ARC: shard the weak-reference table#401
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Per issue gnustep#398. The reference-count fast paths in arc.mm seeded their CAS loops with __sync_fetch_and_add(refCount, 0), a full read-modify-write used only to read the count word, and updated it with sequentially consistent __sync_val_compare_and_swap. The seed only needs a plain load, and the exchanges do not need seq_cst. View the count word through std::atomic<uintptr_t> and operate on it directly. A relaxed load seeds each loop; compare_exchange_weak is acquire-release on the retain and weak-flag paths and release on the decrement; the final release takes an acquire fence before running -dealloc. No functional or ABI change. Isolating just the atomic pattern with no Objective-C involved, on a 32-core x86-64 host with clang 18.1.3: __sync seed + CAS, seq_cst 27.6 ns relaxed load seed + acq/rel CAS 14.4 ns and a retain/release pair on a real object drops from about 31 ns to about 16 ns. All 194 tests pass, along with an 8-thread retain/release balance stress where the count returns to its exact starting value and an 8-thread weak load/store/dealloc race stress.
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This looks more invasive than it should be. The original refactoring to make the type used for a weak ref table distinct was intended to make it easy to provide a replacement that did the sharding internally. |
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Stacked on #399 and depends on it, so the diff currently includes #399's atomic-refcount commit; that drops out once #399 merges.
The weak-reference table used a single global lock, so weak operations on unrelated objects serialised on one mutex and scaled negatively with core count. This splits the table into address-keyed stripes, each with its own lock, so weak traffic on different objects proceeds in parallel.
Follow-up commits make the stripe count configurable (a build flag or the OBJC_WEAK_SHARD_COUNT environment variable), close a lock-selection race the sharding introduced (a use-after-free found under AddressSanitizer, reproducible with more than one shard), and make weak-slot access atomic so the sharding is also sound on weakly-ordered targets (checked under ThreadSanitizer).
Opening as a draft while #399 is in review.