[robobun]

Problem

• ({ [key]() {} })[key].bind().name is "bound ", not "bound one". Bun writes such a key for every decorated member since 1.4.3 (js_parser: lower standard decorators without moving class members bun#40833).
• JSFunction::setFunctionName (runtime/JSFunction.cpp:566) stores the name only in the name property. bind() defers the name, and originalName() reads the executable.
• A static accessor defined by put_getter_by_id skips JSFunction::defineOwnProperty, so bind() of the class ignores a static get length().

Fix

• FunctionRareData::setHasReifiedName() takes Original or AssignedAtRunTime. The second sets the modified-name bit, so bind() reads the name.
• Both bind functions get an arm before the generic steps: the name comes out of the target's own slot, and the length stays deferred.
• First commit: the four single-accessor opcodes call the class's own defineOwnProperty.
• Verified: two new JSTests/stress files fail on main, and CI's 7 Test jobs pass. Self-reviewed: 8 concerns raised, 7 addressed.

Background

• bind() defers name and length while the target's rare data says both are original. The first read takes them from the executable.
• One executable serves every closure of one source function, so it holds no name from a key.
• Weighed: a parse-time name, a stored name, a helper in Bun's decorator runtime (Notes).

Downsides

• bind() of such a target costs 344 → 529 instructions in JIT code. Five decorated methods bound per instance cost 2320 → 3240.
• String(bound) and Bun's console.log(bound) now show the name.
• ENABLE_AOT=0 only: { ...fn } of such a function costs 551 → 2114 instructions.

Notes

Where this comes from. No user reported it. A comparison with Node found it: a.one.bind(a).name of a method with a standard decorator is bound one on Bun 1.4.2 and bound since 1.4.3. Upstream WebKit main has the same three naming sites and the same putGetter, so the fork did not add the defect. No bugs.webkit.org bug is filed for it.

Base. The branch is on main ee59053, the commit that Bun's main pins since oven-sh/bun#48120. So the preview build is that engine plus this change. Before that the branch was on 5a61071, which Bun pinned then. The rebase had no conflict and left both commits as they were (git range-diff). FunctionPrototype.cpp, DFGOperations.cpp, JSBoundFunction.cpp and JSFunction.cpp are the same on both bases.

What ran on which base. The rows, the figures under Cost and the mode runs under Tests are from the branch on 5a61071. There the two new files passed 34 of 34 mode runs and failed on the jsc of the base. On ee59053, CI's 7 Test jobs pass. Bun's copies of the two files fail on a debug build of Bun's main and pass with the preview build (oven-sh/bun#48408).

What each commit is.

commit	change
1	defineAccessorOfLiteral (runtime/JSObject.cpp) takes the method table's defineOwnProperty where the class has its own. JSFunction::defineOwnProperty is what sets the two bits. put_getter_setter_by_id already does the same through putDirectAccessorWithReify.
2	The typed setter at nine sites (three of the plain run time as AssignedAtRunTime, three of the ahead-of-time run time that set both bits by hand, three lazy reifies as Original), and the arm in the two bind functions.

Rows, release jsc of 5a61071 against this branch (node v26.3.0 agrees with the last column).

target, then .bind(null, 1)	pin	branch
({ [key]() {} })[key]	bound	bound one
getter under a key in brackets	bound get	bound get one
method under Symbol("s")	bound	bound [s]
getter function of the custom accessor RegExp.input	bound input	bound get input
class C { constructor(a, b) {} static get length() { return 9 } [key]() {} }, length	1	8
class C { static get name() { return "custom" } [key]() {} }	bound C	bound custom
the class above under a key in brackets	bound	bound custom

Why the arm checks the slots. The two bits only know what went through JSFunction::defineOwnProperty, put and deleteProperty. A property that is written past those leaves them as they were: the four opcodes before commit 1, and putDirectAccessor from an embedder (Bun's spyOn does that, tracked apart). A first cut of the arm trusted the length bit. A class with another name and a static get length() then bound with the length of its constructor, where the pin answered 9. Now the helper takes the name only where the own name is a string in a plain data property, and where the own length is not made yet or holds originalLength() with the attributes of reifyLength(). Everything else takes the generic steps.

