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Restrict sequence locks to multi-word flags and shrink mutation counts. - #2322
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Any `absl::Flag` whose value fits in 8 bytes reads and writes its value atomically without consulting a sequence lock. Even so, every `absl::flags_internal::FlagImpl` in writable data previously paid 8 bytes for a sequence lock just to track modification counts for tests and flag savers. Move `SequenceLock` into the multi-word flag value storage specialization so only types larger than 8 bytes allocate it. Track mutations on `FlagImpl` with a 3-bit saturating counter inside an atomic state byte that overflows into a process-wide map after seven updates, shrinking the base object from 72 bytes to 64 bytes and scalar flags from 80 bytes to 72 bytes. PiperOrigin-RevId: 1006216187
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Restrict sequence locks to multi-word flags and shrink mutation counts.
Any
absl::Flagwhose value fits in 8 bytes reads and writes its value atomically without consulting a sequence lock. Even so, everyabsl::flags_internal::FlagImplin writable data previously paid 8 bytes for a sequence lock just to track modification counts for tests and flag savers.Move
SequenceLockinto the multi-word flag value storage specialization so only types larger than 8 bytes allocate it. Track mutations onFlagImplwith a 3-bit saturating counter inside an atomic state byte that overflows into a process-wide map after seven updates, shrinking the base object from 72 bytes to 64 bytes and scalar flags from 80 bytes to 72 bytes.