OpenSubdiv: Completely avoid possible access to non-existing CPU data

Make it so CCGDM reports 0 number of geometry when it uses GPU backend for
drawing. This screws up a bit statistics in info header and requires to have
some special handle of CCGDM in the drawing code, but makes it so non of the
areas will try to access non-existing geometry.

source/blender/blenkernel/BKE_subsurf.h

@@ -85,6 +85,9 @@ void subsurf_copy_grid_paint_mask(struct DerivedMesh *dm,
                                   const struct MPoly *mpoly, float *paint_mask,
                                   const struct GridPaintMask *grid_paint_mask);
 
+bool subsurf_has_edges(struct DerivedMesh *dm);
+bool subsurf_has_faces(struct DerivedMesh *dm);
+
 typedef enum MultiresModifiedFlags {
 	/* indicates the grids have been sculpted on, so MDisps
 	 * have to be updated */

source/blender/blenkernel/intern/CCGSubSurf.c

@@ -1515,6 +1515,12 @@ int ccgSubSurf_getNumFinalVerts(const CCGSubSurf *ss)
 	                     ss->fMap->numEntries +
 	                     ss->numGrids * ((gridSize - 2) + ((gridSize - 2) * (gridSize - 2))));
 
+#ifdef WITH_OPENSUBDIV
+	if (ss->skip_grids) {
+		return 0;
+	}
+#endif
+
 	return numFinalVerts;
 }
 int ccgSubSurf_getNumFinalEdges(const CCGSubSurf *ss)
@@ -1523,13 +1529,22 @@ int ccgSubSurf_getNumFinalEdges(const CCGSubSurf *ss)
 	int gridSize = ccg_gridsize(ss->subdivLevels);
 	int numFinalEdges = (ss->eMap->numEntries * (edgeSize - 1) +
 	                     ss->numGrids * ((gridSize - 1) + 2 * ((gridSize - 2) * (gridSize - 1))));
-
+#ifdef WITH_OPENSUBDIV
+	if (ss->skip_grids) {
+		return 0;
+	}
+#endif
 	return numFinalEdges;
 }
 int ccgSubSurf_getNumFinalFaces(const CCGSubSurf *ss)
 {
 	int gridSize = ccg_gridsize(ss->subdivLevels);
 	int numFinalFaces = ss->numGrids * ((gridSize - 1) * (gridSize - 1));
+#ifdef WITH_OPENSUBDIV
+	if (ss->skip_grids) {
+		return 0;
+	}
+#endif
 	return numFinalFaces;
 }
 

source/blender/blenkernel/intern/subsurf_ccg.c

@@ -4850,3 +4850,25 @@ void subsurf_calculate_limit_positions(Mesh *me, float (*r_positions)[3])
 
 	dm->release(dm);
 }
+
+bool subsurf_has_edges(DerivedMesh *dm)
+{
+	CCGDerivedMesh *ccgdm = (CCGDerivedMesh *) dm;
+#ifdef WITH_OPENSUBDIV
+	if (ccgdm->useGpuBackend) {
+		return true;
+	}
+#endif
+	return dm->getNumEdges(dm) != 0;
+}
+
+bool subsurf_has_faces(DerivedMesh *dm)
+{
+	CCGDerivedMesh *ccgdm = (CCGDerivedMesh *) dm;
+#ifdef WITH_OPENSUBDIV
+	if (ccgdm->useGpuBackend) {
+		return true;
+	}
+#endif
+	return dm->getNumPolys(dm) != 0;
+}

source/blender/editors/space_view3d/drawobject.c

@@ -74,6 +74,7 @@
 #include "BKE_particle.h"
 #include "BKE_pointcache.h"
 #include "BKE_scene.h"
+#include "BKE_subsurf.h"
 #include "BKE_unit.h"
 #include "BKE_tracking.h"
 
@@ -4027,9 +4028,15 @@ static void draw_mesh_fancy(Scene *scene, ARegion *ar, View3D *v3d, RegionView3D
 	}
 	
 	/* check polys instead of tessfaces because of dyntopo where tessfaces don't exist */
-	no_edges = (dm->getNumEdges(dm) == 0);
-	no_faces = (dm->getNumPolys(dm) == 0);
-	
+	if (dm->type == DM_TYPE_CCGDM) {
+		no_edges = !subsurf_has_edges(dm);
+		no_faces = !subsurf_has_faces(dm);
+	}
+	else {
+		no_edges = (dm->getNumEdges(dm) == 0);
+		no_faces = (dm->getNumPolys(dm) == 0);
+	}
+
 	/* vertexpaint, faceselect wants this, but it doesnt work for shaded? */
 	glFrontFace((ob->transflag & OB_NEG_SCALE) ? GL_CW : GL_CCW);