[JSC] CodeBlock aging: refresh the counter snapshot on every look; drop the per-allocation byte counter - #560
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… optimized code against the last collection that saw allocation #557 added a running total in Heap::didAllocate and a per-CodeBlock MB snapshot to tell whether the mutator had been allocating between full collections. With #558's idle tag that is more machinery than the decision needs: a collection already knows how much was allocated since the previous one (totalBytesAllocatedThisCycle()). - Heap stamps m_lastActiveCollectionTime at the start of any collection that finds more than optimizedCodeAgingQuietAllocationMB allocated since the previous one. Nothing is added to didAllocate(). - CodeBlock::shouldJettisonDueToOldAge for FTL / counter-less DFG code: only an idle-tagged collection may drop it, and only once neither the block's compilation nor an active collection has happened for optimizedCodeAgingQuietSeconds. No per-block state is written; m_leaseStartAllocatedMB is gone (CodeBlock is 4 bytes smaller again). - An embedder's periodic no-op collections while idle (Bun ticks collectAsync() every 30 s) don't see allocation and so don't hold the code alive; a forced or allocation-paced collection still can never drop it. Same behaviour end to end (Bun, express app, idle +120 s: 125 MB anon before and after; idle probe 89 -> 2 CodeBlocks, forced-GC probe 89 -> 84); the stress test additionally checks that an idle collection right after an active one keeps the code.
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Included review availability: 1 review is currently available. Your included PR review attempts over the past 7 days set your current allowance at 2 reviews per hour. WalkthroughChangesThe PR changes optimized code aging from cumulative allocation tracking to mutator quiet-time tracking. The heap records active collection time, CodeBlock checks idle collections, and the stress test verifies retention and later jettisoning. Optimized code aging
Merge Risk: ⚪ Minimal · up to Optimized code aging now uses quiet time after allocation-active collections and idle-tagged collections. No concrete current-head merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 3 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (3 passed)
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In `@JSTests/stress/codeblock-aging-ftl-idle.js`:
- Line 59: After the final idleFullGC() call, assert that the targeted FTL block
has been jettisoned, using the test’s existing block/state inspection mechanism
and matching the expected post-aging result. Keep the assertion immediately
after the quiet-window collection so the test fails when the aging path does not
jettison the block.
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JSTests/stress/codeblock-aging-ftl-idle.jsSource/JavaScriptCore/bytecode/CodeBlock.cppSource/JavaScriptCore/bytecode/CodeBlock.hSource/JavaScriptCore/heap/Heap.cppSource/JavaScriptCore/heap/Heap.hSource/JavaScriptCore/runtime/OptionsList.h
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…tive stamp (a steadily working mutator collected in small cycles still reads as active); guard the test's back-to-back-collections assertion on the measured gap
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LGTM — both points from the earlier pass are addressed: m_bytesAllocatedSinceLastActiveCollection now accumulates across cycles so a steadily-working mutator collected in small increments still stamps as active, and the test's back-to-back-collection assertion is guarded on the measured preciseTime() gap so it can't spuriously fail on a slow runner.
Also checked: default-constructed m_lastActiveCollectionTime is the ApproximateTime epoch, so before any active stamp std::max(m_creationTime, …) correctly falls through to the block's creation time; the preciseTime() guard is conservative in the right direction (activeAt is taken before the stamp, and the check runs after idleFullGC() finishes, so it over-estimates the elapsed time the jettison decision saw); and didAllocate() is back to its base-branch shape with nothing added on the allocation hot path.
Extended reasoning...
Overview
Follow-up commit d265b477e7 responds directly to the two inline comments from the previous run. In Heap::runBeginPhase the per-cycle totalBytesAllocatedThisCycle() is now added into a persistent m_bytesAllocatedSinceLastActiveCollection accumulator that only resets when the threshold is crossed, with a comment calling out the small-cycles case explicitly. In the stress test, activeAt = preciseTime() is captured before the active fullGC(), and the "kept right after an active collection" assertion is skipped when preciseTime() - activeAt >= 0.02, removing the dependence on GC duration. The rest of the diff (removal of m_totalBytesAllocated / m_leaseStartAllocatedMB, shouldJettisonDueToOldAge reading heap.lastActiveCollectionTime(), options help-string rewrites) is unchanged from the first push.
