ARC: retain and release with fetch_add instead of a CAS loop - #402
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ARC: retain and release with fetch_add instead of a CAS loop#402DTW-Thalion wants to merge 2 commits into
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I think this approach is good, but these paths are starting to get quite cluttered. I’d like to have an explicit refcount class that has a from-object static factory method so that the refcount manipulation can be pulled out. That would also let us do the Apple-style refcount-stored-in-the-isa-pointer-with-overflow-in-a-table model. |
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… CAS loop The strong-retain and release fast paths spun a compare-exchange loop that re-tried on every lost race, so under contention they wasted work that a single read-modify-write instruction avoids. A strong retain runs while the caller still owns a reference, so the object cannot be at (or reach) the deallocating sentinel; its increment is therefore a single fetch_add. Release becomes a single fetch_sub, handling the last-reference and saturation edges after the fact. The weak-to-strong retain keeps the compare-exchange loop, because it can race a concurrent final release and has to check-and-increment atomically to avoid resurrecting a dying object. Reserve the bit below the weak flag as a guard, so an optimistic increment can never carry a saturating count into the weak flag. FastRefCount.m mirrors the reference-count layout and is updated for the guard bit; the saturation and weak-at-saturation cases it exercises still pass. Measured on a 32-core machine: retain/release falls from 16.1 to 11.3 ns with no contention, and a single shared object under 24 threads from 2143 to 1124 ns.
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July 29, 2026 22:50
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Stacked on #399 and depends on it being merged.
The strong-retain and release fast paths ran a compare-exchange loop that retried on every lost race. A strong retain holds a live reference, so the object cannot be at the deallocating sentinel and its increment can be a single fetch_add. Release becomes a single fetch_sub, with the last-reference and saturation cases handled after the fact. The weak-to-strong retain keeps its compare-exchange loop, since it can race a concurrent final release and has to check and increment atomically.
Uncontended retain/release drops from 16.1 to 11.3 ns on a (24T active) 32-core machine, and a single shared object under 24 threads from 2143 to 1124 ns.
Opening as a draft while #399 is in review.