedBytecodeFromCache, default on under USE(BUN_JSC_ADDITIONS)). InstructionStream refs read through a span the stream owns; the writer refreshes it after every mutation of its buffer, so owned and borrowed streams share one branch-free access path. UnlinkedCodeBlock reports only the bytes it owns to the GC. The jsc shell can mark its disk-cache files persistent (diskCachePayloadIsPersistentForTesting) so the stress suite's bytecode-cache mode exercises the borrowed path.
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WalkthroughThe pull request adds borrowed payload support, compact and checksummed bytecode-cache records, persistent disk-cache handling, metadata storage compaction, and span-backed instruction-stream references. ChangesBytecode cache pipeline
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I reviewed this PR and didn't find any bugs. Because it refactors how InstructionStream::Ref reads bytes (pointer-to-span with a didMutateBuffer() invariant on every writer mutation) and changes lifetime/ownership semantics for bytecode data on the hot path across all execution tiers, a human look would still be worthwhile.
Checked: didMutateBuffer() coverage across writer mutation sites (reserve/rewind/swap/finalize/setInstructionBuffer/BytecodeRewriter::applyModification); CachePayload defaulted move ctor preserves m_isPersistent; ExpressionInfo::payload() reroutes chapters()/encodedInfo() correctly and byteSize() doesn't over-report borrowed payloads to the GC; CachedVector::borrow() / CachedArray::borrow() return the same encoded bytes the copy path would decode.
Extended reasoning...
Overview
This PR enables zero-copy aliasing of bytecode-cache payloads that the embedder marks persistent. It touches core JSC bytecode infrastructure across 11 files:
InstructionStream.h:BaseRef/Ref/MutableRefnow read through aconst std::span<const uint8_t>*owned by the stream instead of aVector&. Adds aBorrowconstructor,m_isBorrowed, explicit move ctor/assignment, anddidMutateBuffer()which every writer mutation must call to refresh the span.ExpressionInfo.{h,cpp}: addsm_borrowedPayloadandcreateBorrowed();payload()returns the borrowed pointer when set;byteSize()returns header-only size for borrowed instances.CachedTypes.{h,cpp}:Decoder::canBorrowPayload();CachedInstructionStream::decodeandCachedExpressionInfo::decodeconstruct borrowed variants;CachedVector::borrow()/CachedArray::borrow()return raw views of identity-encoded elements.CachePayload.h/CachedBytecode.h:m_isPersistentflag with setter/getter.UnlinkedCodeBlock.cpp: reportsownedSizeInBytes()to the GC instead ofsizeInBytes().BytecodeRewriter.cpp,jsc.cpp,OptionsList.h:didMutateBuffer()after rewrite; test-only shell option; two new JSC options.
Security risks
No auth/crypto/permissions surface. The main risk is memory safety: borrowed InstructionStream/ExpressionInfo hold raw pointers into an external mapping, and Ref holds a pointer into the stream's m_bytes member. Correctness depends on (a) the embedder's persistence promise being true, (b) every writer buffer mutation calling didMutateBuffer(), and (c) no Ref outliving its stream (the new move ctor could invalidate outstanding refs, though the pre-existing Vector& design had the same constraint). These are gated behind an explicit opt-in (setIsPersistent()), and the borrowed path is inert unless USE(BUN_JSC_ADDITIONS) and the embedder sets the flag.
Level of scrutiny
High. InstructionStream is on the hottest path in JSC — every bytecode read in LLInt/Baseline/DFG/FTL bytecode parsing goes through it. The Ref indirection change is unconditional (not behind a flag), so it affects the non-borrowed path too. The didMutateBuffer() invariant is new and must hold at every mutation site now and in future changes; a miss produces a stale span and use-after-free-style reads. This is a well-designed and well-tested change (5,259 stress tests both with and without the borrowed path), but the scope and subtlety of the invariants warrant a human reviewer familiar with BytecodeGenerator/BytecodeRewriter confirming coverage.
