728255 - Replace nsLineBox::LastChild() calls with O(1) equivalents where possible

Status: RESOLVED FIXED

(Core :: Layout: Block and Inline, defect)

Description

[Mats Palmgren (inactive)]

In general we don't need to use nsLineBox::LastChild(), which is O(n)
on the number of frames in the line, except for the last line of the
overflow lines list.

1. the last frame of the last normal line is the same as
   mFrames.LastChild() which is O(1)
2. for any line that isn't the last line, the next line's
   mFirstChild->GetPrevSibling() is our last frame, also O(1)
3. for the last overflow line we must still use nsLineBox::LastChild()
   (this looks like a rare case though)

These invariants holds except in very rare cases when splitting a line
and messing with the mFirstChild / mChildCount.  nsLineBox::LastChild()
probably isn't correct in those cases either.  Anyway, that shouldn't
be the case for the places I'm fixing.

Comment 1

[Mats Palmgren (inactive)]

Attachment: fix

Try run is green:
https://tbpl.mozilla.org/?tree=Try&rev=25542a0f20f3

Comment 2

[Boris Zbarsky [:bzbarsky]]

Comment on attachment 598256 [details] [diff] [review]
fix

r=me

Comment 3

[Mats Palmgren (inactive)]

https://hg.mozilla.org/integration/mozilla-inbound/rev/aaed6b9c8952

Comment 4

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/aaed6b9c8952