[dylan-conway]

What this fixes

A sampling profiler that is started at run time cannot tell where the top frame is once that frame is JIT code. Samples in a function that was inlined go to the function it was inlined into, and the line of the sample is a stale one or none. That is every profiler Bun starts, as far as JavaScript frames go, and startSamplingProfiler() of the jsc shell.

function inner(x) {
    let s = 0;
    for (let i = 0; i < 100; i++)
        s += (x ^ i) * 3;
    return s;
}
function outer(n) {
    let t = 0;
    for (let i = 0; i < n; i++)
        t += inner(i);
    return t;
}
let r = 0;
const end = Date.now() + 1500;
while (Date.now() < end)
    r += outer(10000);

bun --cpu-prof, self samples by function:

	inner	outer	(module)
Bun 1.4.2	9	838	632
Bun 1.4.2 with BUN_JSC_alwaysGeneratePCToCodeOriginMap=1	1,458	10	0
debug build of Bun, WebKit at the base of this branch	289	1,129	4
the same with this branch	1,391	20	5

It is also why stress/sampling-profiler-line-column.js.ftl-eager fails now and then on Test bun-webkit-linux-amd64-lto, for branches that have nothing to do with it (afed06b, 07b2aaf, 68efad5, 152b406).

Cause

For the top frame the profiler has a PC and nothing else. SamplingProfiler::processUnverifiedStackTraces() asks CodeBlock::findPC(), which reads the code block's PCToCodeOriginMap. The three JITs build that map only if VM::shouldBuilderPCToCodeOriginMapping() says so, and that flag is set in three places: the VM constructor under --useSamplingProfiler, the same under --alwaysGeneratePCToCodeOriginMap, and Debugger::attach(). VM::ensureSamplingProfiler(), the one place a profiler is created, does not set it.

Without a map findPC() finds nothing for a PC in the body of the function (it still knows inline caches, OSR exit stubs and lazy slow paths), and the frame is treated like a caller's: its position is taken from the call site index in the frame header. The top frame is not at a call, so that index is what the last call left there, or in FTL nothing valid. With a line of instrumentation at that point, on the test below, every code block had no map, Baseline, DFG and FTL alike.

The flaky test loops over Math.tan(i) in a function test and waits up to 50 s for a frame named test with a line inside it. A copy of it that tallies its traces, 32 runs of 3 s, with the options of the failing CI run:

top frame	base	base with --alwaysGeneratePCToCodeOriginMap=true
test, FTL, without line and column	3,782	1
test, FTL, with them	3	3,730
runs in which no frame was acceptable	4 of 32	0 of 32

So it passes only if a sample catches test before the FTL code is installed, in the first milliseconds, and otherwise never: a run takes 0.2 s at most or the whole 50 s.

Where the code comes from, and why the fix is here

All of it is upstream's, and upstream main (c468f0d) has the same text. The flag and its three setters are from 2016. The commit that added the map (https://bugs.webkit.org/show_bug.cgi?id=152629) states the rule: "We only build the mapping if a debugger has ever been attached to any global object", and what the map is for: "This allows us to correctly observe inlining. It also allows us to observe expression-level line/column information for the top frame."

WebKit's own host keeps that rule without trying: Web Inspector attaches a debugger before it starts a profiler. Two users do not. Bun starts a profiler with no debugger, at four call sites of ensureSamplingProfiler(): Bun::startCPUProfiler() (--cpu-prof, Profiler.start of node:inspector, CPU profiles of a Worker), the V8 CpuProfiler shim, and profile() and startSamplingProfiler() of bun:jsc. Upstream's jsc shell does the same in startSamplingProfiler(), which is what the sampling-profiler-*.js tests use.

Bun could set the flag itself at its four call sites, but nobody wants a profiler that cannot read the top frame, the shell and its tests would stay as they are, and the next caller would have to know. The VM constructor already pairs the two by hand. So the flag is set where the profiler is created, and the line in the constructor goes.