Other factors
CachePayload(CachePayload&&) = defaultcorrectly copiesm_isPersistent(bool member).RELEASE_ASSERT(!stream.isBorrowed())inCachedInstructionStream::encodeguards against re-encoding a borrowed stream whosem_instructionsvector is empty.RELEASE_ASSERT(!m_isBorrowed)indidMutateBuffer()catches any attempt to mutate a borrowed stream.- No prior human or bot review comments to address; coderabbitai was rate-limited.
Preview Builds
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…breadth-first, expression info last The encoder recursed into a nested function's body as soon as it wrote that function's executable record, so the N records a code block reads when it is decoded were interleaved with N whole subtrees. Decoding one block of a mapped payload touched pages across most of the file (measured: decoding only the top-level block of a 75 MB cache made 33 MB of it resident). The encoder now writes a block's own arrays and all of its children's executable records back to back, queues the children's bodies and emits them breadth-first, and emits every block's ExpressionInfo (read only when a stack trace is built) after all bodies. Offsets in the format are relative, so the decoder is unchanged and decoded objects are identical. Same 75 MB cache: top-level decode 32.7 MB resident / 143 ms cold -> 1.9 MB / 10 ms; one level deeper 50.4 MB / 280 ms -> 17.8 MB / 117 ms; full decode 785 ms -> 465 ms.
…tadata offset table - VariableLengthObjectBase::m_offset is int32_t: every CachedPtr is 4 bytes and every CachedVector 8 (a payload is one code block tree, nowhere near 2 GB; safeCast asserts). - Encoder::malloc aligns each allocation to alignof(T) instead of rounding both offset and size up to 16. - CachedMetadataTable stored the 51-entry cumulative offset table as unsigned[51] in every code block (212 bytes, present even when the block has no metadata). It now stores only the entries where the running offset changes, as (index << 24 | delta); a typical function has a handful. Same 12 MB bundle: 75.2 MB -> 47.8 MB cache; full decode 465 ms -> 352 ms.
Jarred-Sumner
changed the title
…characters, no unused offset fields - CachedFunctionExecutable and CachedCodeBlock keep only pointers/vectors (and, for the executable, the fields CachedBytecode::commitUpdates patches) in the fixed record; every count, register, position and flag is written as a LEB128 tail right after the record (positions as deltas from the function start) and read once into the object being constructed. - CachedUniquedStringImpl is a 4-byte header followed directly by the characters, instead of a header, an offset, and a separately aligned allocation. - CachedString, CachedIdentifier, CachedHashMap and CachedInlineMap derived from VariableLengthObject without ever allocating through it, so each carried a dead offset field (CachedIdentifier was 12 bytes for a 4-byte pointer). Same 12 MB bundle: 47.8 MB -> 40.5 MB.
Instruction streams, expression info, jump tables and metadata step lists whose encoding is a plain copy of their bytes are looked up by content in the encoder; a repeat points at the first copy. Decoded objects are still one per code block (or, with a persistent payload, alias the same immutable bytes). Same 12 MB bundle: 40.5 MB -> 37.9 MB.
…is linked finalize() (and the bytecode-cache decoder) allocated the full value-profile + LinkingData + metadata buffer up front and the UnlinkedCodeBlock kept it for its whole life, including after every CodeBlock linked from it had been destroyed, and for cache-decoded functions that were never linked at all. The unlinked table now holds just the offset table; link() allocates the full buffer (zeroed, as before) and unlink() of the owning MetadataTable shrinks back to the offset table. Decoding every function of a 75 MB bytecode cache (48k UnlinkedCodeBlocks, none linked): peak RSS 217 MB -> 175 MB.
…ion info; regenerate from source on mismatch Each code block's own bytes (record, varint tail, its arrays, its children's record slots), each function executable record, and each ExpressionInfo carry a CRC-32C computed by the encoder once every deferred slot is final. The decoder checks a block's checksum and its children's records before constructing anything (hardware crc32c on x86-64/ARMv8, table elsewhere; Options::verifyBytecodeCacheChecksums). A damaged block decodes as absent, which every caller already handles by generating from source; damaged expression info decodes as empty. Pointers that come from slots CachedBytecode::commitUpdates rewrites are bounds-checked against the payload, and commitUpdates re-seals the records it patches. addFunctionUpdate ignores executables that were not recorded as leaves instead of dereferencing end(). Encoder pages are zero-initialized so alignment gaps are deterministic in the file. Cost on the 12 MB corpus: +0.96 MB (2.5%); decode time and resident pages unchanged within noise. 244 single-byte corruptions swept across a cache file: no crashes, correct results.
