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https://github.com/oven-sh/bun
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## Summary
Add a fast path for `structuredClone` and `postMessage` when the root
value is a dense array of primitives or strings. This bypasses the full
`CloneSerializer`/`CloneDeserializer` machinery by keeping data in
native C++ structures instead of serializing to a byte stream.
**Important:** This optimization only applies when the root value passed
to `structuredClone()` / `postMessage()` is an array. Nested arrays
within objects still go through the normal serialization path.
## Implementation
Three tiers of array fast paths, checked in order:
| Tier | Indexing Type | Strategy | Applies When |
|------|--------------|----------|--------------|
| **Tier 1** | `ArrayWithInt32` | `memcpy` butterfly data | Dense int32
array, no holes, no named properties |
| **Tier 2** | `ArrayWithDouble` | `memcpy` butterfly data | Dense
double array, no holes, no named properties |
| **Tier 3** | `ArrayWithContiguous` | Copy elements into
`FixedVector<variant<JSValue, String>>` | Dense array of
primitives/strings, no holes, no named properties |
All tiers fall through to the normal serialization path when:
- The array has holes that must forward to the prototype
- The array has named properties (e.g., `arr.foo = "bar"`) — checked via
`structure->maxOffset() != invalidOffset`
- Elements contain non-primitive, non-string values (objects, arrays,
etc.)
- The context requires wire-format serialization (storage, cross-process
transfer)
### Deserialization
- **Tier 1/2:** Allocate a new `Butterfly` via `vm.auxiliarySpace()`,
`memcpy` data back, create array with `JSArray::createWithButterfly()`.
Falls back to normal deserialization if `isHavingABadTime` (forced
ArrayStorage mode).
- **Tier 3:** Pre-convert elements to `JSValue` (including `jsString()`
allocation), then use `JSArray::tryCreateUninitializedRestricted()` +
`initializeIndex()`.
## Benchmarks
Apple M4 Max, comparing system Bun 1.3.8 vs this branch (release build):
| Benchmark | Before | After | Speedup |
|-----------|--------|-------|---------|
| `structuredClone([10 numbers])` | 308.71 ns | 40.38 ns | **7.6x** |
| `structuredClone([100 numbers])` | 1.62 µs | 86.87 ns | **18.7x** |
| `structuredClone([1000 numbers])` | 13.79 µs | 544.56 ns | **25.3x** |
| `structuredClone([10 strings])` | 642.38 ns | 307.38 ns | **2.1x** |
| `structuredClone([100 strings])` | 5.67 µs | 2.57 µs | **2.2x** |
| `structuredClone([10 mixed])` | 446.32 ns | 198.35 ns | **2.3x** |
| `structuredClone(nested array)` | 1.84 µs | 1.79 µs | 1.0x (not
eligible) |
| `structuredClone({a: 123})` | 95.98 ns | 100.07 ns | 1.0x (no
regression) |
Int32 arrays see the largest gains (up to 25x) since they use a direct
`memcpy` of butterfly memory. String/mixed arrays see ~2x improvement.
No performance regression on non-eligible inputs.
## Bug Fix
Also fixes a correctness bug where arrays with named properties (e.g.,
`arr.foo = "bar"`) would lose those properties when going through the
array fast path. Added a `structure->maxOffset() != invalidOffset` guard
to fall back to normal serialization for such arrays.
Fixed a minor double-counting issue in `computeMemoryCost` where
`JSValue` elements in `SimpleArray` were counted both by `byteSize()`
and individually.
## Test Plan
38 tests in `test/js/web/structured-clone-fastpath.test.ts` covering:
- Basic array types: empty, numbers, strings, mixed primitives, special
numbers (`-0`, `NaN`, `Infinity`)
- Large arrays (10,000 elements)
- Tier 2: double arrays, Int32→Double transition
- Deep clone independence verification
- Named properties on Int32, Double, and Contiguous arrays
- `postMessage` via `MessageChannel` for Int32, Double, and mixed arrays
- Edge cases: frozen/sealed arrays, deleted elements (holes), `length`
extension, single-element arrays
- Prototype modification (custom prototype, indexed prototype properties
with holes)
- `Array` subclass identity loss (per spec)
- `undefined`-only and `null`-only arrays
- Multiple independent clones from the same source
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>