Security risks
None. This is a performance/lifetime heuristic for jettisoning optimized CodeBlocks, entirely inside #if USE(BUN_JSC_ADDITIONS). There is no attacker-controlled input, no auth/crypto/permission surface, and the worst failure mode is either retaining FTL code longer than ideal (memory) or dropping it early (recompilation cost) — not a correctness or safety issue.
Level of scrutiny
Moderate. It touches Heap and CodeBlock, but the change is narrow: one accumulator + timestamp read at the start of each collection, and a read-only comparison during CodeBlock visiting. Nothing is added to didAllocate() (the hot allocation path is restored to base). The default-constructed ApproximateTime is the epoch, so std::max(m_creationTime, lastActiveCollectionTime()) degrades to the existing creation-time behaviour before the first active stamp. The size_t accumulator resets on threshold cross (default 1 MB) so overflow is not a practical concern.
Other factors
The stress test covers the three relevant transitions (non-idle GC keeps, idle-right-after-active keeps, idle-after-quiet drops) and the -off.js / execution-count companions were reported passing. No third-party reviewer left a CHANGES_REQUESTED state; the only other bot review was COMMENTED. The author replied on both of this bot's earlier threads and the follow-up commit matches those replies. Given the small scope, the Bun-specific guard, and both prior concerns cleanly resolved, approving is appropriate.
…erfly slots through libc memset (oven-sh/WebKit#564) (#41407) ### What Bumps WebKit from `fbd894680a6c` to [`2e2aa2290fac`](oven-sh/WebKit@2e2aa22), the oven-sh/WebKit#564 merge commit on `main`. The range contains two changes: **oven-sh/WebKit#564 — concurrent marker could read a torn JSValue on musl builds.** `JSArray::setLength` and `JSArray::shiftCountWithAnyIndexingType` — the Int32/Contiguous paths behind `array.length = n`, `Array.prototype.splice` and `shift` — move elements with `gcSafeMemmove()` and then cleared the vacated butterfly slots with a `WriteBarrier::clear()` loop, which clang compiles to a call to libc `memset()`. The GC's parallel marker scans that same butterfly on a helper thread without a lock, and libc makes no promise about store width: musl's `memset` writes the head and tail of the range with 1-, 2- and 4-byte and unaligned 8-byte stores (glibc happens to use 8-byte-or-wider stores for these sizes). So on `*-musl` builds (Alpine images, musl `--compile` targets) a marker thread could load a half-cleared slot — the low byte, or the low five bytes, of the pointer that was there — treat it as a `JSCell*`, and: - SIGSEGV on a `HeapHelper` thread in `JSObject::visitChildren` / `SlotVisitor::appendHiddenSlow` at `<garbage & ~0x3fff> + 0x20` (e.g. `0x20`, `0x8f6c954020`), or - SIGSEGV at `0xd0` in `SlotVisitor::drain` after the torn pointer was marked and pushed, or - a mark bit / cell-state byte written at a non-cell offset in an unrelated block (later crashes anywhere), or - a hung process with a marker parked in `MarkedBlock::aboutToMarkSlow` → `CountingLock::lockSlow` while the mutator waits in `Heap::runFixpointPhase`. A 25-line script (64 arrays of 3000 objects, `a.splice(N - 6, 3)` + refill in a loop) crashes `oven/bun:1.4.1-alpine` within seconds with default options, 6/6 runs; the glibc image and the same musl image with an `LD_PRELOAD` memset that only does 8-byte stores both run clean. The clears now use `gcSafeZeroMemory()`, and the unlocked in-place ArrayStorage grow path in `JSObject::increaseVectorLength` fences before publishing the larger vector length. **oven-sh/WebKit#560 — CodeBlock aging** drops the per-allocation byte counter added in oven-sh/WebKit#557 and ages idle optimized code against the last collection that saw allocation instead. ### Related reports - Fixes #19895 — musl x64, `HeapHelper` thread in `JSObject::visitChildren` → `visitButterfly` under `SlotVisitor::drain`; the fault address `0x7623000020` is a butterfly pointer with bytes 0–2 and 5–6 zeroed, which is exactly musl `memset`'s intermediate state when clearing a single 8-byte slot. - Fixes #17792 — musl x64-baseline, long-running `next start`, parallel marker `SlotVisitor::drain` → `visitChildren` faulting at `0xd0`, i.e. a torn pointer that was marked and then visited. - Not this bug, deliberately not marked: the glibc reports with marker-thread stacks (#32109, #30205, #30191, #34476, #26678, #28990) — glibc's `memset` does not tear these slots, and their fault addresses (`0x703b1a408020`, `0x800000020`, `0xe00000020`, …) point at a stale block or an `IndexingHeader` read rather than a torn pointer.
