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SpiderMonkey's optimizing JIT, IonMonkey, uses different optimization strategies to speed up various operations. The most typical operations that are relevant for fast program execution are property accesses and function calls.
This page documents the meaning of different optimization outcomes.
General Outcomes
General outcomes shared between various optimization strategies.
GenericFailure
The optimization attempt failed, and the reason was not recorded.
GenericSuccess
Optimization succeeded.
Disabled
The optimization has been explicitly disallowed.
NoTypeInfo
Optimization failed because there was no type information associated with an object containing the property. This failure mode is unlikely and occurs if the target object is obtained in some roundabout way.
NoAnalysisInfo
TODO
NoShapeInfo
The baseline compiler recorded no usable shape information for this operation.
UnknownObject
The type of the object is not known. This can happen if the operation sees many different types of objects, and so the type of the input to the operation cannot be resolved to a single type.
UnknownProperties
Optimization failed because the object containing the property was marked as having unknown properties. This can happen if too many properties are defined on the object, or if delete is used to remove one of the object's properties.
Singleton
One of the types present in the typeset was a singleton type, preventing the optimization from being enabled.
NotSingleton
Optimization failed because the object containing the property did not have a 'singleton' type. Singleton types are assigned to objects that are "one of a kind", such as global objects, literal objects declared in the global scope, and top-level function objects.
NotFixedSlot
The property being accessed is not stored at a known location in the object. This can occur if one of the expected types of objects to be used in this operation has unknown properties, or if different instances of the object store the property at different locations (for example, some instances have the property assigned in a different order than others).
InconsistentFixedSlot
The property being accessed is not stored at a known location in the object. This can occur if the operation is polymorphic on different object types and one or more of the object types contain the property at a different slot than the others.
NotObject
Optimization failed because the stored in the property could potentially be a non-object value. Since only objects can be uniquely typed, the optimization strategy fails in this case.
NotStruct
The object holding the property is not a typed struct object.
NotUnboxed
The object whose property is being accessed is not formatted as an unboxed object.
UnboxedConvertedToNative
The object whose property is being accessed was previously unboxed but was deoptimized and converted to a native object.
StructNoField
The unboxed property being accessed does not correspond to a field on the typed object.
InconsistentFieldType
The type of an unboxed field is not consistent across all the different types of objects it could be accessed from.
InconsistentFieldOffset
The offset of an unboxed field is not consistent across all the different types of objects it could be accessed from.
NeedsTypeBarrier
Optimization failed because somehow the property was accessed in a way that returned a different type than the expected constant. This is an unlikely failure mode, and should not occur.
InDictionaryMode
The object whose property is being accessed is in dictionary mode. Objects which are used in ways that suggest they are hashtables are turned into dictionary objects and their types marked as such.
NoProtoFound
A prototype object was not found for all the object used by this operation.
MultiProtoPaths
Objects used in this operation had differing prototypes.
NonWritableProperty
The property being assigned to is not writable for some types of objects which are used in this operation.
ProtoIndexedProps
The object being accessed has indexed properties that are exotic (for example, defined as a property on a prototype object and left as a hole in the underlying object).
ArrayBadFlags
The array being accessed may have flags that the optimization strategy cannot handle. For example, if the array has sparse indexes, or has indexes that overflow the array's length, the optimization strategy may fail.
ArrayDoubleConversion
The type-system indicates that some arrays at this site should be converted to packed arrays of doubles while others should not. The optimization strategy fails for this condition.
ArrayRange
Could not accurately calculate the range attributes of an inline array creation.
ArraySeenNegativeIndex
Arrays at this element access location have seen negative indexes.
TypedObjectNeutered
The typed object being accessed at this location may have been neutered (a neutered typed object is one where the underlying byte buffer has been removed or transferred to a worker).
TypedObjectArrayRange
Failed to do range check of element access on a typed object.
AccessNotDense
AccessNotSimdObject
The observed type of the target of the property access doesn't guarantee that it is an SIMD object.
AccessNotTypedObject
The observed type of the target of the property access doesn't guarantee that it is a TypedObject.
AccessNotTypedArray
AccessNotString
StaticTypedArrayUint32
StaticTypedArrayCantComputeMask
OutOfBounds
GetElemStringNotCached
NonNativeReceiver
IndexType
SetElemNonDenseNonTANotCached
NoSimdJitSupport
Optimization failed because SIMD JIT support was not enabled.
SimdTypeNotOptimized
The type observed as being retrieved from this property access did not match an optimizable type.
