1927062 - Optimize bidi [re-]resolution for long continuation chains

Status: RESOLVED FIXED

(Core :: Layout, enhancement)

Description

[Jonathan Kew [:jfkthame]]

nsBidiPresUtils::ResolveParagraph may become terribly expensive when there are very long continuation chains involved, e.g. in bug 1925916.

As ResolveParagraph passes through the text, it calls RemoveBidiContinuation to clear any pre-existing non-fluid continuation that may be present when it reaches the end of a textframe but the direction run continues. Note, however, that RemoveBidiContinuation is called here even if the next continuation is already fluid (i.e. not a bidi continuation). That seems unnecessary.

It wouldn't matter much, except that one of the things RemoveBidiContinuation does is to make the next continuation non-fluid (and propagate this up through its inline ancestors).

But it's highly likely, in the case of a large amount of mostly-same-direction text, that the direction run will continue through the next line, and the next, etc. So the non-fluid continuation that RemoveBidiContinuation just imposed will then be removed when we reach the end of the next textframe, and a new one set for the next line, and so on until the actual end of the direction run is reached.

All this isn't a problem during the initial bidi processing of a block, when we're doing bidi resolution and line-breaking for the first time; but it becomes really expensive when reflowing such a block, as ResolveParagraph causes continuations to be made non-fluid and then revert to fluid again, rippling through a list of potentially thousands of lines.

I think we may be able to avoid this by making RemoveBidiContinuation a no-op for the case where the continuation is already fluid.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1927062 - Try to avoid redundant flipping between fluid and non-fluid continuations during bidi paragraph resolution. r=#layout

Comment 2

[Mayank Bansal]

From the try build, profile for the testcase in bug 1925916: https://share.firefox.dev/40pDeHw (between 3s-7s)

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/f81869ef8776
Try to avoid redundant flipping between fluid and non-fluid continuations during bidi paragraph resolution. r=layout-reviewers,emilio

Comment 4

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/f81869ef8776