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Bug 852229 - Add a comment explaining zones (r=jorendorff)

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Bill McCloskey 2013-03-18 11:10:23 -07:00
parent 0262903d6f
commit 48a54a9fd5
1 changed files with 46 additions and 0 deletions
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46
js/src/gc/Zone.h
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@ -57,6 +57,52 @@ typedef Vector<JSCompartment *, 1, SystemAllocPolicy> CompartmentVector;
namespace JS {
/*
* A zone is a collection of compartments. Every compartment belongs to exactly
* one zone. In Firefox, there is roughly one zone per tab along with a system
* zone for everything else. Zones mainly serve as boundaries for garbage
* collection. Unlike compartments, they have no special security properties.
*
* Every GC thing belongs to exactly one zone. GC things from the same zone but
* different compartments can share an arena (4k page). GC things from different
* zones cannot be stored in the same arena. The garbage collector is capable of
* collecting one zone at a time; it cannot collect at the granularity of
* compartments.
*
* GC things are tied to zones and compartments as follows:
*
* - JSObjects belong to a compartment and cannot be shared between
* compartments. If an object needs to point to a JSObject in a different
* compartment, regardless of zone, it must go through a cross-compartment
* wrapper. Each compartment keeps track of its outgoing wrappers in a table.
*
* - JSStrings do not belong to any particular compartment, but they do belong
* to a zone. Thus, two different compartments in the same zone can point to a
* JSString. When a string needs to be wrapped, we copy it if it's in a
* different zone and do nothing if it's in the same zone. Thus, transferring
* strings within a zone is very efficient.
*
* - Shapes and base shapes belong to a compartment and cannot be shared between
* compartments. A base shape holds a pointer to its compartment. Shapes find
* their compartment via their base shape. JSObjects find their compartment
* via their shape.
*
* - Scripts are also compartment-local and cannot be shared. A script points to
* its compartment.
*
* - Type objects and IonCode objects belong to a compartment and cannot be
* shared. However, there is no mechanism to obtain their compartments.
*
* A zone remains alive as long as any GC things in the zone are alive. A
* compartment remains alive as long as any JSObjects, scripts, shapes, or base
* shapes within it are alive.
*
* We always guarantee that a zone has at least one live compartment by refusing
* to delete the last compartment in a live zone. (This could happen, for
* example, if the conservative scanner marks a string in an otherwise dead
* zone.)
*/
struct Zone : private JS::shadow::Zone, public js::gc::GraphNodeBase<JS::Zone>
{
JSRuntime *rt;