Cost. Local release builds of 5a61071 and of this branch, same flags, no LTO. Instructions in user space per operation, the slope between 1e6 and 3e6 operations (1e5 and 3e5 for instances), least of 5 runs.

bind() of	interpreter	JIT, no concurrent JIT
a target with a name in the code	466.3 → 468.2	34.9 → 34.9
the same after a read of its name	481.2 → 483.3	344.5 → 344.3
a target with a name from a key in brackets	481.9 → 664.2	344.2 → 528.5
the getter function of a custom accessor	481.4 → 664.5	344.2 → 528.5
a target that the program renamed	1301.9 → 663.8	1161.7 → 528.7
a bound function	1211.3 → 1235.5	1071.4 → 1096.6
a proxy	3664.9 → 3659.3

• operationFunctionBind calls per 200,000 binds of a target with a name in the code: 0 → 0 (breakpoint count). The fold to NewBoundFunction is untouched.
• A closure under a key in brackets: 1974.3 → 1977.3 instructions interpreted, 612.6 → 616.5 in JIT code, 171.65 bytes on both. A literal with a getter under a key in brackets: 2993.4 → 3005.1. bind() allocates the same cells, and the target has no length property afterwards.
• new A() of a class whose decorator binds 5 methods to the instance (the decorators proposal's @bound): 2319.9 → 3239.8 in JIT code, 11571.1 → 12483.1 interpreted. The same class as Bun 1.4.2 lowered it (plain keys): 764.7 → 764.2. Node v26.3.0 on the same two bundles: 2339.2 with plain keys, 21901.2 with keys in brackets.
• jsc text: 51,890,148 → 51,890,276 bytes (+128). With ENABLE_AOT=0: 46,362,548 → 46,362,676 (+128).
• With ENABLE_AOT=0, canPerformFastEnumerablePropertyEnumeration() reads the same bits: { ...fn } of a function under a key in brackets costs 550.9 → 2114.3 instructions interpreted, 488.8 → 2046.5 in JIT code. A name in the code: 532.8 → 532.7. The builds that Bun ships have that path off for every function.

Tests.

• stress/bound-function-name-of-run-time-named-target.js: 28 kinds of target by 4 ways to reach bind(), each with the name, the name bound twice and the three lengths. 29 orders of change, read and bind. A loop that asks $vm.getConcurrently(target, "length") behind bind(): 0 through the arm, 1 through the generic steps. It fails if the arm goes from either bind function.
• stress/function-bind-class-with-static-name-or-length-accessor.js: 15 classes, each as written and with another name, the count of getter calls, and getters that throw.
• Two rows differ from V8 on purpose: a class under a key in brackets with a static member called name. V8 names that class behind its static elements.
• Related upstream tests on a debug ASAN jsc of this branch: 3138 mode runs, 0 wrong answers. 4 hard timeouts, all of stress/buffer-accessor-jit-differential.js in eager modes. That test takes 337 s alone on the debug jsc of 5a61071, against a limit of 300 s.
• LayoutTests/js/script-tests/class-syntax-declaration.js passes in its 7 modes: static get ['prototype']() {} still throws the TypeError.

Text that changes. For a bound function of these targets, Function.prototype.toString gives function one() { [native code] } where it gave function () { [native code] }, as it already does for a target with a name in the code. Node prints no name there.

Left as it is.

• A class under a key in brackets that binds itself in its own static block is still bound . The class gets its name behind its static elements.
• getCalculatedDisplayName and JSFunction::name() still read the executable alone. Bun's console.log(fn) prints [Function] for an unbound function under a key in brackets. inspect: display a class or function name set with Object.defineProperty bun#38922 is the open change for that reader. It reads the bit that this change now sets from the start.
• Creators of a bound function that pass a null name (JSBoundFunction::create from native code) still get the name from originalName().
• bind() of a decorated method stays out of line. Two ways back to the inline path are open: a name at parse time for a key that ends in a literal, and a fold that knows the structure of a function with nothing but a name.

Why not a stored name, or a helper in Bun. A name kept in FunctionRareData for originalName() adds a field that the collector visits to every rare data (72 to 80 bytes), and the bound name then depends on when it is first read where the target is named behind the bind(). A helper in Bun's decorator runtime that defines name again reaches decorated members only: not a key that the program writes in brackets, not a custom accessor, and not the member that holds the decorator list of a #private neighbour. It costs 711 to 2,362 instructions for each decorated member of each class.

Why not a name at parse time. A key in brackets and a custom accessor have no name when the parser runs, so the mark is needed in any case. For [(_dec = [dec], "one")], which is what Bun's lowering writes, the parser can see the name. That change would give decorated methods their 1.4.2 cost back. It also changes what frames, console.log and the ahead-of-time parity programs print for those members, so it is a change of its own.

Other open work.

