919349 - JS debugger: remove uses of 'delete' that put objects in dictionary mode gratuitously

Status: RESOLVED FIXED

(DevTools :: Debugger, defect)

Description

[Jim Blandy :jimb]

The built-in JavaScript debugger code uses 'delete' in several places in a way that wastes memory; those uses could be replaced with assignments that set the given properties to 'null' or 'undefined'.

Deleting properties from objects puts them in 'dictionary mode'. Objects in dictionary mode do not share shape metadata with other similar objects. For objects being used as hash tables, this is fine; such sharing would not be helpful anyway. But for an object of which the program will use only a small, fixed vocabulary of properties, sharing that metadata can save memory, and enable optimizations like inline caches. If the intention is simply to help the GC free the property's referent, it's better to simply set the property to 'null' or 'undefined'.

Comment 1

[Jim Blandy :jimb]

Attachment: Remove infelicitous uses of 'delete' from the JavaScript debugger server.

Comment 2

[Jim Blandy :jimb]

https://hg.mozilla.org/integration/mozilla-inbound/rev/93f1537b3a9d

Comment 3

[Phil Ringnalda (:philor)]

https://hg.mozilla.org/mozilla-central/rev/93f1537b3a9d