[Fontconfig] Excessive short-lived allocations during FcFontSet

[Behdad Esfahbod]

Thanks Chris!  Finally...

I suggest keeping the FcCharSetOperate name and call the new one
FcCharSetOperateInPlace().  Other than that, looks good to me.  Keith,
this single change is worth a release on itself.


Cheers,

behdad

PS.  Chris, given that you have some familiarity with the fontconfig
code now, if you feel like hacking, there's some useful and easy bugs to
close for 2.6:

  https://bugs.freedesktop.org/show_bug.cgi?id=14062

On Wed, 2008-04-23 at 09:38 +0100, Chris Wilson wrote:
> As everyone who has profiled the memory allocation behaviour of a Pango
> using app knows (hi Behdad!) FcCharSetFindLeafCreate dominates the
> profile during initialisation. And then every time Pango has to load a
> new fontset.
> 
> For a very simple GTK+ application just opening a window:
> Allocator		nAllocs		nBytes
> FcCharSetFindLeafCreate 62886		2012352
> FcCharSetPutLeaf        9361		11441108
> 
> For gedit loading a UTF-8 demo:
> FcCharSetFindLeafCreate 124872		3995904
> FcCharSetPutLeaf        18656		22372816
> 
> AIUI, the current behaviour of FcSortWalk() is to create a new FcCharSet
> on each iteration that is the union of the previous iteration with the
> next FcCharSet in the font set. This causes the existing FcCharSet to be
> duplicated in its entirety and then allocates fresh leaves for the new
> FcCharSet. In essence the number of allocations is quadratic wrt the
> number of fonts required.
> 
> By introducing a new FcCharSet operator to merge a new FcCharSet into an
> existing FcCharset, we can reduce the allocations to cover the leaves in
> all the fonts in the set and no more.
> 
> For gedit loading the UTF-8 demo, after the patch:
> FcCharSetFindLeafCreate 1940		62080
> FcCharSetPutLeaf        281		190336
> 
> Please review, thanks.
> --
> Chris Wilson
-- 
behdad
http://behdad.org/

"Those who would give up Essential Liberty to purchase a little
 Temporary Safety, deserve neither Liberty nor Safety."
        -- Benjamin Franklin, 1759