The second change: FTL asks the VM twice

Setting the flag at run time is not new, Debugger::attach() does it, but it becomes common, and FTL cannot take it. On the compiler thread FTL::State reads the flag to decide whether B3 keeps its origins (and whether the DFG graph that they point into is freed after lowering), and FTL::compile() reads it again, after the whole B3 pipeline, to take the map: releasePCToOriginMap() has RELEASE_ASSERT(needsPCToOriginMap()) (both since https://bugs.webkit.org/show_bug.cgi?id=226187, 2021). A compilation that is between the two when the flag is set aborts the process.

300 small functions on their way up the tiers, the profiler started at a round from 1 to 40, eager thresholds, concurrent JIT:

	aborted
the flag set in ensureSamplingProfiler(), FTL::compile() unchanged	168 of 400
this branch	0 of 400

FTL::compile() now goes by what the procedure was told. From the code, the base has the same window when a debugger is attached while FTL compiles; I have not reproduced that. Both conditions are needed: with --useIRDump the procedure keeps origins while the VM wants no map. Baseline, LOL and DFG read the flag once, when the compiler object is made.

The flag was a plain bool, written by the mutator and read by compiler threads. That race is upstream's too, but the write is now routine, and the guard counts on the third read, in the constructor of PCToCodeOriginMapBuilder, not seeing false after the guard saw true (RELEASE_ASSERT(builder.didBuildMapping())), which the language promises only for atomics. It is now a std::atomic<bool> with relaxed accesses, like m_numberOfActiveJITPlans in the same header; nothing is published through it. In a Release x86_64 build that changes two functions, FTL::State::State() and FTL::compile(), where cmpb $0, mem becomes movzbl and testb.

What the flag does to each tier

Measured with --alwaysGeneratePCToCodeOriginMap, which sets the flag with no profiler, so that the maps are apart from the sampling. Release jsc, x86_64, unless it says otherwise.

Who reads the maps. One place in the engine: SamplingProfiler::processUnverifiedStackTraces(), through CodeBlock::findPC(). No trap, exception, OSR or disassembly path looks at them, so that they exist changes nothing else.

The generated code is the same, in every tier. From the source: Baseline, LOL and DFG only append to a side table, and evaluate the labels they pass to it whatever the flag says. In FTL the flag also makes B3 and Air keep their origins, and Air only copies an origin on to the instructions it inserts, or reads it for the C calls, patchpoints and checks that are kept in both cases. Measured: the 1,695 files of JSTests/microbenchmarks, no concurrent JIT or GC, no ASLR, every blob of JIT code taken from --useJITDump, disassembled, with addresses masked and nops dropped (Baseline puts one at random at the start of a function). Each file runs twice alike, as the control, and a third time. Of the files whose control agrees, about 7.4 M instructions:

the third run	files that differ	blobs that differ: FTL / DFG / Baseline
has the flag on	105 of 1,189	96 / 14 / 4
has the flag off and --reportCompileTimes=1, which only prints	103 of 1,181	80 / 13 / 14
is the same again	84 of 1,195	67 / 17 / 9

What differs is which of several absolute addresses B3 takes as a base for the others, and the registers that follow from it, with the same number of instructions in 88 of 116 blobs and at most 8 apart. It comes with any disturbance of the allocator and with none.

Compile time, summed over those files (3,356 Baseline, 3,341 DFG and 2,087 FTL compilations), flag on over flag off, and the control: Baseline 1.056 (1.006), DFG 1.008 (0.997), FTL 1.012 (0.996).

Memory that stays, all of JetStream2:

	compilations	code	maps	maps / code
Baseline	9,170	44.3 MB	2.9 MB	6.6%
DFG	10,316	22.5 MB	2.8 MB	12.3%
FTL	2,710	8.3 MB	2.5 MB	30.2%

A range inside an inlined function holds a pointer to its InlineCallFrame, which is what makes FTL's the largest. That pointer is into the table of the same JITCode.