UnknownSimdProperty
The property being accessed on the object is not one of the optimizable property names.
HasCommonInliningPath
Inlined
DOM
Monomorphic
Polymorphic
Inline Cache Outcomes
Outcomes describing inline cache stubs that were generated.
ICOptStub_GenericSuccess
ICGetPropStub_ReadSlot
ICGetPropStub_CallGetter
ICGetPropStub_ArrayLength
ICGetPropStub_UnboxedRead
ICGetPropStub_UnboxedReadExpando
ICGetPropStub_UnboxedArrayLength
ICGetPropStub_TypedArrayLength
ICGetPropStub_DOMProxyShadowed
ICGetPropStub_DOMProxyUnshadowed
ICGetPropStub_GenericProxy
ICGetPropStub_ArgumentsLength
ICSetPropStub_Slot
ICSetPropStub_GenericProxy
ICSetPropStub_DOMProxyShadowed
ICSetPropStub_DOMProxyUnshadowed
ICSetPropStub_CallSetter
ICSetPropStub_AddSlot
ICSetPropStub_SetUnboxed
ICGetElemStub_ReadSlot
ICGetElemStub_CallGetter
ICGetElemStub_ReadUnboxed
ICGetElemStub_Dense
ICGetElemStub_DenseHole
ICGetElemStub_TypedArray
ICGetElemStub_ArgsElement
ICGetElemStub_ArgsElementStrict
ICSetElemStub_Dense
ICSetElemStub_TypedArray
ICNameStub_ReadSlot
ICNameStub_CallGetter
Call Inlining Outcomes
Optimization outcomes of attempts to inline function calls.
CantInlineGeneric
CantInlineClassConstructor
CantInlineExceededDepth
CantInlineExceededTotalBytecodeLength
CantInlineBigCaller
CantInlineBigCallee
CantInlineBigCalleeInlinedBytecodeLength
CantInlineNotHot
CantInlineNotInDispatch
CantInlineNativeBadType
CantInlineNoTarget
Unable to inline function call. The callee (target) function, f in f(x), is not known at JIT time.
CantInlineNotInterpreted
Unable to inline function call. The callee function is not an interpreted function. For example, it could be a native function for which Ion has no built-in specialization.
CantInlineNoBaseline
Unable to inline function call. The interpreted callee function could not be compiled by the Baseline compiler.
CantInlineLazy
Unable to inline function call. The interpreted callee function has a lazy, compiled on-demand script instead of an already compiled script.
CantInlineNotConstructor
Unable to inline function call. Rare. The interpreted callee function is invoked with new but cannot be called as a constructor. Property accessors, Function.prototype, and arrow (=>) functions cannot be called as constructors.
CantInlineDisableIon
Unable to inline function call. The interpreted callee function has been explicitly blacklisted against Ion compilation.
CantInlineTooManyArgs
Unable to inline function call. The interpreted callee function either has too many parameters or is called with too many arguments. These thresholds are subject to change.
CantInlineRecursive
Unable to inline function call. The interpreted callee function recurs for more than one level. The first level of recursion is inlinable.
CantInlineHeavyweight
Unable to inline function call. The interpreted callee function contains variables bindings that are closed over. For example, function f() { var x; return function () { x++; } } closes over x in f.
CantInlineNeedsArgsObj
Unable to inline function call. The interpreted callee function requires an arguments object to be created.
CantInlineDebuggee
Unable to inline function call. The interpreted callee function is being debugged by the Debugger API.
CantInlineUnknownProps
Unable to inline function call. The engine knows nothing definite about the type of the callee function object.
CantInlineUnreachable
Unable to inline function call. The call site has not been observed to have ever been executed. It lacks observed type information for its arguments, its return value, or both.
CantInlineBound
Unable to inline function call. The interpreted callee is a bound function generated from Function.prototype.bind that failed some sub-checks. (expand)
CantInlineNativeNoSpecialization
Unable to inline function call. Ion does not have built-in specialization for the native (implemented in C++) callee function.
CantInlineNativeBadForm
Unable to inline function call. The native callee function was called with an unsupported number of arguments, or calling non-constructing functions with new.
CantInlineBadType
Unable to inline function call. The native callee function was called with arguments with types that the built-in specialization does not support. For example, calling Math functions on objects.
CantInlineNativeNoTemplateObj
Unable to inline function call. Cannot inline a native constructor (e.g., new Array) because no template object was cached by the Baseline compiler. (expand)