[JSC] A StringObject with immutable properties keeps the compilers' fast conversion to a string - #765
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…ast conversion to a string The DFG and the FTL turn a StringObject into its string without a call (ToString with StringObjectUse or StringOrStringObjectUse) behind a CheckStructure for the realm's stringObjectStructure(), which says that the object has no toString, valueOf or Symbol.toPrimitive of its own and still has String.prototype. JSObject::makePropertiesImmutable() gives the object another structure, so the check failed, and Graph::canOptimizeStringObjectAccess() gives up on a site for good once it has a BadCache exit: the site made a call from then on, for ordinary StringObjects too. The structure makePropertiesImmutable() makes of stringObjectStructure() says as much about an object, so the check accepts it. - JSGlobalObject keeps that structure once there is one (stringObjectStructureWithImmutableProperties()), alive, so that it is not collected and made again as another Structure. - FixupPhase::addCheckStructureForOriginalStringObjectUse(), which every such check comes from, adds it to the StructureSet. - While there is none, the set has the one structure, and the code watches stringObjectImmutablePropertiesWatchpointSet(), which JSGlobalObject::didMakePropertiesImmutable() fires when it sets the structure. Code compiled before that is thrown away, which leaves no exit against its sites, and is compiled again with both. A realm in which no StringObject has immutable properties gets the code it got.
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Included review availability: This review used your included allowance. Your plan provides up to 5 included reviews per hour; 0 remain after this review. WalkthroughThe change tracks the immutable-properties structure for each realm’s original StringObject. DFG fixup uses the recorded structure and watchpoint when building structure checks. A stress test covers string operations across immutable objects, realms, overrides, prototype changes, and garbage collection. ChangesImmutable String Object Handling
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Inline comments:
Review comments at @Source/JavaScriptCore/dfg/DFGFixupPhase.cpp:
- Line 4007: Select the conversion use-site realm when building the structure
check and immutable-properties watchpoint: update the globalObject lookup in the
relevant fixup logic to use origin.semantic instead of node->origin.semantic,
keeping the conversion eligibility checks aligned with the realm they protect.
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JSTests/stress/immutable-properties-string-object.jsSource/JavaScriptCore/dfg/DFGFixupPhase.cppSource/JavaScriptCore/runtime/JSGlobalObject.cppSource/JavaScriptCore/runtime/JSGlobalObject.hSource/JavaScriptCore/runtime/JSObject.cpp
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also checked the compiler-thread read of stringObjectStructureWithImmutableProperties() in DFGFixupPhase.cpp for a null or stale value — the storeStoreFence before fireAll in didMakePropertiesImmutable pairs with the loadLoadFence after isStillValid() returns false, and addLazily re-validates the set at plan finalization, so the ordering holds. I also checked that the realm-wide jettison on the first immutable StringObject is a one-time event per realm (the structure is set once and the set is never re-armed), matching the regExpLastIndexWritableWatchpointSet arrangement.
Extended reasoning...
The change touches DFG fixup, JSGlobalObject, and JSObject::makePropertiesImmutable to add a second accepted structure for StringObject fast string conversion, guarded by a new watchpoint set; it touches no auth, injection, or data-exposure surface. One confirmed inline finding (realm taken from the child node's origin) is posted separately; the ruled-out note records the concurrency publication check that the repository's CLAUDE.md specifically calls out for data the DFG reads from JSGlobalObject.
… conversion FixupPhase::addCheckStructureForOriginalStringObjectUse() took the realm from the origin of the node that produces the object, and its callers ask Graph::canOptimizeStringObjectAccess() about the origin of the conversion, which is where String.prototype is watched. The two differ when the object comes from an inlined function of another realm: the check accepted that realm's StringObjects while nobody watched its String.prototype, so compiled code went on returning the string after that realm replaced String.prototype.valueOf. (So it did before there were immutable properties.) The structures and the watchpoint set now come from the origin of the conversion.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding on the test, I also checked the compiler-thread protocol in addCheckStructureForOriginalStringObjectUse: the structure is stored before the storeStoreFence/fireAll in didMakePropertiesImmutable and read only after isStillValid() returns false plus a loadLoadFence, and a set fired between the isStillValid() check and plan finalization makes DesiredWatchpoints::reallyAdd fail the plan rather than install stale code, so that race is covered. The cross-realm origin point from the previous round is addressed by ba43cb2 taking the realm from origin.semantic.
Extended reasoning...
The second push changes DFGFixupPhase to take the realm from the conversion's origin and adds a per-realm immutable StringObject structure, watchpoint set, and fence protocol in JSGlobalObject/JSObject; no security-sensitive surface. One inline finding on the new stress test remains, and the fence/watchpoint race and the prior cross-realm issue were examined and found handled.