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Additional findings (outside current diff — PR may have been updated during review):
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Source/JavaScriptCore/runtime/CachedTypes.cpp:580-592—allocateOrShareBytesignores itsalignmentargument on the share-existing path: an align-4 array (e.g.m_steps,m_jumpTargets,CachedExpressionInfo::m_storage) can be pointed at storage that was allocated for a byte-identical align-1 array (instruction stream,SourceCodeRepresentationvector) at an odd offset now that the encoder uses natural alignment. On decodebuffer<T>()firesASSERT(!(uintptr_t % alignof(T)))in debug and is a misaligned load under UBSan. Skip sharing when*existing % alignment != 0(or key/store alignment inm_arraysByHash).Extended reasoning...
What the bug is.
VariableLengthObject::allocateOrShareBytesreceives analignmentparameter but uses it only on the fresh-allocation branch. On the share-existing branch it does:if (auto existing = encoder.existingIdenticalArray(bytes, hash)) { m_offset = safeCast<Offset>(*existing - encoder.offsetOf(&m_offset)); return; }
existingIdenticalArraylooks up inm_arraysByHash, which is a single pool keyed only by content hash and compared only bysize+ byte equality (CachedTypes.cpp:228-237).registerArraystores only(offset, size). Alignment is neither stored nor checked before reusing.How the misalignment arises.
allocateOrShareBytesis called from bothCachedVector<T>::encodeandCachedArray<T>::encodefor every trivially-copyableT == Sourceinstantiation. The pool is therefore shared across:- align-1 element types:
CachedVector<uint8_t, …>inCachedInstructionStream, andCachedVector<SourceCodeRepresentation>(SourceCodeRepresentationisenum : uint8_t); - align-4 element types:
CachedVector<uint32_t, 16> m_stepsinCachedMetadataTable,CachedVector<JSInstructionStream::Offset> m_jumpTargets,CachedArray<unsigned> m_storageinCachedExpressionInfo.
Before this PR every allocation was rounded up so everything was 16-byte aligned. Now
Page::mallocusesroundUpToMultipleOf(alignment, m_offset)with the caller'salignof(T), and inline string tails / varint tails leavem_offsetat arbitrary byte positions. So an align-1 array can genuinely land at an odd byte offset. If a later align-4 array happens to be byte-identical (same length, same bytes),allocateOrShareBytespoints it at the earlier odd-offset storage.Why nothing catches it. There is no defensive check anywhere on the sharing path:
existingIdenticalArrayis content-only, andallocateOrShareBytesreturns immediately after settingm_offset. On decode,CachedVector<T>::decode()/borrow()andCachedArray<T>::borrow()callthis->template buffer<T>(), which doesASSERT(!(std::bit_cast<uintptr_t>(buffer()) % alignof(T)))and thenbit_cast<const T*>and reads T-typed elements.Step-by-step example.
- Function A's
m_constantsSourceCodeRepresentationis a 4-element vector[Integer, Integer, Integer, Integer]= bytes{01,01,01,01}.CachedVector<SourceCodeRepresentation>::encodecallsallocateOrShareBytes(encoder, {ptr,4}, /*alignof*/1). The current page'sm_offsetis odd (e.g. right after an odd-length inline string tail), soencoder.malloc(4, 1)places the 4 bytes at, say, offset0x1235.registerArray(hash, 0x1235, 4)records it. - Function B's
m_jumpTargetsis a single-entryVector<unsigned>containing0x01010101.CachedVector<unsigned>::encodecallsallocateOrShareBytes(encoder, {ptr,4}, /*alignof*/4). The bytes hash-match and byte-compare equal to A's array, soexistingIdenticalArrayreturns0x1235andm_offsetis set to point there — with no check that0x1235 % 4 == 0. - On decode,
CachedVector<unsigned>::decodecallsbuffer<unsigned>(), which asserts!(0x…1235 % 4)→ fires in debug; in release/UBSan it's a misalignedunsignedload. In the borrowed-payload path,CachedExpressionInfo::decodehandsm_storage.borrow()out asExpressionInfo::m_borrowedPayload, from whichChapter*/EncodedInfo*are derived, so the misaligned pointer escapes into longer-lived structures.