Memory during a compilation. Only FTL: the DFG graph is not freed after lowering, nor the B3 values. It does not raise the peak. Maximum RSS, one function compiled on the main thread, 3 runs each: 87.7 to 88.6 MB with the flag off and 88.3 to 88.9 MB with it on for 34 KB of FTL code, about the largest function FTL takes by default; 2.31 to 2.45 GB and 2.45 GB for 645 KB, with the limits raised. Code size and compile time are equal.

The assertions in PCToCodeOriginMap's constructor (ranges strictly increasing, none empty) are RELEASE_ASSERTs, and every compilation of a profiled process now goes through them. The harness runs the whole suite with the flag on in one mode, ftl-no-cjit-validate-sampling-profiler, without concurrent compilation, where a profiled Bun has it. All of JSTests/stress in the two concurrent modes, default and ftl-eager, on an assertion build: 11,205 pass and 5 fail with JSC_alwaysGeneratePCToCodeOriginMap=1, 11,206 and 4 with =0. The one more is a second failure of auxiliary-evacuation-with-concurrent-compiles.js, which on that build and a loaded machine fails 55 of 160 runs with the flag on and 58 of 160 with it off.

Lifetimes. A sample holds raw pointers and is interpreted later, in a marking constraint (visitSamplingProfiler() in Heap.cpp), so before anything of that cycle is swept; a jettisoned code block keeps its code and its inline call frames until then. Walking the inline stack is not new, the callers' frames always did through their call site index. Without a map every JIT top frame went on to search the inline caches under the code block's lock; with one, the lookup returns before taking it.

How right the positions are. A function whose time is 10, 20, 40 and 30% on known lines, the last in a helper that DFG and FTL inline, 2,000 samples:

	10%	20%	40%	30%, inlined	no position
LLInt	11.9	17.8	39.6	29.4	1.1
Baseline	9.9	18.1	41.2	30.8	0
DFG	10.0	19.9	39.0	31.1	0
FTL	8.4	24.1	38.0	29.4	0

What a map does not cover. The prologue, which Air emits before the first entry (AirGenerate.cpp), and shared thunks, which are outside the function's code. Such a frame falls back to its call site index. That is the 38% on an assertion build with --useDFGJIT=0: all of those PCs are in generateConsistencyCheck, a thunk that Baseline calls at every bytecode in assertion builds only. FTL's late paths have their origins.

A lookup is linear in the size of the map. Per sample, with the top frame in one Baseline function, which has no limit of size, JSON included: 3 us for 27 KB of code, 10 us for 0.3 MB, 64 us for 2 MB, 241 us for 8 MB, 1,030 us for 30 MB.

Baseline code is shared. It is cached on the UnlinkedCodeBlock and given to new code blocks of the same function (LLIntSlowPaths.cpp, ScriptExecutable.cpp), so a code block made after the profiler can get code compiled before it, without a map, as on the base. The cache is filled only when it is empty (CodeBlock::setupWithUnlinkedBaselineCode()), so to pass over a stale entry at those two places would keep the new code out of it; what deletes all code blocks already clears it (Heap.cpp). The maps can be shared: a Baseline origin has no pointer in it.

The flag can only be turned on. FTL::compile() and then the builder's constructor read it; off after on would fail RELEASE_ASSERT(builder.didBuildMapping()). There is no way to clear it, and the member now says so.

LLInt (which finds its position from its own PC register), Yarr and the WebAssembly tiers do not read the flag.

