Performance drop on XDrawRectangles()

[Thomas Richter <richter@rus.uni-stuttgart.de>]

Hi folks,

during the changes from 3.12rc7 to 3.13rc4, the performance of 
XDrawRectangles() dropped considerably. Interestingly, it is not the raw 
rectangles drawing operations that are slow, but it seems that the 
"per-call" overhead has increased by one magnitude. In specific, if you 
use the unmodified "x11pref" program:

x11pref -rect10

no substiantial changes are visible. However, if the rectangles are 
drawn one by one by changing:

/* snip: old version, lines 86ff of do_rects.c of the x11perf program */

void
DoRectangles(XParms xp, Parms p, int reps)
{
     int i;

     for (i = 0; i != reps; i++) {
         XFillRectangles(xp->d, xp->w, pgc, rects, p->objects);
         if (pgc == xp->bggc)
             pgc = xp->fggc;
         else
             pgc = xp->bggc;
	CheckAbort ();
     }
}

/* to the following : */

void
DoRectangles(XParms xp, Parms p, int reps)
{
     int i;
     int j;

     for (i = 0; i != reps; i++) {
         for(j = 0;j < p->objects;j++) {
           XFillRectangles(xp->d, xp->w, pgc, rects+j, 1);
         }
         if (pgc == xp->bggc)
             pgc = xp->fggc;
         else
             pgc = xp->bggc;
	CheckAbort ();
     }
}

by instead drawing the rectangles one by one, the performance is 
decreased to one eigths of the original performance:

400000 trep @   0.0687 msec ( 14600.0/sec): 10x10 rectangle (new)
2500000 trep @   0.0107 msec ( 93900.0/sec): 10x10 rectangle (old)

Thus, apparently, not the actual hardware acceleration degraded, but 
there is something in the call path that slowed down the call considerably.

Any idea what changed?

Greetings,
	Thomas