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LibJS: Promote Holey arrays to Packed when all holes are filled

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Arrays created via new Array(N) or by setting .length start as Holey
since their elements are not present. After sequential fill (e.g.
for (i=0; i<N; i++) a[i]=v), all holes are filled but the array
remained Holey, preventing the Packed fast paths in the asm
interpreter from triggering.

Now, whenever indexed_put() writes to the last index of a Holey
array, we scan for remaining holes and promote to Packed if none
are found. Only checking on writes to the last index avoids O(N^2)
scanning on partial fills while still catching the common
sequential fill pattern.
This commit is contained in:
Andreas Kling 2026-03-17 10:02:34 -05:00 committed by Andreas Kling
parent 5895cacc21
commit 5f586ae406
1 changed files with 14 additions and 0 deletions
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14
Libraries/LibJS/Runtime/Object.cpp
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@ -1857,6 +1857,20 @@ void Object::indexed_put(u32 index, Value value, PropertyAttributes attributes)
m_indexed_storage_kind = IndexedStorageKind::Holey;
m_indexed_elements[index] = value;
// Promote Holey -> Packed when filling the last hole.
// Only check when writing to the last index to avoid O(N^2) scanning.
if (m_indexed_storage_kind == IndexedStorageKind::Holey && index == m_indexed_array_like_size - 1) {
bool has_holes = false;
for (u32 i = 0; i < m_indexed_array_like_size; ++i) {
if (m_indexed_elements[i].is_special_empty_value()) {
has_holes = true;
break;
}
}
if (!has_holes)
m_indexed_storage_kind = IndexedStorageKind::Packed;
}
}
bool Object::indexed_has(u32 index) const