Follow-up to #557 / #558: same behaviour, less machinery, nothing on the allocation path.
#557 kept a running
m_totalBytesAllocated(a+=inHeap::didAllocate) and a per-CodeBlock MB snapshot to decide whether the mutator had been allocating between full collections. With #558's idle tag that's more than the decision needs — a collection already knows how much was allocated since the previous one.What
Heapstampsm_lastActiveCollectionTimeat the start of any collection that finds more thanoptimizedCodeAgingQuietAllocationMBallocated since the previous one (totalBytesAllocatedThisCycle(), which already exists).didAllocate()is back to what it was;m_totalBytesAllocated/totalBytesAllocated()are removed.CodeBlock::shouldJettisonDueToOldAgefor FTL / counter-less DFG code becomes: only an idle-tagged collection (GCRequest::isIdle) may drop it, and only oncenow − max(m_creationTime, heap.lastActiveCollectionTime()) ≥ optimizedCodeAgingQuietSeconds. No per-block state is read or written any more;m_leaseStartAllocatedMBis gone (CodeBlock −4 bytes).collectAsync()every 30 s) see no allocation and so don't keep the code alive, whereas any collection that follows real allocation does; forced / allocation-paced / memory-pressure collections still can never drop it (not tagged idle).Based on
6119947(what Bun currently pins); rebases cleanly onto the upstream merge.Verification
JSTests/stress/codeblock-aging-ftl-idle.jsupdated: plainfullGC()s never drop the block; anidleFullGC()right after a collection that saw 4 MB allocated keeps it; anidleFullGC()after a quiet window drops it; allocation between idle collections keeps it.-off.jsandcodeblock-aging-execution-count.jspass.FunctionCodeBlocks, forced-Bun.gc()probe 89 → 84,test/js/bun/gc/gc-controller-cadence.test.ts10/10; express app after a 5,000-request burst, idle +120 s: 125 MB anon with this vs 125 MB with the merged [JSC] CodeBlock aging: let idle FTL code (and counter-less DFG code) age out once the mutator goes quiet #557/[JSC] CodeBlock aging: only an embedder-tagged idle collection may let counter-less optimized code go #558 build, same burst CPU.Update (8aa8f4e): the bigger win turned out to be a snapshot-ordering bug in the counter path.
Instrumenting Claude Code at idle showed ~1,000 DFG blocks (+ their ~1,000 Baseline alternatives) surviving both of Bun's idle collections with their tier-up counters unchanged. Reason:
shouldJettisonDueToOldAge()returned early ontimeSinceCreation() < ttlbefore recording the counter, so idle GC #1 (~10 s after the last turn, blocks still "too young") kept a pre-turn snapshot, and idle GC #2 (~75 s) sawcurrent != previous— the turn's execution — and renewed every block for another lease that no third collection ever expired. Diagnostic counters over the two idle GCs: beforetooYoung=1250 → counterMoved=1088, agedOut=0; aftertooYoung=257 → agedOut=1159.Fix: check the counter and refresh the snapshot on every look, then apply the TTL. Also route an optimizing block that aged out this cycle through
JettisonDueToOldAgeinstead ofJettisonDueToWeakReference(an unmarked optimizing block always matched the latter).Claude Code (compiled CLI, 20-turn website fixture, then idle), after the second idle collection:
codeblock-aging-execution-count.js(actively running code is never jettisoned across 30 GCs) still passes.