697931 - Remove KidHashes when possible

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Nicholas Nethercote [inactive]]

Attachment: patch

Looking at Gmail and TechCrunch I see that 13% of all KidHashes contain 0 or
1 elements.  (If you weight KidHashes by their capacity, it's about 10% of
the space used by KidsHash entries.)  This can happen because
Shape::removeChild() does not convert HASH form KidsPointers back to SHAPE
form when possible.

This patch implements the conversion.  This saves roughly 30--40KB for a
page like Gmail.  It also adds capacity() to Hash{Table,Map,Set}, which
aren't used in the patch but I used them for measurement and they seem like
useful additions in general.

Comment 1

[Luke Wagner [:luke]]

Comment on attachment 570177 [details] [diff] [review]
patch

Looks reasonable.  I was initially worried about thrashing back 'n forth but then I remembered that Shape::removeChild is only called from GC.

Comment 2

[Nicholas Nethercote [inactive]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/127d6851a322

Comment 3

[Matt Brubeck (:mbrubeck)]

https://hg.mozilla.org/mozilla-central/rev/127d6851a322

Comment 4

[Brendan Eich [:brendan]]

Thanks, njn -- I should have caught this transition and covered it in the landing last summer.

/be