JavaScript Runtimes

JSC Options.md

Option	Default	Description
validateOptions	false	crashes if mis-typed JSC options were passed to the VM
dumpOptions	0	dumps JSC options (0 = None, 1 = Overridden only, 2 = All, 3 = Verbose)
configFile	""	file to configure JSC options and logging location
useLLInt	true	allows the LLINT to be used if true
useJIT	true	allows the executable pages to be allocated for JIT and thunks if true
useBaselineJIT	true	allows the baseline JIT to be used if true
useDFGJIT	true	allows the DFG JIT to be used if true
useRegExpJIT	true	allows the RegExp JIT to be used if true
useDOMJIT	true	allows the DOMJIT to be used if true
reportMustSucceedExecutableAllocations	false
maxPerThreadStackUsage	4194304	Max allowed stack usage by the VM
softReservedZoneSize	131072	A buffer greater than reservedZoneSize that reserves space for stringifying exceptions.
reservedZoneSize	65536	The amount of stack space we guarantee to our clients (and to interal VM code that does not call out to clients).
crashIfCantAllocateJITMemory	false
jitMemoryReservationSize	0	Set this number to change the executable allocation size in ExecutableAllocatorFixedVMPool. (In bytes.)
useSeparatedWXHeap	false
forceCodeBlockLiveness	false
forceICFailure	false
repatchCountForCoolDown	8
initialCoolDownCount	20
repatchBufferingCountdown	8
dumpGeneratedBytecodes	false
dumpBytecodeLivenessResults	false
validateBytecode	false
forceDebuggerBytecodeGeneration	false
dumpBytecodesBeforeGeneratorification	false
useFunctionDotArguments	true
useTailCalls	true
optimizeRecursiveTailCalls	true
alwaysUseShadowChicken	false
shadowChickenLogSize	1000
shadowChickenMaxTailDeletedFramesSize	128
dumpDisassembly	false	dumps disassembly of all JIT compiled code upon compilation
asyncDisassembly	false
dumpDFGDisassembly	false	dumps disassembly of DFG function upon compilation
dumpFTLDisassembly	false	dumps disassembly of FTL function upon compilation
dumpAllDFGNodes	false
bytecodeRangeToJITCompile		bytecode size range to allow compilation on, e.g. 1:100
bytecodeRangeToDFGCompile		bytecode size range to allow DFG compilation on, e.g. 1:100
bytecodeRangeToFTLCompile		bytecode size range to allow FTL compilation on, e.g. 1:100
jitWhitelist	""	file with list of function signatures to allow compilation on
dfgWhitelist	""	file with list of function signatures to allow DFG compilation on
ftlWhitelist	""	file with list of function signatures to allow FTL compilation on
dumpSourceAtDFGTime	false	dumps source code of JS function being DFG compiled
dumpBytecodeAtDFGTime	false	dumps bytecode of JS function being DFG compiled
dumpGraphAfterParsing	false
dumpGraphAtEachPhase	false
dumpDFGGraphAtEachPhase	false	dumps the DFG graph at each phase of DFG compilation (note this excludes DFG graphs during FTL compilation)
dumpDFGFTLGraphAtEachPhase	false	dumps the DFG graph at each phase of DFG compilation when compiling FTL code
dumpB3GraphAtEachPhase	false	dumps the B3 graph at each phase of compilation
dumpAirGraphAtEachPhase	false	dumps the Air graph at each phase of compilation
verboseDFGBytecodeParsing	false
safepointBeforeEachPhase	true
verboseCompilation	false
verboseFTLCompilation	false
logCompilationChanges	false
useProbeOSRExit	false
printEachOSRExit	false
validateGraph	false
validateGraphAtEachPhase	false
verboseValidationFailure	false
verboseOSR	false
verboseDFGOSRExit	false
verboseFTLOSRExit	false
verboseCallLink	false
verboseCompilationQueue	false
reportCompileTimes	false	dumps JS function signature and the time it took to compile in all tiers
reportBaselineCompileTimes	false	dumps JS function signature and the time it took to BaselineJIT compile
reportDFGCompileTimes	false	dumps JS function signature and the time it took to DFG and FTL compile