• [JSC] Store the name default for an anonymous default export #950 rewrites JSFunction::reifyName(VM&, JSGlobalObject*). The resolution is its return with FunctionRareData::ReifiedName::Original as the last argument.
• node: $toClass installs the prototype of built-in constructors; EventEmitter reads it with $getPrototypeOf bun#46197 adds a call of setHasReifiedName() without an argument next to setHasModifiedNameForBoundOrNonHostFunction(). It becomes setHasReifiedName(FunctionRareData::ReifiedName::AssignedAtRunTime).
• The isNumber guard that operationFunctionBind lacks is upstream's [JSC] Read the bind target's prototype before length and name WebKit/WebKit#75885 (bug 326072). It is not in this change.

Self-review. Addressed: a wrong bound length through the arm (slot checks, commit 1, rows in both tests and in an ahead-of-time parity program in Bun), rows for targets that are no functions of the program, a row that fails without the arm, one frozen commit for every figure, the second commit of an earlier cut (out, it is upstream's), two switches in Bun's tests that hid these rows. Not taken at the first push: a rebase onto main. The branch is on main now (Base).

[robobun]

Status

• Reproduced on the engine of main ee59053, the commit that Bun's main pins. Bun's copies of the two new files ran on a debug build of Bun's main. bound-function-name-of-run-time-named-target.js stops at its first row with expected "bound bound one" but got "bound bound ". function-bind-class-with-static-name-or-length-accessor.js stops with the length: expected 9 but got 2. The jsc of the earlier base 5a61071 stopped at the same two rows.
• In Bun the same shows as a.one.bind(a).name === "bound " for class A { @keep one() {} } since 1.4.3 ("bound one" on 1.4.2).
• The Bun pull request that pins the preview build of this one and adds the tests is Bump WebKit (oven-sh/WebKit#1023 preview): bind() takes over the name of a function that was named by a computed key bun#48408.

[coderabbitai]

Walkthrough

The changes track whether function names are original or runtime-assigned, update bind paths to handle runtime-assigned names, and add stress tests for bound functions and classes with static name or length accessors.

Changes

Function names and binding

Layer / File(s)	Summary
Track function name origin Source/JavaScriptCore/runtime/FunctionRareData.h, Source/JavaScriptCore/runtime/JSFunction.*, Source/JavaScriptCore/runtime/JSFunctionInlines.h, Source/JavaScriptCore/aot/object/*, Source/JavaScriptCore/runtime/JSCustomGetterFunction.cpp, Source/JavaScriptCore/runtime/JSCustomSetterFunction.cpp	FunctionRareData now distinguishes original and runtime-assigned reified names. JSFunction and the AOT, getter, and setter name-setting paths pass that classification.
Handle runtime-assigned names in bind Source/JavaScriptCore/runtime/FunctionPrototype.cpp, Source/JavaScriptCore/dfg/DFGOperations.cpp, JSTests/stress/bound-function-name-of-run-time-named-target.js, JSTests/BUN-TEST-DIFFERENCES.md	The runtime and DFG bind paths use a non-original own name and defer length computation. Stress tests cover target naming and binding cases. The test index lists both added tests.
Define and bind classes with static accessors Source/JavaScriptCore/runtime/JSObject.cpp, JSTests/stress/function-bind-class-with-static-name-or-length-accessor.js	defineAccessorOfLiteral now uses a non-default defineOwnProperty method-table entry. Stress tests cover bound-class names, lengths, getter calls, and getter exceptions.

Possibly related PRs

• oven-sh/WebKit#950: Changes to JSFunction name representation and reification overlap with this PR’s updates to name reification and bound-function naming.

Merge Risk: Minimal · up to 79f76

The change fixes the name and length that bind() reports for run-time-named functions and for classes with static accessors. No concrete merge-blocking problem was found. The author reports a JIT instruction-count increase on some paths, and a few unverified compile-time and dispatch questions remain for the author to confirm.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly identifies the main change: bind() reads the name of a target that received its name at run time.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description clearly explains the problem, fix, implementation choices, tests, and tradeoffs. It does not include a Bugzilla issue link, a “Reviewed by” line, or the template’s changed-file and fun…

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[claude]

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 new bound-function state (name set, length still PNaN): JSBoundFunction::lengthSlow falls through to the target's originalLength(), which ownNameIfOnlyNameIsNotOriginalSlow has just verified against the own length slot, and a re-bind of such a bound function takes the generic path because bound functions never get the modified bits. The DFG NewBoundFunction fold and the AOT emitFunctionBind stub both key on an untransitioned structure, which a run-time-named target no longer has, so neither reaches the new arm. No build or test run was done in this review; the new stress tests were checked by reading only.

Extended reasoning...

