ARC: retain and release with fetch_add instead of a CAS loop#402
Draft
DTW-Thalion wants to merge 2 commits into
Draft
ARC: retain and release with fetch_add instead of a CAS loop#402DTW-Thalion wants to merge 2 commits into
DTW-Thalion wants to merge 2 commits into
Conversation
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.
… 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.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
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.