Not changed

• A VM in which no profiler was created and no debugger attached: the flag stays off. VM::disableSamplingProfiler() creates a profiler in order to pause it, so it now sets the flag too.
• The flag is never cleared, as before. After one profile() of bun:jsc, every later compilation in that VM builds its map.
• Code that was compiled before the profiler was created has no map until it is compiled again, and nothing is recompiled here. That includes an FTL compilation that is in flight at that moment, which the guard above lets finish without one. Shareable Baseline code is not compiled again even for a new code block, as long as its cache lives: see "Baseline code is shared" above. --cpu-prof starts the profiler before any code runs. The others can start it on code that is already hot, and for code that is not compiled again they get what they got before. profile() of bun:jsc, top frames, where spin calls outer and the time is spent in inner: on cold code outer 678 and inner 4 in Bun 1.4.2, inner 659 and outer 15 in a debug build with this branch; after 700 ms of the same work, spin 677 in Bun 1.4.2, and with this branch outer 470 and inner 204 in one run, outer 661 and inner 0 in another.
• WebAssembly: the maps that give a Wasm frame its offset depend on Options::useSamplingProfiler() (WasmBBQJIT.cpp, WasmCallee.cpp) and not on this flag.
• A sample taken inside a C function that JIT code calls without storing topCallFrame (Math.tan in DFG and FTL) is still attributed from vm.topCallFrame, that is to a caller. About two thirds of the samples of the flaky test are of that kind.
• With a map, a few PCs still have no position, a prologue for one: 9 of 1,500 top frames in FTL with inlining off. See above for the 38% on an assertion build.
• Inlined frames are named as they always were, which now shows: an inlined accessor is val where the interpreter and Baseline say get val and set val, and an inlined tail call keeps the frame that a real one drops.

Cost

Only once a profiler has been created. The generated code is the same; B3 keeps its origins and the DFG graph for the length of the compilation, and the maps stay with the code.

JetStream2 tests that compile a lot, with startSamplingProfiler() before them, Release jsc, score, higher is better, median of 9, on two reserved cores with their SMT siblings. "base again" is a second copy of the base binary.

	base 4b724e9	this branch	base again
typescript	20.5	21.1	20.9
acorn-wtb	51.8	50.7	51.9
Babylon	974.7	976.0	967.0
Air	707.9	709.8	712.6
ML	148.6	148.7	150.2
delta-blue	1,281.7	1,313.8	1,314.8
richards	948.7	934.3	734.2
total	279.3	277.3	272.4

Memory, from the review: 3,000 live functions taken through the tiers with eager thresholds, maximum RSS of 3 runs each, in MB. With a profiler: 292 to 299 on the base, 297 to 299 on its second copy, 308 on this branch. Without one: 287 to 291, 287 to 292, 287 to 293.

Tests

stress/sampling-profiler-line-column.js is unchanged: it was right. With the options of the failing CI run, 200 runs each: 16 fail on the base, 0 on this branch.

It finds the bug only by timing, so there are two new tests.

stress/sampling-profiler-inlined-top-frame.js is the program above, with a longer loop in inner(). It lets outer() reach the top tier, drops the samples taken so far, takes 40 whose top frame is inner or outer, and asks that inner has more of them. It waits for samples and not for time; if they have not come after 50 s, the limit of the suite's runTest(), it throws.

Release jsc, each mode with the options the harness gives it (bytecode-cache, which goes through a helper script, only in the harness):

	base, 50 runs each	this branch, 350 runs each
default, dfg-eager, dfg-eager-no-cjit-validate, eager-jettison-no-cjit, ftl-eager, ftl-no-cjit-no-put-stack-validate, ftl-no-cjit-small-pool, mini-mode, no-cjit-collect-continuously, no-ftl, no-llint	50 fail	0 fail
ftl-eager-no-cjit, which fails compilations at random	36 fail	0 fail
lockdown (no JIT), ftl-no-cjit-validate-sampling-profiler (--useSamplingProfiler=true), ftl-no-cjit-no-inline-validate (no inlining), no-cjit-validate-phases (inner is not inlined there: the frames are inner/DFG < outer/DFG)	0 fail	0 fail

How far it is from failing on this branch: 20 of the 40 samples would have to be outer's. Over 4,940 runs (those 16 modes, --useDFGJIT=0, Release and assertion builds, and 1,140 of them with 8 processes of the assertion build pinned to one CPU) the most was 6.

