722102 - SpiderMonkey should provide debugging names for builtin class instances' reserved slots

Status: RESOLVED INCOMPLETE

(Core :: JavaScript Engine, defect)

Description

[Jim Blandy :jimb]

Attachment: In SpiderMonkey, provide debugging names for built-in classes' reserved slots.

It's nice when the GC can provide proper names for edges in the graph. However, at the moment, objects' reserved slots get named "**UNKNOWN SLOT 0**", which is less than helpful. Proper names also make things like findReferences more useful in tests, because the names can remain stable.

Comment 1

[Jim Blandy :jimb]

Attachment: In SpiderMonkey, provide debugging names for built-in classes' reserved slots.

I think this covers everything.
try: https://tbpl.mozilla.org/?tree=Try&rev=1845f0b22fbf

Comment 2

[Jim Blandy :jimb]

If this patch lands, I'll write the docs for it as well.

Comment 3

[Jim Blandy :jimb]

Attachment: In SpiderMonkey, provide debugging names for built-in classes' reserved slots.

Fixed a test that wasn't checking that findReferences is available.
New try: https://tbpl.mozilla.org/?tree=Try&rev=a412183ca6ec

Comment 4

[Jim Blandy :jimb]

Comment on attachment 595079 [details] [diff] [review]
In SpiderMonkey, provide debugging names for built-in classes' reserved slots.

Review of attachment 595079 [details] [diff] [review]:
-----------------------------------------------------------------

Try looks good.

Comment 5

[Jeff Walden [:Waldo]]

Comment on attachment 595079 [details] [diff] [review]
In SpiderMonkey, provide debugging names for built-in classes' reserved slots.

Review of attachment 595079 [details] [diff] [review]:
-----------------------------------------------------------------

The dynamicity of implementation here smells funny, given that reserved slots are a purely static function of object class.  Can't this just be encoded in a static const array per class, indexed by slot number, bounds-checked by JSSLOT_FREE(clasp)?  This would require a little trickiness if this is supposed to be part of the public API, but not much at all.  Could you try something along those lines, please?

Comment 6

[Michael Wu [:mwu]]

This is possibly interesting for magic DOM since it uses a lot of reserved slots.

A static const array per class is enough for many but not all cases. It's possible to have something that's effectively a union, so the names of the slots would change based on the content of some other slot. Not having support for that isn't a big problem for most of my cases though - those kinds of slots seem rare.