Impact. Cross-type byte collisions are rare (need exact length + content match across semantically different data), so this is unlikely to bite in practice, and on x86-64/ARM64 release builds the misaligned load produces the correct value. But the dedup runs over every array of every function of a 48k-function bundle, so the birthday-style probability isn't zero over time; when it hits, debug builds assert and
-fsanitize=alignmentflags it. It also violates an invariant the code explicitly asserts.Fix. One-liner: before reusing, check that the stored offset satisfies the requested alignment, e.g.
if (auto existing = encoder.existingIdenticalArray(bytes, hash); existing && !(*existing % alignment)) { m_offset = safeCast<Offset>(*existing - encoder.offsetOf(&m_offset)); return; }
(or store the allocation's alignment alongside
(offset, size)inm_arraysByHashand requirestoredAlign >= requestedAlign). - align-1 element types:
…hecksum field; assert re-sealed record bounds
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Re the dedup alignment finding: fixed in 33c3372 — |
…eeds compiler-rt's __cpu_model, absent in the macOS cross-link)
… checksum, cover an executable record's own rare data / name / TDZ allocations, and seal the cache entry header - Content-sharing of arrays only happens for the four block-level arrays the decoder knows how to follow (metadata steps, instruction stream, source-code representations, jump targets) plus the self-checked ExpressionInfo storage; when one of them points outside the block's own bytes, both encoder and decoder continue the block's CRC over it, so a shared array is verified by whoever reads it, not only by its first writer. - A function executable's checksum extent runs to the end of what its encode() allocated (rare data, name string, TDZ environment when this record wrote them), not just record + tail. - The cache entry header (version, boot session string, source code key) is checksummed and a mismatch is a cache miss. - CachedBytecode::commitUpdates reads the record's stored extent when re-sealing it. Every single-byte corruption of a test cache (all 1,808 offsets) now runs correctly.
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In `@Source/JavaScriptCore/bytecode/UnlinkedMetadataTable.cpp`:
- Around line 159-163: Update the 16-bit offset-table allocation in
UnlinkedMetadataTable’s link path to use MetadataTableMalloc::zeroedMalloc
instead of malloc, preserving the existing size and copy loop so trailing
padding bytes are initialized.
In `@Source/JavaScriptCore/bytecode/UnlinkedMetadataTable.h`:
- Around line 154-167: Synchronize MetadataTable state transitions in link(),
unlink(), and setDidOptimize() with the same protocol used by
sizeInBytesForGC(MetadataTable&) and CodeBlock::visitChildren(), protecting the
packed m_isLinked bitfield and m_rawBuffer consistently. Alternatively, enforce
a lifecycle guarantee that prevents these operations from overlapping, while
preserving correct GC accounting and buffer access.
In `@Source/JavaScriptCore/runtime/CachedBytecode.cpp`:
- Around line 70-99: Update findIn inside recordBytes to require decoded extent
to be at least CachedFunctionExecutableOffsets::fixedSize() as well as no
greater than record.size(), rejecting records that are too short before
returning record.first(extent). Preserve the existing RELEASE_ASSERT-based
validation and patch flow.
