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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* vim: set ts=8 sts=4 et sw=4 tw=99:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef vm_ArgumentsObject_h
#define vm_ArgumentsObject_h
#include "mozilla/MemoryReporting.h"
#include "gc/Barrier.h"
#include "vm/NativeObject.h"
namespace js {
class AbstractFramePtr;
class ScriptFrameIter;
namespace jit {
class JitFrameLayout;
} // namespace jit
* ArgumentsData stores the initial indexed arguments provided to the
* corresponding and that function itself. It is used to store arguments[i]
* and arguments.callee -- up until the corresponding property is modified,
* when the relevant value is flagged to memorialize the modification.
struct ArgumentsData
* numArgs = Max(numFormalArgs, numActualArgs)
* The array 'args' has numArgs elements.
uint32_t numArgs;
/* Size of ArgumentsData and data allocated after it. */
uint32_t dataBytes;
* arguments.callee, or MagicValue(JS_OVERWRITTEN_CALLEE) if
* arguments.callee has been modified.
HeapValue callee;
/* The script for the function containing this arguments object. */
JSScript* script;
* Pointer to an array of bits indicating, for every argument in 'slots',
* whether the element has been deleted. See isElementDeleted comment.
size_t* deletedBits;
* This array holds either the current argument value or the magic
* forwarding value. The latter means that the function has both a
* CallObject and an ArgumentsObject AND the particular formal variable is
* aliased by the CallObject. In such cases, the CallObject holds the
* canonical value so any element access to the arguments object should load
* the value out of the CallObject (which is pointed to by MAYBE_CALL_SLOT).
HeapValue args[1];
/* For jit use: */
static ptrdiff_t offsetOfArgs() { return offsetof(ArgumentsData, args); }
/* Iterate args. */
HeapValue* begin() { return args; }
const HeapValue* begin() const { return args; }
HeapValue* end() { return args + numArgs; }
const HeapValue* end() const { return args + numArgs; }
// Maximum supported value of arguments.length. This bounds the maximum
// number of arguments that can be supplied to Function.prototype.apply.
// This value also bounds the number of elements parsed in an array
// initializer.
static const unsigned ARGS_LENGTH_MAX = 500 * 1000;
* ArgumentsObject instances represent |arguments| objects created to store
* function arguments when a function is called. It's expensive to create such
* objects if they're never used, so they're only created when they are
* potentially used.
* Arguments objects are complicated because, for non-strict mode code, they
* must alias any named arguments which were provided to the function. Gnarly
* example:
* function f(a, b, c, d)
* {
* arguments[0] = "seta";
* assertEq(a, "seta");
* b = "setb";
* assertEq(arguments[1], "setb");
* c = "setc";
* assertEq(arguments[2], undefined);
* arguments[3] = "setd";
* assertEq(d, undefined);
* }
* f("arga", "argb");
* ES5's strict mode behaves more sanely, and named arguments don't alias
* elements of an arguments object.
* ArgumentsObject instances use the following reserved slots:
* Stores the initial value of arguments.length, plus a bit indicating
* whether arguments.length has been modified. Use initialLength() and
* hasOverriddenLength() to access these values. If arguments.length has
* been modified, then the current value of arguments.length is stored in
* another slot associated with a new property.
* Stores an ArgumentsData*, described above.
class ArgumentsObject : public NativeObject
static const uint32_t INITIAL_LENGTH_SLOT = 0;
static const uint32_t DATA_SLOT = 1;
static const uint32_t MAYBE_CALL_SLOT = 2;
static const uint32_t LENGTH_OVERRIDDEN_BIT = 0x1;
static const uint32_t PACKED_BITS_COUNT = 1;
template <typename CopyArgs>
static ArgumentsObject* create(JSContext* cx, HandleFunction callee, unsigned numActuals,
CopyArgs& copy);
ArgumentsData* data() const {
return reinterpret_cast<ArgumentsData*>(getFixedSlot(DATA_SLOT).toPrivate());
static bool obj_delProperty(JSContext* cx, HandleObject obj, HandleId id,
ObjectOpResult& result);
static const uint32_t RESERVED_SLOTS = 3;
static const gc::AllocKind FINALIZE_KIND = gc::AllocKind::OBJECT4_BACKGROUND;
/* Create an arguments object for a frame that is expecting them. */
static ArgumentsObject* createExpected(JSContext* cx, AbstractFramePtr frame);
* Purposefully disconnect the returned arguments object from the frame
* by always creating a new copy that does not alias formal parameters.
* This allows function-local analysis to determine that formals are
* not aliased and generally simplifies arguments objects.
static ArgumentsObject* createUnexpected(JSContext* cx, ScriptFrameIter& iter);
static ArgumentsObject* createUnexpected(JSContext* cx, AbstractFramePtr frame);
static ArgumentsObject* createForIon(JSContext* cx, jit::JitFrameLayout* frame,
HandleObject scopeChain);
static ArgumentsObject* createTemplateObject(JSContext* cx, bool mapped);
* Return the initial length of the arguments. This may differ from the
* current value of arguments.length!
uint32_t initialLength() const {
uint32_t argc = uint32_t(getFixedSlot(INITIAL_LENGTH_SLOT).toInt32()) >> PACKED_BITS_COUNT;
return argc;
/* The script for the function containing this arguments object. */
JSScript* containingScript() const {
return data()->script;
/* True iff arguments.length has been assigned or its attributes changed. */
bool hasOverriddenLength() const {
const Value& v = getFixedSlot(INITIAL_LENGTH_SLOT);
return v.toInt32() & LENGTH_OVERRIDDEN_BIT;
void markLengthOverridden() {
uint32_t v = getFixedSlot(INITIAL_LENGTH_SLOT).toInt32() | LENGTH_OVERRIDDEN_BIT;
setFixedSlot(INITIAL_LENGTH_SLOT, Int32Value(v));
* Because the arguments object is a real object, its elements may be
* deleted. This is implemented by setting a 'deleted' flag for the arg
* which is read by argument object resolve and getter/setter hooks.
