Re: Analysis of redisplay performance on Windows

[Jason Rumney <jasonr@gnu.org>]

Chong Yidong wrote:

>> There may also be a problem with the setting of
>> row->contains_overlapping_glyphs_p on Windows. The above functions
>> should only be called when that is set, but after inserting debugging
>> code I can see them being called frequently even when using fonts that
>> contain no overlapping glyphs (confirmed by further debugging code in
>> w32font_text_metrics).

> 
> But I don't understand why the problem of left_overwriting and
> right_overwriting, if that's indeed the culprit, would be specific to
> Windows.

That is why I suspect there may be a problem with the setting of
row->contains_overlapping_glyphs_p on Windows. We avoid calling those
functions if that variable is not set. It may also be much more
expensive to look up the glyph index points in Windows, that is the case
for obtaining text extents, so it wouldn't surprise me.

Another factor is that we do this all one character at a time, and each
time on Windows we select the font into the current GC, then reselect
the previously set font. This operation is also expensive, so it would
help to have functions in the font backend interface for selecting and
deselecting fonts. I already tried using prepare_face and done_face for
this, but the calls overlap - ie we prepare a second face without
calling done_face for the first.