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📒 Files selected for processing (15)
Source/JavaScriptCore/bytecode/BytecodeRewriter.cppSource/JavaScriptCore/bytecode/ExpressionInfo.cppSource/JavaScriptCore/bytecode/ExpressionInfo.hSource/JavaScriptCore/bytecode/InstructionStream.hSource/JavaScriptCore/bytecode/UnlinkedCodeBlock.cppSource/JavaScriptCore/bytecode/UnlinkedMetadataTable.cppSource/JavaScriptCore/bytecode/UnlinkedMetadataTable.hSource/JavaScriptCore/bytecode/UnlinkedMetadataTableInlines.hSource/JavaScriptCore/jsc.cppSource/JavaScriptCore/runtime/CachePayload.hSource/JavaScriptCore/runtime/CachedBytecode.cppSource/JavaScriptCore/runtime/CachedBytecode.hSource/JavaScriptCore/runtime/CachedTypes.cppSource/JavaScriptCore/runtime/CachedTypes.hSource/JavaScriptCore/runtime/OptionsList.h
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…tent below as well
…ed offsets) before decoding, so a rejected block falls back to source instead of leaving the offsets behind as a cell pointer
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I reviewed the latest revision (738c4bc) and the bug hunting system found no new issues — the earlier findings on shared-array checksum coverage, executable-record extent, the 16-bit offset-table padding, the extent lower bound, and the construct-slot union clearing all look addressed. Given the scope (a new on-disk cache format, borrowed-pointer lifetimes into mmap'd payloads, UnlinkedMetadataTable link/unlink buffer lifecycle, and 32-bit relative offsets throughout CachedTypes), a human pass is still worthwhile.
Checked in this run:
decodeCachedCodeBlocksnow clears the construct slot before decode, so a rejected block falls back to source cleanly.regionIsIntactfolds externally-shared arrays into the CRC in encoder order, matchingencodeDeferred;ShareableArrayScopelimits sharing to the four arrays the decoder follows.InstructionStreammove/assign re-derivem_bytesfrom the moved-into vector rather than the source span; every writer mutation path callsdidMutateBuffer().VarintReader::u32bounds the shift, andCachedMetadataTable::expandbounds indices before writing the offset table.
Extended reasoning...
Overview
This PR reworks the JavaScriptCore bytecode cache format across ~16 files, with the bulk in CachedTypes.cpp (~1,500 lines changed). It introduces: (1) borrowed InstructionStream/ExpressionInfo payloads that alias a persistent mmap instead of copying, (2) a breadth-first region layout so decoding one block touches contiguous pages, (3) 32-bit relative offsets and alignof(T)-driven allocation replacing 16-byte rounding, (4) LEB128 varint tails for per-function scalar fields, (5) content-addressed array deduplication, (6) CRC-32C region/record checksums with hardware paths, (7) a sparse CachedMetadataTable encoding, and (8) an UnlinkedMetadataTable lifecycle change that defers the value-profile/metadata buffer to link() and releases it at unlink(). It also refactors InstructionStream::Ref to read through an owned std::span and adds move semantics and didMutateBuffer() hooks.
Security risks
The cache is deserialized from disk / an executable section, so this is inherently a parser of semi-trusted binary input. The PR adds defense (per-region CRC-32C, payloadContains bounds checks, RELEASE_ASSERTs on the sparse-table indices, varint shift bound, extent lower/upper bounds in commitUpdates), which is strictly better than main where none of this existed. Remaining risk surface: 32-bit relative offsets (safeCast guards encode-side, payloadContains guards decode-side of the followed slots, but not every pointer target is self-checksummed — a deliberate trade-off the author documented); borrowed payloads rely entirely on the embedder's setIsPersistent() promise (correct by design, but a lifetime contract that spans process boundaries). No injection/auth/crypto concerns.
Level of scrutiny
High. This is core VM infrastructure: the bytecode the interpreter/JIT executes may now be a raw pointer into an mmap'd file, UnlinkedMetadataTable::link()/unlink() reallocate a buffer that GC marking threads compare pointers against, and the on-disk format is being redefined. Mistakes here manifest as memory corruption or wrong-code execution, not exceptions. The change is large, touches multiple interacting subsystems, and encodes several design trade-offs (checksum coverage vs. size, 32-bit offset ceiling, which arrays participate in dedup) that deserve a human maintainer's sign-off.