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- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @JSTests/stress/immutable-properties-string-object.js:
- Line 117: Update the factory assigned to fresh so it applies change to each
newly created StringObject in the other realm while preserving the existing
cross-realm call path; keep the addFresh assertions exercising those immutable
objects.
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JSTests/stress/immutable-properties-string-object.jsSource/JavaScriptCore/dfg/DFGFixupPhase.cpp
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…l can Each was checked against an engine with the line it is for taken out. - "The structure is kept" ran after String.prototype.valueOf had been replaced, which ends the fast conversion for every site of the realm compiled afterwards; and the structure never died, because earlier parts still held objects and code that use it. It has a realm of its own now, whose only such object is made in a function that has returned, and other structures take what a collected one would have left. It fails without the visitor.append() in JSGlobalObject::visitChildrenImpl(). The replacement of valueOf goes last. - With an object from another realm that has immutable properties, what produced the object was in this realm's code once everything was inlined (the function that makes the properties immutable, and the caller's own argument). It is a call that is not inlined and a load from an object now, both in the other realm's code, and the factory applies the change to what it makes. Each fails with the realm taken from the node's origin.
FixupPhase::addCheckStructureForOriginalStringObjectUse(): the comment on the realm and the parameter's name are main's; the structure with immutable properties and its watchpoint set are added after them, from the same realm.
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Follow-up to #759. A
StringObjectwhose properties were made immutable lost the optimizing compilers' fast conversion to a string, and so did every ordinaryStringObjectat a site that had seen one.What happened
The DFG and the FTL turn a
StringObjectinto its string without a call (ToStringwithStringObjectUseorStringOrStringObjectUse) behind aCheckStructurefor the realm'sstringObjectStructure(). That structure says the object has notoString,valueOforSymbol.toPrimitiveof its own and still hasString.prototype.JSObject::makePropertiesImmutable()gives the object another structure, so the check failed.Graph::canOptimizeStringObjectAccess()gives up on a site for good once it has aBadCacheexit, so from then on the site made a call, whatever object it was given.The change
The structure
makePropertiesImmutable()makes ofstringObjectStructure()says as much about an object (and the object can never gain a property), so the check accepts it.JSGlobalObjectkeeps that structure once there is one,stringObjectStructureWithImmutableProperties(). It is kept alive so that it is not collected and made again as a differentStructure.FixupPhase::addCheckStructureForOriginalStringObjectUse(), which every such check comes from, adds it to theStructureSet.stringObjectImmutablePropertiesWatchpointSet(), whichJSGlobalObject::didMakePropertiesImmutable()fires when it sets the structure. Code compiled before that is thrown away, which leaves no exit against its sites, and is compiled again with both. This is the arrangementregExpLastIndexWritableWatchpointSet()has.The compiled conversion itself checks the cell type and loads the internal value, so it needs no change.
The structure and the watchpoint set come from the realm of the conversion, as
stringObjectStructure()does there since #766.A realm in which no
StringObjecthas immutable properties gets the code it got. Forobject + suffixthe DFG's graph and machine code are the same, line for line (499 lines); the FTL's differ by less than two runs ofmaindiffer from each other, plus one more entry in the list of watchpoint sets the code depends on.Measurements
Bun built with this engine,
mainagainst this branch, same Bun revision and build profile. Median ns per call over 8 processes, each pinned to its own cores, every configuration of an operation run back to back.mainmainmaino + ss + o`${o}${s}`String(o)o.lengtho.concat(s)o.charAt(i)o[i]When the realm's first such object appears after the site is hot, the immutable object runs at 3.13, 3.56, 3.10, 0.32, 0.27 and 3.08 ns for the first six rows (
main: 29.62, 29.77, 26.62, 23.13, 10.02, 30.68).103 workloads that never call the operation: geometric mean 1.0004 of the time before #759 (that build against itself: 1.0017).
Tests
JSTests/stress/immutable-properties-string-object.js:StringObject, hot, with ordinary objects, objects with immutable properties, and both: the same results.reoptimizationRetryCount()of a site that sees such objects, later or from the start, is no more than that of a site with the same number of calls on ordinary objects alone. Onmainit is 2 against 0. (Each site has source text of its own: functions with the same source share their exit profile.)toStringandvalueOf, another prototype, and a derived class, each with immutable properties, are not taken for untouched objects.String.prototype.valueOfreplaced while the site is hot. Each immutable case fails with the realm taken from the child's origin.visitor.append().String.prototype.valueOfreplaced while a site is hot on such an object (last, because it ends the fast conversion for the realm).The test fails on
main. Throughrun-jsc-stress-tests: 25 runs of its 17 modes on a release build and 25 on a build with assertions, no failure; it takes 71 ms in the default mode and 167 ms in the slowest. The other tenimmutable-properties-*tests pass in all modes on both builds. All ofJSTests/stress(5,931 files) ends as it does onmain, apart from this test and two tests that fail now and then onmainas well.