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Update comments on the structure of arguments objects in jsobj.h to account for strict mode arguments. r=commentsonlyfix

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This commit is contained in:
Jeff Walden 2010-12-29 16:45:44 -06:00
parent 6107b87ed3
commit e083e5490a
1 changed files with 36 additions and 11 deletions
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47
js/src/jsobj.h
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@ -795,25 +795,50 @@ struct JSObject : js::gc::Cell {
private:
/*
* Reserved slot structure for Arguments objects:
* We represent arguments objects using js_ArgumentsClass and
* js::StrictArgumentsClass. The two are structured similarly, and methods
* valid on arguments objects of one class are also generally valid on
* arguments objects of the other.
*
* Arguments objects of either class store arguments length in a slot:
*
* private - the function's stack frame until the function
* returns; also, JS_ARGUMENTS_OBJECT_ON_TRACE if
* arguments was created on trace
* JSSLOT_ARGS_LENGTH - the number of actual arguments and a flag
* indicating whether arguments.length was
* overwritten. This slot is not used to represent
* arguments.length after that property has been
* assigned, even if the new value is integral: it's
* always the original length.
* JSSLOT_ARGS_DATA - pointer to an ArgumentsData structure containing
* the arguments.callee value or JSVAL_HOLE if that
* was overwritten, and the values of all arguments
* once the function has returned (or as soon as a
* strict arguments object has been created).
*
* Argument index i is stored in ArgumentsData.slots[i], accessible via
* {get,set}ArgsElement().
* Both arguments classes use a slot for storing arguments data:
*
* JSSLOT_ARGS_DATA - pointer to an ArgumentsData structure
*
* ArgumentsData for normal arguments stores the value of arguments.callee,
* as long as that property has not been overwritten. If arguments.callee
* is overwritten, the corresponding value in ArgumentsData is set to
* MagicValue(JS_ARGS_HOLE). Strict arguments do not store this value
* because arguments.callee is a poison pill for strict mode arguments.
*
* The ArgumentsData structure also stores argument values. For normal
* arguments this occurs after the corresponding function has returned, and
* for strict arguments this occurs when the arguments object is created,
* or sometimes shortly after (but not observably so). arguments[i] is
* stored in ArgumentsData.slots[i], accessible via getArgsElement() and
* setArgsElement(). Deletion of arguments[i] overwrites that slot with
* MagicValue(JS_ARGS_HOLE); subsequent redefinition of arguments[i] will
* use a normal property to store the value, ignoring the slot.
*
* Non-strict arguments have a private:
*
* private - the function's stack frame until the function
* returns, when it is replaced with null; also,
* JS_ARGUMENTS_OBJECT_ON_TRACE while on trace, if
* arguments was created on trace
*
* Technically strict arguments have a private, but it's always null.
* Conceptually it would be better to remove this oddity, but preserving it
* allows us to work with arguments objects of either kind more abstractly,
* so we keep it for now.
*/
static const uint32 JSSLOT_ARGS_DATA = 1;