The version in the first commit also asked every sample for a line inside its function. That cannot hold, see "Not changed", and it failed 12 of 100 runs in ftl-no-cjit-no-inline-validate on this branch. Both reviews found it.

stress/sampling-profiler-start-during-ftl-compilation.js is the race, in ftl-eager only: it means nothing without concurrent compilation. It makes testLoopCount calls, going round 300 functions, and starts the profiler a quarter of the way in. It guards the second change against the first, so it passes on the base. With the options of that mode:

	aborted
this branch, Release	0 of 500
this branch, assertions	0 of 300
this branch with FTL::compile() as on the base	101 of 200

The 300 is what the test lives by: with 200 functions it caught the missing guard in 22 of 60 runs, with 300 in 44, with 400 in 30. In the harness it takes 13 ms at most (10 runs, 4 jobs on 4 CPUs), 70 ms on an assertion build, and 0.5 to 0.9 s in an unoptimized ASAN build, where an empty script takes 0.2 to 0.3 s.

Tools/Scripts/run-jsc-stress-tests --report-execution-time, the two new tests and sampling-profiler-line-column.js, Release jsc of this branch, 5 times: 35 of 35 runs pass each time, the slowest in 140 ms (bytecode-cache, which runs the file twice). With this branch minus the line in ensureSamplingProfiler(), ftl-no-cjit-validate-sampling-profiler fails as well, 30 of 30: --useSamplingProfiler now gets the flag from there.

The same with --filter sampling-profiler, which goes by the name of the configuration too: the 19 sampling-profiler-*.js in their configurations (211 runs) and every other stress test in ftl-no-cjit-validate-sampling-profiler, that is with a profiler running (5,233 runs). Three times, none fails.

Time, since the lanes do not record it: the 19 files, 4 jobs on 4 CPUs (taskset, --child-processes 4), 5 times. inlined-top-frame: median 72 ms, slowest 153 ms (bytecode-cache), 0 of 85 runs over 200 ms. Most of its time is the wait for 40 samples at the profiler's interval of 1 ms. With 32 jobs on those 4 CPUs its median is 353 ms.

Two independent reviews with the binaries of the base, of this branch and of this branch without the change to FTL::compile(). One went through every reader of the flag. The other tried to make this branch crash: the profiler started at 80 points among 200 functions, a very large one and gc(), under eight sets of options; 4 agents with VMs of their own, half of them with a profiler; WebAssembly modules compiled in a loop; recursion, stack overflow, exceptions, accessors, bound functions, generators, eval; FTL compilations for OSR entry; 8 s runs with constant recompilation. No run of this branch, Release or assertions, ended other than with 0, in more than 9,000. The binary without the guard aborted in 20% to 40% of the runs of most of the same programs. It also compared the top frames of ten shapes of program with the interpreter's: with the profiler started first, this branch agrees with the interpreter (3 functions deep: the innermost has 89% in the interpreter, 92% here, and on the base the outermost has 98%), and no shape is worse than on the base.

stress/sampling-profiler-bound-function-name.js.ftl-eager-no-cjit, which fails now and then on the arm64 lane, has another cause and is not fixed by this: 14 of 200 runs fail on the base, 10 of 200 with the maps, 0 of 200 with --useExecutableAllocationFuzz=false.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[github-actions]

Preview build of 9027169: autobuild-preview-pr-804-90271690

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• Source/JavaScriptCore/runtime/VM.h

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Walkthrough

The change moves PC-to-code-origin mapping setup to sampling-profiler initialization and restricts FTL map creation to procedures that retain origin data. It adds two stress tests for top-frame sampling and profiler startup during FTL compilation.

