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CBL-8494: Use bit 1 instead of the high bit for AtomicWrapper busy flag #308

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Merged
pasin merged 1 commit into from
Jun 21, 2026
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CBL-8494: Use bit 1 instead of the high bit for AtomicWrapper busy flag #308

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7 changes: 5 additions & 2 deletions Fleece/Support/RefCounted.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -135,8 +135,11 @@ namespace fleece {

namespace internal {
/// Tag bit that's added to `_ref` while accessing it.
/// We can't use the low bit (1) because mutable Fleece Values already use that as a tag.
static constexpr uintptr_t kBusyMask = uintptr_t(1) << (8 * sizeof(uintptr_t) - 1);
/// We use bit 1 (value 2): RefCounted has an `atomic<int32_t>` member, so it's always
/// >= 4-aligned, guaranteeing bits 0 and 1 are zero in any stored pointer on every ABI.
/// Bit 0 (value 1) is reserved because mutable Fleece Values already use it as a tag.
/// (The previous high-bit choice collided with Android's arm64 tagged pointers; see CBL-8494.)
static constexpr uintptr_t kBusyMask = 0b10;

AtomicWrapper::AtomicWrapper(uintptr_t ref) noexcept
:_ref(ref)
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