mi: "Cache" arc span data for dashed arcsarcology

This avoids recomputing the span data for every dash. x11perf thinks
this is a pretty modest speedup:

    832919.4       840471.1 ( 1.009)   100-pixel dashed ellipse
    672353.1       680652.2 ( 1.012)   100-pixel double-dashed ellipse
     13748.9        24287.9 ( 1.767)   100-pixel wide dashed ellipse
      9236.3        21298.2 ( 2.306)   100-pixel wide double-dashed ellipse

But part of the reason it's so modest there is that the arcs are
relatively small (100 pixel diameter at line width 10, so ~6000 pixels)
and the dashes relatively large (30 on 20 off so ~6 dashes per
quadrant).

With larger arcs and finer dashes this is much more impressive. A fairly
trivial testcase of a single 15000x13000 arc with the default {2, 2}
dash pattern drops from ~3500 milliseconds to 10 milliseconds.

Back in 946f664b I'd removed a more elaborate LRU cache for arcs. This
change doesn't so much "introduce a cache" as "preserves context".

Signed-off-by: Adam Jackson <ajax@redhat.com>

1 files changed, 11 insertions, 1 deletions

diff --git a/mi/miarc.c b/mi/miarc.c
index d6be99000..71df4ab64 100644
--- a/mi/miarc.c
+++ b/mi/miarc.c
@@ -1021,6 +1021,7 @@ miWideArc(DrawablePtr pDraw, GCPtr pGC, int narcs, xArc * parcs)
     join[0] = join[1] = 0;
     for (iphase = (pGC->lineStyle == LineDoubleDash); iphase >= 0; iphase--) {
         miArcSpanData *spdata = NULL;
+        xArc lastArc;
         ChangeGCVal gcval;
 
         if (iphase == 1) {
@@ -1037,10 +1038,17 @@ miWideArc(DrawablePtr pDraw, GCPtr pGC, int narcs, xArc * parcs)
             miArcDataPtr arcData;
 
             arcData = &polyArcs[iphase].arcs[i];
+            if (spdata) {
+                if (lastArc.width != arcData->arc.width ||
+                    lastArc.height != arcData->arc.height) {
+                    free(spdata);
+                    spdata = NULL;
+                }
+            }
+            memcpy(&lastArc, &arcData->arc, sizeof(xArc));
             spdata = miArcSegment(pDrawTo, pGCTo, arcData->arc,
                                   &arcData->bounds[RIGHT_END],
                                   &arcData->bounds[LEFT_END], spdata);
-            free(spdata);
             if (polyArcs[iphase].arcs[i].render) {
                 fillSpans(pDrawTo, pGCTo);
                 /* don't cap self-joining arcs */
@@ -1097,6 +1105,8 @@ miWideArc(DrawablePtr pDraw, GCPtr pGC, int narcs, xArc * parcs)
                 }
             }
         }
+        free(spdata);
+        spdata = NULL;
     }
     miFreeArcs(polyArcs, pGC);