Changes

Sampling profiler behavior

Layer / File(s)	Summary
Profiler mapping lifecycle Source/JavaScriptCore/runtime/VM.cpp, Source/JavaScriptCore/runtime/VM.h, Source/JavaScriptCore/ftl/FTLCompile.cpp	ensureSamplingProfiler() requests PC-to-code-origin mapping when it first creates the profiler. The mapping flag uses relaxed atomic operations. FTL compilation creates the map only when the VM option is enabled and the procedure needs the map.
Sampling profiler stress coverage JSTests/stress/sampling-profiler-inlined-top-frame.js, JSTests/stress/sampling-profiler-start-during-ftl-compilation.js	One test checks top-frame sample counts for inner and outer, with a timeout. The other starts the profiler during repeated execution of generated functions with eager FTL compilation.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 90271

The supplied evidence establishes no user-facing regression or concrete merge blocker. Whether mapping continues after disabling the profiler remains unconfirmed.

Pre-merge checks: 4

Passed checks: 4 passed

Check name	Status	Explanation
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.
Title check		The title clearly identifies the primary change: enabling PC-to-CodeOrigin maps for sampling-profiler use in a VM.
Description check		The description provides a detailed explanation of the bug, implementation, scope, limitations, performance impact, and test results. It omits the template's explicit Bugzilla URL, reviewer line, and …

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[claude]

I re-reviewed the two follow-up commits and found no bugs; the earlier nit about the unbounded sample loop is addressed by the deadline and throw now in JSTests/stress/sampling-profiler-inlined-top-frame.js. Since this changes what the FTL compiler threads do when the flag flips mid-compilation, a human look at the engine side and at test runtime is still worthwhile.

What was reviewed:

• The new guard in FTLCompile.cpp matches the freeDFGIRAfterLowering condition at FTLCompile.cpp:180, so the DFG nodes the map builder dereferences are still alive whenever a map is built.
• Baseline, DFG and LOL capture the flag once in PCToCodeOriginMapBuilder's constructor and gate on didBuildMapping(), so they have no equivalent mid-compile window.
• VM.cpp: ensureSamplingProfiler is the sole creation path (constructor, enable/disableSamplingProfiler, jsc shell startSamplingProfiler all go through it), and the alwaysGeneratePCToCodeOriginMap and Debugger::attach setters are untouched.
• Test runtime: the ftl-eager test compiles 300 functions over 30 rounds and the inlined-top-frame test follows the suite's 50 s time-to-fail pattern; whether that fits the JSTests 200 ms guidance on the CI lanes is a judgment call I did not measure.

Extended reasoning...

The PR moves setShouldBuildPCToCodeOriginMapping() from the VM constructor's useSamplingProfiler branch into VM::ensureSamplingProfiler(), and adds a state.proc->needsPCToOriginMap() check before releasePCToOriginMap() in FTL::compile() so a compile that started before the flag was set does not hit the RELEASE_ASSERT in B3Procedure.h:274; two JSTests/stress tests are added. It touches no security-sensitive surface (no auth, injection or data exposure), only JIT metadata generation and the sampling profiler. The engine change is three lines and internally consistent with the existing FTLState.cpp:75 and FTLCompile.cpp:180 reads of the same flag, and the follow-up commits only reworded comments and bounded the test loop. Deferring rather than approving because the behavior involves a plain bool written on the main thread and read on concurrent compiler threads (a pre-existing pattern shared with Debugger::attach) and because the new tests' wall-clock cost against the JSTests 200 ms guidance was not measured here.

[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @Source/JavaScriptCore/runtime/VM.cpp:
- Line 810: Make m_shouldBuildPCToCodeOriginMapping and its accessors in VM use
atomic access or shared synchronization, so runtime updates through
setShouldBuildPCToCodeOriginMapping() cannot race with concurrent FTL
compilation reads.

