Speed up pixman_region{,32}_contains_rectangle()

When someone selects some text in Firefox under a non-composited X
server and initiates a drag, a shaped window is created with a complex
shape corresponding to the outline of the text. Then, on every mouse
movement pixman_region_contains_rectangle() is called many times on
that complicated region. And pixman_region_contains_rectangle() is
doing a linear scan through the rectangles in the region, although the
scan does exit when it finds the first box that can't possibly
intersect the passed-in rectangle.

This patch changes the loop so that it uses a binary search to skip
boxes that don't overlap the current y position.  The performance
improvement for the text dragging case is easily noticable.

V2: Use the binary search for the "getting up to speed or skippping
remainder of band" as well.

1 files changed, 42 insertions, 6 deletions

diff --git a/pixman/pixman-region.c b/pixman/pixman-region.c
index 142493b..71995fd 100644
--- a/pixman/pixman-region.c
+++ b/pixman/pixman-region.c
@@ -2086,6 +2086,40 @@ PIXMAN_EXPORT PREFIX (_inverse) (region_type_t *new_reg,  /* Destination region
     return TRUE;
 }
 
+/* In time O(log n), locate the first box whose y2 is greater than y.
+ * Return @end if no such box exists.
+ */
+static box_type_t *
+find_box_for_y (box_type_t *begin, box_type_t *end, int y)
+{
+    box_type_t *mid;
+
+    if (end == begin)
+	return end;
+
+    if (end - begin == 1)
+    {
+	if (begin->y2 > y)
+	    return begin;
+	else
+	    return end;
+    }
+
+    mid = begin + (end - begin) / 2;
+    if (mid->y2 > y)
+    {
+	/* If no box is found in [begin, mid], the function
+	 * will return @mid, which is then known to be the
+	 * correct answer.
+	 */
+	return find_box_for_y (begin, mid, y);
+    }
+    else
+    {
+	return find_box_for_y (mid, end, y);
+    }
+}
+
 /*
  *   rect_in(region, rect)
  *   This routine takes a pointer to a region and a pointer to a box
@@ -2102,7 +2136,6 @@ PIXMAN_EXPORT PREFIX (_inverse) (region_type_t *new_reg,  /* Destination region
  *   partially in the region) or is outside the region (we reached a band
  *   that doesn't overlap the box at all and part_in is false)
  */
-
 pixman_region_overlap_t
 PIXMAN_EXPORT PREFIX (_contains_rectangle) (region_type_t *  region,
                                             box_type_t *     prect)
@@ -2139,12 +2172,15 @@ PIXMAN_EXPORT PREFIX (_contains_rectangle) (region_type_t *  region,
 
     /* can stop when both part_out and part_in are TRUE, or we reach prect->y2 */
     for (pbox = PIXREGION_BOXPTR (region), pbox_end = pbox + numRects;
-         pbox != pbox_end;
-         pbox++)
+	 pbox != pbox_end;
+	 pbox++)
     {
-
-        if (pbox->y2 <= y)
-	    continue;   /* getting up to speed or skipping remainder of band */
+	/* getting up to speed or skipping remainder of band */
+	if (pbox->y2 <= y)
+	{
+	    if ((pbox = find_box_for_y (pbox, pbox_end, y)) == pbox_end)
+		break;
+	}
 
         if (pbox->y1 > y)
         {