105859 - further reduce refcounting of atoms

Status: NEW

(Core :: Layout, defect, P4)

Description

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

The fix for bug 92141 greatly reduced the (threadsafe, and thus expensive)
refcounting of atoms by using a permanent atom implementation for all the atoms
in the atom lists.  However, there are probably a few other key places where we
could use permanent atoms for additional performance gain.

A profile of ibench with that fix should that the top places where we AddRef
atoms are (by timer counts):

                 11 nsFontMetricsGTK::GetLangGroup(nsIAtom**)
                  7 nsLanguageAtom::GetLanguageGroup(nsIAtom**)
                  3 nsNodeInfo::GetNameAtom(nsIAtom*&)
                  1 nsTextFrame::TextStyle::TextStyle(nsIPresContext*, etc.)
                  1 nsNodeInfoManager::GetNodeInfo(nsAString const&, etc.)
                  1 nsXULAttribute::GetValueAsAtom(nsIAtom**)
                  1 AtomKey::AtomKey(AtomKey_base const&)
                  1 NS_NewAtom(unsigned short const*)
                  1 NS_NewAtom(nsAString const&)
                  1 nsFontCache::GetMetricsFor(nsFont const&, nsIAtom*, etc.)

The places where we release atoms are similar.

Three of these hits are associated with CSS classes (the three hard-to-attribute
ones), a few with XUL attribute values, a few are associated with other things,
but many are associated with language groups and fonts.

Comment 1

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

There are also some very commonly used attribute values, such as "true", that
are not in any atom list.  Probably some attribute names, like "hidden" or
"collapsed" (one of them -- I forget which) as well.

Comment 2

[Frank Tang]

Not sure what does this bug really mean. Can you put a simple step-by-step
change here.