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• JSTests/stress/sampling-profiler-inlined-top-frame.js
• JSTests/stress/sampling-profiler-start-during-ftl-compilation.js
• Source/JavaScriptCore/ftl/FTLCompile.cpp
• Source/JavaScriptCore/runtime/VM.cpp

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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 nit, I checked the other readers of the flag for the same window the FTL guard closes: Baseline (JIT.cpp:77), DFG (DFGJITCompiler.cpp:57) and LOL read it once into their PCToCodeOriginMapBuilder member and key the map on didBuildMapping(), so they are consistent; the JSTag builder constructor in PCToCodeOriginMap.cpp:111 re-reads the VM flag, but the flag is never cleared, so it cannot see false after FTLCompile.cpp:324 saw true. The graph-free condition at FTLCompile.cpp:180 keys on the same proc->needsPCToOriginMap() as the new guard, so a procedure that kept no origins also keeps no graph for them to point into. The deadline added to sampling-profiler-inlined-top-frame.js in eb1826c covers what I raised earlier.

Extended reasoning...

The change moves setShouldBuildPCToCodeOriginMapping() from the VM constructor into VM::ensureSamplingProfiler(), makes the flag a relaxed std::atomic, adds a proc->needsPCToOriginMap() guard in FTL::compile(), and adds two stress tests; it touches no security-sensitive surface. The one inline finding is a test-convention nit, and the remaining race-window checks across tiers were examined and ruled out as noted in the body.

[claude]

The latest pushes address the points I raised earlier (the deadline in sampling-profiler-inlined-top-frame.js, testLoopCount in sampling-profiler-start-during-ftl-compilation.js), and this run found no bugs in the current head. Since the C++ side changes what compiler threads observe mid-compilation in JavaScriptCore, a human look from someone who knows the JIT tiers is still worthwhile.

What was reviewed:

• Every reader of shouldBuilderPCToCodeOriginMapping(): Baseline, LOL and DFG read it once in the PCToCodeOriginMapBuilder constructor and gate on didBuildMapping(), so a flag flipping mid-compilation only affects FTL, which the new needsPCToOriginMap() guard in FTLCompile.cpp covers; that guard also keeps the DFG graph alive (freeDFGIRAfterLowering is skipped when the procedure needs origins) for the builder's dfgOrigin() walk.
• The relaxed atomic: the flag only goes false to true and nothing is published through it, so the third read in PCToCodeOriginMapBuilder cannot see false after FTL::compile() saw true.
• The new tests: trace.frames[0] is never undefined because the profiler only records a trace when the walk returned at least one frame; the 50 s deadline matches the suite's own samplingProfiler.js helper.

Extended reasoning...

The change moves setShouldBuildPCToCodeOriginMapping() into VM::ensureSamplingProfiler(), makes the backing bool a relaxed std::atomic in VM.h, and adds a needsPCToOriginMap() guard before releasePCToOriginMap() in FTLCompile.cpp, plus two new JSTests/stress tests. It touches no security-sensitive surface (no auth, injection or data exposure), but it does alter what concurrent DFG/FTL compiler threads observe, in paths covered by the repository's CODEOWNERS for Source/JavaScriptCore and JSTests. The bug hunt ran dry with no findings, all of my earlier inline notes were addressed by later commits, and no third-party objection is outstanding, but the CODEOWNERS coverage and the compiler-thread semantics are why this is a defer rather than an approve.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[robobun]

I measured this change against the flaky test with the published binaries (bin/jsc of bun-webkit-linux-amd64-lto). Linux x64, 12 cores, 36 processes at once, the options of the ftl-eager mode:

binary	stress/sampling-profiler-line-column.js fails
autobuild-7d0c8a7d09 (main)	8 of 360
autobuild-preview-pr-804-90271690 (this pull request)	0 of 2,520

Each failure on main runs the full 50 s and ends with Error: Bad stack trace. The test still fails on Test bun-webkit-linux-amd64-lto for changes that do not touch it (run 39376715720 of #849).

The other intermittent failure of that lane from the same day is a different bug: stress/waitasync-wait-infinity-notify-half-multi-workers.js.dfg-eager-no-cjit-validate with exit 134. #882 has it.