reportFTLCompileTimes	false	dumps JS function signature and the time it took to FTL compile
reportTotalCompileTimes	false
reportParseTimes	false	dumps JS function signature and the time it took to parse
reportBytecodeCompileTimes	false	dumps JS function signature and the time it took to bytecode compile
verboseExitProfile	false
verboseCFA	false
verboseDFGFailure	false
verboseFTLToJSThunk	false
verboseFTLFailure	false
alwaysComputeHash	false
testTheFTL	false
verboseSanitizeStack	false
useGenerationalGC	true
useConcurrentBarriers	true
useConcurrentGC	true
collectContinuously	false
collectContinuouslyPeriodMS	1
forceFencedBarrier	false
verboseVisitRace	false
optimizeParallelSlotVisitorsForStoppedMutator	false
largeHeapSize	33554432
smallHeapSize	1048576
smallHeapRAMFraction	0.25
smallHeapGrowthFactor	2
mediumHeapRAMFraction	0.5
mediumHeapGrowthFactor	1.5
largeHeapGrowthFactor	1.24
miniVMHeapGrowthFactor	1.27
criticalGCMemoryThreshold	0.8	percent memory in use the GC considers critical. The collector is much more aggressive above this threshold
minimumMutatorUtilization	0
maximumMutatorUtilization	0.7
epsilonMutatorUtilization	0.01
concurrentGCMaxHeadroom	1.5
concurrentGCPeriodMS	2
useStochasticMutatorScheduler	true
minimumGCPauseMS	0.3
gcPauseScale	0.3
gcIncrementBytes	10000
gcIncrementMaxBytes	100000
gcIncrementScale	0
scribbleFreeCells	false
sizeClassProgression	1.4
largeAllocationCutoff	100000
dumpSizeClasses	false
useBumpAllocator	true
stealEmptyBlocksFromOtherAllocators	true
tradeDestructorBlocks	true
eagerlyUpdateTopCallFrame	false
useOSREntryToDFG	true
useOSREntryToFTL	true
useFTLJIT	true	allows the FTL JIT to be used if true
useFTLTBAA	true
validateFTLOSRExitLiveness	false
defaultB3OptLevel	2
b3AlwaysFailsBeforeCompile	false
b3AlwaysFailsBeforeLink	false
ftlCrashes	false
clobberAllRegsInFTLICSlowPath	false
enableJITDebugAssertions	false
useAccessInlining	true
maxAccessVariantListSize	8
usePolyvariantDevirtualization	true
usePolymorphicAccessInlining	true
usePolymorphicCallInlining	true
usePolymorphicCallInliningForNonStubStatus	false
maxPolymorphicCallVariantListSize	15
maxPolymorphicCallVariantListSizeForTopTier	5
maxPolymorphicCallVariantListSizeForWebAssemblyToJS	5
maxPolymorphicCallVariantsForInlining	5
frequentCallThreshold	2
minimumCallToKnownRate	0.51
createPreHeaders	true
useMovHintRemoval	true
usePutStackSinking	true
useObjectAllocationSinking	true
useArityFixupInlining	true
logExecutableAllocation	false
useConcurrentJIT	true	allows the DFG / FTL compilation in threads other than the executing JS thread
numberOfDFGCompilerThreads	2
numberOfFTLCompilerThreads	7
priorityDeltaOfDFGCompilerThreads	0
priorityDeltaOfFTLCompilerThreads	0
priorityDeltaOfWasmCompilerThreads	0
useProfiler	false
disassembleBaselineForProfiler	true
useArchitectureSpecificOptimizations	true
breakOnThrow	false
maximumOptimizationCandidateInstructionCount	100000
maximumFunctionForCallInlineCandidateInstructionCount	190
maximumFunctionForClosureCallInlineCandidateInstructionCount	100
maximumFunctionForConstructInlineCandidateInstructionCount	100
maximumFTLCandidateInstructionCount	20000
maximumInliningDepth	5	maximum allowed inlining depth. Depth of 1 means no inlining
maximumInliningRecursion	2
maximumInliningCallerSize	10000
maximumVarargsForInlining	100
usePolyvariantCallInlining	true
usePolyvariantByIdInlining	true
useMaximalFlushInsertionPhase	false	Setting to true allows the DFG's MaximalFlushInsertionPhase to run.
maximumBinaryStringSwitchCaseLength	50
maximumBinaryStringSwitchTotalLength	2000
jitPolicyScale	1	scale JIT thresholds to this specified ratio between 0.0 (compile ASAP) and 1.0 (compile like normal).