Other factors
Several rounds of review feedback have already been folded in (commits 33c3372, 61c5f15, 43e3144, 738c4bc), and the author reports JSTests/stress parity through the disk cache plus a byte-by-byte corruption sweep. All prior inline threads are resolved. No outstanding reviewer comments remain. That said, the size and criticality put this well outside the auto-approval bar.
…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.
…s skip the atom table (#506) * Bytecode cache: number the strings so a decoder looks each up in the 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). * String constants decode as plain StringImpls (aliasing the payload when long) instead of going through the atom table; only identifiers need to be atoms 60k-function decode: 991M -> 874M cycles; decode-everything 1846M -> 1781M. * Payloads appended by addFunctionUpdate leave their strings unnumbered 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. * HashTable::reserveCapacity: invalidate iterators before rehashing, like add/remove * Count cache-backed instruction streams, expression info and metadata 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.
…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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The bytecode-cache parts of the startup-snapshot branch (#397) that help every
bun build --compile --bytecodeexecutable, plus the format work that came out of measuring them, plus theUnlinkedMetadataTablechange (was #496). Supersedes #187; #495/#496 folded in here. Bun side: oven-sh/bun#40201.Test corpus for all numbers: a 12 MB minified bundle (typescript, babel, prettier, zod, …), 48k functions, encoded as one module the way Bun's
--bytecodedoes; jsc shell, x64 Linux; "resident" =mincoreon the cache file mapped cold.--compile --bytecodebinary after running 4,000 functionsCommits
InstructionStream/ExpressionInfoalias a persistent payload instead of copying it.CachePayload::setIsPersistent()is an explicit embedder promise (Bun: bytecode embedded in the running executable's section).InstructionStream::Refreads through a span the stream owns (writer refreshes it after every mutation) so owned/borrowed share one branch-free path;UnlinkedCodeBlockreports only owned bytes to the GC.useBorrowedBytecodeFromCache(default on); shell testing switchdiskCachePayloadIsPersistentForTesting.ExpressionInfogoes to a cold tail. Encoder-only (offsets are relative). This is the 32.7 MB → 1.9 MB row by itself.alignof(T)alignment, sparse metadata offset table.CachedPtr8→4 B,CachedVector16→8 B; no more rounding every allocation to 16;CachedMetadataTablestores only the entries where the cumulative offset changes instead ofunsigned[51](212 B) in every code block. 75.2 → 47.8 MB.CachedCodeBlockandCachedFunctionExecutableare a LEB128 tail after the fixed record (positions as deltas from the function start), read once into the constructed object; the fieldsCachedBytecode::commitUpdatespatches stay fixed.CachedUniquedStringImplis a 4-byte header followed by the characters.CachedString/CachedIdentifier/CachedHashMap/CachedInlineMapno longer carry an unusedVariableLengthObjectoffset (CachedIdentifierwas 12 bytes for a 4-byte pointer). 47.8 → 40.5 MB.crc32cbefore a block is decoded (only over bytes that decode reads anyway). A damaged block decodes as absent → generated from source; damaged ExpressionInfo → empty. Pointers from slots the shell'scommitUpdatesrewrites are bounds-checked, andcommitUpdatesre-seals what it patches. Cost: +0.96 MB (2.5%), decode time and residency unchanged within noise; 244 single-byte corruptions swept across a cache file with no crashes.verifyBytecodeCacheChecksums(default on).UnlinkedMetadataTableholds only the offset table while noCodeBlockis linked; the value-profile/metadata buffer is allocated atlink()and released when the owning table is unlinked (was: allocated at finalize/decode and kept for theUnlinkedCodeBlock's life).Testing
JSTests/stressthrough the disk cache (write run +JSC_forceDiskCacheread run, all 5,259 tests without special mode requirements), with and without the persistent-payload switch, after each commit: identical pass/fail tomain(thev8-*finalizationregistry*and a couple of stack-depth regexp tests are flaky in cache mode onmaintoo). Bun built against the preview:bun-build-compile,bundler_compile -t bytecode,bundler_compile_splittingtest files pass, plus the new RSS test in oven-sh/bun#40201.