* NB: an element, once deleted, stays deleted. Thus:
* function f(x) { delete arguments[0]; arguments[0] = 42; return x }
* assertEq(f(1), 1);
* This works because, once a property is deleted from an arguments object,
* it gets regular properties with regular getters/setters that don't alias
* ArgumentsData::slots.
bool isElementDeleted(uint32_t i) const {
MOZ_ASSERT(i < data()->numArgs);
if (i >= initialLength())
return false;
return IsBitArrayElementSet(data()->deletedBits, initialLength(), i);
bool isAnyElementDeleted() const {
return IsAnyBitArrayElementSet(data()->deletedBits, initialLength());
void markElementDeleted(uint32_t i) {
SetBitArrayElement(data()->deletedBits, initialLength(), i);
* An ArgumentsObject serves two roles:
* - a real object, accessed through regular object operations, e.g..,
* GetElement corresponding to 'arguments[i]';
* - a VM-internal data structure, storing the value of arguments (formal
* and actual) that are accessed directly by the VM when a reading the
* value of a formal parameter.
* There are two ways to access the ArgumentsData::args corresponding to
* these two use cases:
* - object access should use elements(i) which will take care of
* forwarding when the value is the magic forwarding value;
* - VM argument access should use arg(i) which will assert that the
* value is not the magic forwarding value (since, if such forwarding was
* needed, the frontend should have emitted JSOP_GETALIASEDVAR).
const Value& element(uint32_t i) const;
inline void setElement(JSContext* cx, uint32_t i, const Value& v);
const Value& arg(unsigned i) const {
MOZ_ASSERT(i < data()->numArgs);
const Value& v = data()->args[i];
return v;
void setArg(unsigned i, const Value& v) {
MOZ_ASSERT(i < data()->numArgs);
HeapValue& lhs = data()->args[i];
lhs = v;
* Attempt to speedily and efficiently access the i-th element of this
* arguments object. Return true if the element was speedily returned.
* Return false if the element must be looked up more slowly using
* getProperty or some similar method. The second overload copies the
* elements [start, start + count) into the locations starting at 'vp'.
* NB: Returning false does not indicate error!
bool maybeGetElement(uint32_t i, MutableHandleValue vp) {
if (i >= initialLength() || isElementDeleted(i))
return false;
return true;
inline bool maybeGetElements(uint32_t start, uint32_t count, js::Value* vp);
* Measures things hanging off this ArgumentsObject that are counted by the
* |miscSize| argument in JSObject::sizeOfExcludingThis().
size_t sizeOfMisc(mozilla::MallocSizeOf mallocSizeOf) const {
return mallocSizeOf(data());
size_t sizeOfData() const {
return data()->dataBytes;
static void finalize(FreeOp* fop, JSObject* obj);
static void trace(JSTracer* trc, JSObject* obj);
static size_t objectMovedDuringMinorGC(JSTracer* trc, JSObject* dst, JSObject* src);
/* For jit use: */
static size_t getDataSlotOffset() {
return getFixedSlotOffset(DATA_SLOT);
static size_t getInitialLengthSlotOffset() {
return getFixedSlotOffset(INITIAL_LENGTH_SLOT);
static Value MagicScopeSlotValue(uint32_t slot) {
// When forwarding slots to a backing CallObject, the slot numbers are
// stored as uint32 magic values. This raises an ambiguity if we have
// also copied JS_OPTIMIZED_OUT magic from a JIT frame or
// JS_UNINITIALIZED_LEXICAL magic on the CallObject. To distinguish
// normal magic values (those with a JSWhyMagic) and uint32 magic
// values, we add the maximum JSWhyMagic value to the slot
// number. This is safe as ARGS_LENGTH_MAX is well below UINT32_MAX.
return JS::MagicValueUint32(slot + JS_WHY_MAGIC_COUNT);
static uint32_t SlotFromMagicScopeSlotValue(const Value& v) {
return v.magicUint32() - JS_WHY_MAGIC_COUNT;
static bool IsMagicScopeSlotValue(const Value& v) {
return v.isMagic() && v.magicUint32() > JS_WHY_MAGIC_COUNT;
static void MaybeForwardToCallObject(AbstractFramePtr frame, ArgumentsObject* obj,
ArgumentsData* data);
static void MaybeForwardToCallObject(jit::JitFrameLayout* frame, HandleObject callObj,
ArgumentsObject* obj, ArgumentsData* data);
class MappedArgumentsObject : public ArgumentsObject
static const Class class_;
* Stores arguments.callee, or MagicValue(JS_ARGS_HOLE) if the callee has
* been cleared.
const js::Value& callee() const {
return data()->callee;
/* Clear the location storing arguments.callee's initial value. */
void clearCallee() {
data()->callee = MagicValue(JS_OVERWRITTEN_CALLEE);
static bool obj_enumerate(JSContext* cx, HandleObject obj);
static bool obj_resolve(JSContext* cx, HandleObject obj, HandleId id, bool* resolvedp);
class UnmappedArgumentsObject : public ArgumentsObject
static const Class class_;
static bool obj_enumerate(JSContext* cx, HandleObject obj);
static bool obj_resolve(JSContext* cx, HandleObject obj, HandleId id, bool* resolvedp);
} // namespace js
inline bool
JSObject::is<js::ArgumentsObject>() const
return is<js::MappedArgumentsObject>() || is<js::UnmappedArgumentsObject>();
#endif /* vm_ArgumentsObject_h */