forceEagerCompilation	false
thresholdForJITAfterWarmUp	500
thresholdForJITSoon	100
thresholdForOptimizeAfterWarmUp	1000
thresholdForOptimizeAfterLongWarmUp	1000
thresholdForOptimizeSoon	1000
executionCounterIncrementForLoop	1
executionCounterIncrementForEntry	15
thresholdForFTLOptimizeAfterWarmUp	100000
thresholdForFTLOptimizeSoon	1000
ftlTierUpCounterIncrementForLoop	1
ftlTierUpCounterIncrementForReturn	15
ftlOSREntryFailureCountForReoptimization	15
ftlOSREntryRetryThreshold	100
evalThresholdMultiplier	10
maximumEvalCacheableSourceLength	256
randomizeExecutionCountsBetweenCheckpoints	false
maximumExecutionCountsBetweenCheckpointsForBaseline	1000
maximumExecutionCountsBetweenCheckpointsForUpperTiers	50000
likelyToTakeSlowCaseMinimumCount	20
couldTakeSlowCaseMinimumCount	10
osrExitCountForReoptimization	100
osrExitCountForReoptimizationFromLoop	5
reoptimizationRetryCounterMax	21	(default: 0)
minimumOptimizationDelay	1
maximumOptimizationDelay	5
desiredProfileLivenessRate	0.75
desiredProfileFullnessRate	0.35
doubleVoteRatioForDoubleFormat	2
structureCheckVoteRatioForHoisting	1
checkArrayVoteRatioForHoisting	1
maximumDirectCallStackSize	200
minimumNumberOfScansBetweenRebalance	100
numberOfGCMarkers	8
useParallelMarkingConstraintSolver	true
opaqueRootMergeThreshold	1000
minHeapUtilization	0.8
minMarkedBlockUtilization	0.9
slowPathAllocsBetweenGCs	0	force a GC on every Nth slow path alloc, where N is specified by this option
percentCPUPerMBForFullTimer	0.0003125
percentCPUPerMBForEdenTimer	0.0025
collectionTimerMaxPercentCPU	0.05
forceWeakRandomSeed	false
forcedWeakRandomSeed	0
useZombieMode	false	debugging option to scribble over dead objects with 0xbadbeef0
useImmortalObjects	false	debugging option to keep all objects alive forever
sweepSynchronously	false	debugging option to sweep all dead objects synchronously at GC end before resuming mutator
logGC	None	debugging option to log GC activity (0 = None, 1 = Basic, 2 = Verbose)
useGC	true
gcAtEnd	false	If true, the jsc CLI will do a GC before exiting
forceGCSlowPaths	false	If true, we will force all JIT fast allocations down their slow paths.
gcMaxHeapSize	0
forceRAMSize	0
recordGCPauseTimes	false
logHeapStatisticsAtExit	false
forceCodeBlockToJettisonDueToOldAge	false	If true, this means that anytime we can jettison a CodeBlock due to old age, we do.
useEagerCodeBlockJettisonTiming	false	If true, the time slices for jettisoning a CodeBlock due to old age are shrunk significantly.
useTypeProfiler	false
useControlFlowProfiler	false
useSamplingProfiler	false
sampleInterval	1000	Time between stack traces in microseconds.
collectSamplingProfilerDataForJSCShell	false	This corresponds to the JSC shell's --sample option.
samplingProfilerTopFunctionsCount	12	Number of top functions to report when using the command line interface.
samplingProfilerTopBytecodesCount	40	Number of top bytecodes to report when using the command line interface.
samplingProfilerPath	""	The path to the directory to write sampiling profiler output to. This probably will not work with WK2 unless the path is in the whitelist.
sampleCCode	false	Causes the sampling profiler to record profiling data for C frames.
alwaysGeneratePCToCodeOriginMap	false	This will make sure we always generate a PCToCodeOriginMap for JITed code.
verifyHeap	false
numberOfGCCyclesToRecordForVerification	3
exceptionStackTraceLimit	100	Stack trace limit for internal Exception object
defaultErrorStackTraceLimit	100	The default value for Error.stackTraceLimit
useExceptionFuzz	false
fireExceptionFuzzAt	0
validateDFGExceptionHandling	false	Causes the DFG to emit code validating exception handling for each node that can exit
dumpSimulatedThrows	false	Dumps the call stack of the last simulated throw if exception scope verification fails
validateExceptionChecks	false	Verifies that needed exception checks are performed.