The change makes Function.prototype.bind read the own name slot of a target whose name was assigned at run time (computed keys, custom accessor functions) while still deferring length, routes the four single-accessor opcodes through the class's own defineOwnProperty, and threads a new ReifiedName enum through nine setter call sites across runtime, dfg and aot. It touches no security-sensitive surface. One inline finding stands (the non-AOT { ...fn } slowdown, which the PR's Downsides section already acknowledges), so this note only records what else was examined and ruled out.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟡 Source/JavaScriptCore/runtime/JSFunctionInlines.h — Users of a non-AOT build (bun --aot=off, lanes "release", "lto", "lto-hardened") get a roughly 4x slower { ...fn } and Object.assign({}, fn) for every function named at run time. JSFunctionInlines.h:296 asks canAssumeNameAndLengthAreOriginal, and FunctionRareData.h:144 now sets the modified-name bit at reify time for every computed-key method, every class-field arrow (NodesCodegen.cpp:5918) and every custom accessor function. Fix: let the enumeration fast path key on hasReifiedName/structure state instead of the bind-only modified-name bit, or add a separate AssignedAtRunTime bit that bind() reads and the enumeration path ignores.

  Why this was flagged

  Trigger: any { ...fn }, Object.assign(target, fn) or copyDataProperties where fn got its name from set_function_name: a computed key, a class field handler = () => {}, or a JSCustomGetterFunction (JSCustomGetterFunction.cpp:74). On an ENABLE_AOT=0 build (.github/scripts/lanes.mjs:93,95,115 mark "release", "lto", "lto-hardened" as aot: false) JSFunctionInlines.h:296 calls canAssumeNameAndLengthAreOriginal, which at JSFunctionInlines.h:260 returns false once FunctionRareData.h:144 has set m_hasModifiedNameForBoundOrNonHostFunction, which the new ReifiedName::AssignedAtRunTime does for every such function at creation. JSGlobalObjectFunctions.cpp:852 then refuses the fast enumeration path. On the base branch the bit stayed clear and the fast path ran. The PR's own numbers: 551 -> 2114 instructions interpreted, 489 -> 2047 in JIT code. Remedy: separate the bit bind() needs from the one the enumeration path reads.

  Verification: JSFunction::setFunctionName now calls reifyName(..., FunctionRareData::ReifiedName::AssignedAtRunTime), and FunctionRareData::setHasReifiedName (Source/JavaScriptCore/runtime/FunctionRareData.h:142-144) sets m_hasModifiedNameForBoundOrNonHostFunction = true. On the base commit the same call set only m_hasReifiedName, so the fast path was taken.

[github-actions]

Preview build of 79f76b4: autobuild-preview-pr-1023-79f76b47

[robobun]

On the review's finding about { ...fn } in a build with ENABLE_AOT=0: it holds, and it is the third bullet under Downsides. Measured on release builds without the compiler, instructions per spread of a function under a key in brackets: 550.9 → 2114.3 interpreted, 488.8 → 2046.5 in JIT code.

Two corrections to its reach:

• Object.assign({}, fn) does not change (2538.3 → 2538.7). It does not take that path.
• A class field with a plain key (handler = () => {}) does not get the mark. The parser gives that function its name and no op_set_function_name runs for it: $vm.hasOwnLengthProperty() answers true for it on this branch, and false for [key] = () => {}. The mark goes to a function under a key in brackets and to the getter and setter functions of a custom accessor.

The gate of the enumeration path stays as it is in this change. The builds that Bun ships have that path off for every function (#if ENABLE(AOT) in canPerformFastEnumerablePropertyEnumeration), and a bit of its own for "assigned at run time" would be one more state that every naming site has to keep right. The Notes name it under Cost.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between b3049a8 and 79f76b4.

Files selected for processing: 15

• JSTests/BUN-TEST-DIFFERENCES.md
• JSTests/stress/bound-function-name-of-run-time-named-target.js
• JSTests/stress/function-bind-class-with-static-name-or-length-accessor.js
• Source/JavaScriptCore/aot/object/AOTClassTable.cpp
• Source/JavaScriptCore/aot/object/AOTClosure.cpp
• Source/JavaScriptCore/aot/object/AOTFunctionTables.cpp
• Source/JavaScriptCore/dfg/DFGOperations.cpp
• Source/JavaScriptCore/runtime/FunctionPrototype.cpp
• Source/JavaScriptCore/runtime/FunctionRareData.h
• Source/JavaScriptCore/runtime/JSCustomGetterFunction.cpp
• Source/JavaScriptCore/runtime/JSCustomSetterFunction.cpp
• Source/JavaScriptCore/runtime/JSFunction.cpp
• Source/JavaScriptCore/runtime/JSFunction.h
• Source/JavaScriptCore/runtime/JSFunctionInlines.h
• Source/JavaScriptCore/runtime/JSObject.cpp

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