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refactor(allocator): improve documentation on pointer alignment #12307

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graphite-app merged 1 commit into from
Jul 15, 2025
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refactor(allocator): improve documentation on pointer alignment #12307

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18 changes: 12 additions & 6 deletions crates/oxc_allocator/src/fixed_size.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,12 +66,18 @@ impl FixedSizeAllocator {
};

// Get pointer to use for allocator chunk, aligned to 4 GiB.
// SAFETY: `offset` is either 0 or `TWO_GIB`.
// We allocated 4 GiB of memory, so adding `offset` to `alloc_ptr` is in bounds.
let chunk_ptr = unsafe {
let offset = alloc_ptr.as_ptr() as usize % FOUR_GIB;
alloc_ptr.add(offset)
};
// `alloc_ptr` is aligned on 2 GiB, so `alloc_ptr % FOUR_GIB` is either 0 or `TWO_GIB`.
//
// * If allocation is already aligned on 4 GiB, `offset == 0`.
// Chunk occupies 1st half of the allocation.
// * If allocation is not aligned on 4 GiB, `offset == TWO_GIB`.
// Adding `offset` to `alloc_ptr` brings it up to 4 GiB alignment.
// Chunk occupies 2nd half of the allocation.
//
// Either way, `chunk_ptr` is aligned on 4 GiB.
let offset = alloc_ptr.as_ptr() as usize % FOUR_GIB;
// SAFETY: We allocated 4 GiB of memory, so adding `offset` to `alloc_ptr` is in bounds
let chunk_ptr = unsafe { alloc_ptr.add(offset) };

debug_assert!(chunk_ptr.as_ptr() as usize % BUFFER_ALIGN == 0);

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