unexpectedExceptionStackTraceLimit	100	Stack trace limit for debugging unexpected exceptions observed in the VM
useExecutableAllocationFuzz	false
fireExecutableAllocationFuzzAt	0
fireExecutableAllocationFuzzAtOrAfter	0
verboseExecutableAllocationFuzz	false
useOSRExitFuzz	false
fireOSRExitFuzzAtStatic	0
fireOSRExitFuzzAt	0
fireOSRExitFuzzAtOrAfter	0
logPhaseTimes	false
rareBlockPenalty	0.001
airLinearScanVerbose	false
airLinearScanSpillsEverything	false
airForceBriggsAllocator	false
airForceIRCAllocator	false
airRandomizeRegs	false
coalesceSpillSlots	true
logAirRegisterPressure	false
useB3TailDup	true
maxB3TailDupBlockSize	3
maxB3TailDupBlockSuccessors	3
useDollarVM	false	installs the $vm debugging tool in global objects
functionOverrides	""	file with debugging overrides for function bodies
useSigillCrashAnalyzer	false	logs data about SIGILL crashes
watchdog	0	watchdog timeout (0 = Disabled, N = a timeout period of N milliseconds)
usePollingTraps	false	use polling (instead of signalling) VM traps
useMachForExceptions	true	Use mach exceptions rather than signals to handle faults and pass thread messages. (This does nothing on platforms without mach)
useICStats	false
prototypeHitCountForLLIntCaching	2	Number of prototype property hits before caching a prototype in the LLInt. A count of 0 means never cache.
dumpCompiledRegExpPatterns	false
dumpModuleRecord	false
dumpModuleLoadingState	false
exposeInternalModuleLoader	false	expose the internal module loader object to the global space for debugging
useSuperSampler	false
useSourceProviderCache	true	If false, the parser will not use the source provider cache. It's good to verify everything works when this is false. Because the cache is so successful, it can mask bugs.
useCodeCache	true	If false, the unlinked byte code cache will not be used.
useWebAssembly	true	Expose the WebAssembly global object.
enableSpectreMitigations	true	Enable Spectre mitigations.
enableSpectreGadgets	false	enable gadgets to test Spectre mitigations.
usePoisoning	true	Poison is randomized at load time when true, and initialized to 0 if false which defeats some Spectre and type confusion mitigations, but allows tools such as leak detectors to function better.
zeroStackFrame	false	Zero stack frame on entry to a function.
useAsyncIterator	true	Allow to use Async Iterator in JS.
failToCompileWebAssemblyCode	false	If true, no Wasm::Plan will sucessfully compile a function.
webAssemblyPartialCompileLimit	5000	Limit on the number of bytes a Wasm::Plan::compile should attempt before checking for other work.
webAssemblyBBQOptimizationLevel	1	B3 Optimization level for BBQ Web Assembly module compilations.
webAssemblyOMGOptimizationLevel	2	B3 Optimization level for OMG Web Assembly module compilations.
useBBQTierUpChecks	true	Enables tier up checks for our BBQ code.
webAssemblyOMGTierUpCount	5000	The countdown before we tier up a function to OMG.
webAssemblyLoopDecrement	15	The amount the tier up countdown is decremented on each loop backedge.
webAssemblyFunctionEntryDecrement	1	The amount the tier up countdown is decremented on each function entry.
useWebAssemblyFastMemory	true	If true, we will try to use a 32-bit address space with a signal handler to bounds check wasm memory.
logWebAssemblyMemory	false
webAssemblyFastMemoryRedzonePages	128	WebAssembly fast memories use 4GiB virtual allocations, plus a redzone (counted as multiple of 64KiB WebAssembly pages) at the end to catch reg+imm accesses which exceed 32-bit, anything beyond the redzone is explicitly bounds-checked
crashIfWebAssemblyCantFastMemory	false	If true, we will crash if we can't obtain fast memory for wasm.
maxNumWebAssemblyFastMemories	4
useFastTLSForWasmContext	true	If true, we will store context in fast TLS. If false, we will pin it to a register.
useWebAssemblyStreamingApi	false	Allow to run WebAssembly's Streaming API
useCallICsForWebAssemblyToJSCalls	true	If true, we will use CallLinkInfo to inline cache Wasm to JS calls.
useEagerWebAssemblyModuleHashing	false	Unnamed WebAssembly modules are identified in backtraces through their hash, if available.
useObjectRestSpread	true	If true, we will enable Object Rest/Spread feature.
useBigInt	false	If true, we will enable BigInt support.
useIntlNumberFormatToParts	false	If true, we will enable Intl.NumberFormat.prototype.formatToParts
useIntlPluralRules	false	If true, we will enable Intl.PluralRules.
useArrayAllocationProfiling	true	If true, we will use our normal array allocation profiling. If false, the allocation profile will always claim to be undecided.
forcePolyProto	false	If true, create_this will always create an object with a poly proto structure.
forceMiniVMMode	false	If true, it will force mini VM mode on.
useTracePoints	false