Refactor Mesh split_faces() code to use loop normal spaces.

Finding which loop should share its vertex with which others is not easy
with regular Mesh data (mostly due to lack of advanced topology info, as
opposed with BMesh case).

Custom loop normals computing already does that - and can return 'loop
normal spaces', which among other things contain definitions of 'smooth
fans' of loops around vertices.

Using those makes it easy to find vertices (and then edges) that needs
splitting.

This commit also adds support of non-autosmooth meshes, where we want to
split out flat faces from smooth ones.

3 files changed, 262 insertions, 237 deletions

diff --git a/source/blender/blenkernel/BKE_mesh.h b/source/blender/blenkernel/BKE_mesh.h
index d41878825bb..e42692622f4 100644
--- a/source/blender/blenkernel/BKE_mesh.h
+++ b/source/blender/blenkernel/BKE_mesh.h
@@ -131,7 +131,6 @@ bool BKE_mesh_uv_cdlayer_rename(struct Mesh *me, const char *old_name, const cha
 
 float (*BKE_mesh_vertexCos_get(const struct Mesh *me, int *r_numVerts))[3];
 
-void BKE_mesh_calc_normals_split(struct Mesh *mesh);
 void BKE_mesh_split_faces(struct Mesh *mesh);
 
 struct Mesh *BKE_mesh_new_from_object(struct Main *bmain, struct Scene *sce, struct Object *ob,
@@ -228,6 +227,9 @@ void BKE_lnor_space_custom_normal_to_data(MLoopNorSpace *lnor_space, const float
 
 bool BKE_mesh_has_custom_loop_normals(struct Mesh *me);
 
+void BKE_mesh_calc_normals_split(struct Mesh *mesh);
+void BKE_mesh_calc_normals_split_ex(struct Mesh *mesh, struct MLoopNorSpaceArray *r_lnors_spacearr);
+
 void BKE_mesh_normals_loop_split(
         const struct MVert *mverts, const int numVerts, struct MEdge *medges, const int numEdges,
         struct MLoop *mloops, float (*r_loopnors)[3], const int numLoops,
diff --git a/source/blender/blenkernel/intern/mesh.c b/source/blender/blenkernel/intern/mesh.c
index 73fcd7615f7..7a27c43e28f 100644
--- a/source/blender/blenkernel/intern/mesh.c
+++ b/source/blender/blenkernel/intern/mesh.c
@@ -39,7 +39,9 @@
 
 #include "BLI_utildefines.h"
 #include "BLI_math.h"
+#include "BLI_linklist.h"
 #include "BLI_listbase.h"
+#include "BLI_memarena.h"
 #include "BLI_edgehash.h"
 #include "BLI_string.h"
 
@@ -2053,7 +2055,7 @@ void BKE_mesh_mselect_active_set(Mesh *me, int index, int type)
 	           (me->mselect[me->totselect - 1].type  == type));
 }
 
-void BKE_mesh_calc_normals_split(Mesh *mesh)
+void BKE_mesh_calc_normals_split_ex(Mesh *mesh, MLoopNorSpaceArray *r_lnors_spacearr)
 {
 	float (*r_loopnors)[3];
 	float (*polynors)[3];
@@ -2088,251 +2090,249 @@ void BKE_mesh_calc_normals_split(Mesh *mesh)
 	BKE_mesh_normals_loop_split(
 	        mesh->mvert, mesh->totvert, mesh->medge, mesh->totedge,
 	        mesh->mloop, r_loopnors, mesh->totloop, mesh->mpoly, (const float (*)[3])polynors, mesh->totpoly,
-	        (mesh->flag & ME_AUTOSMOOTH) != 0, mesh->smoothresh, NULL, clnors, NULL);
+	        (mesh->flag & ME_AUTOSMOOTH) != 0, mesh->smoothresh, r_lnors_spacearr, clnors, NULL);
 
 	if (free_polynors) {
 		MEM_freeN(polynors);
 	}
 }
 
-/* Split faces helper functions. */
-
-enum {
-	/* Vertex is adjacent to some loop which normal is different,
-	 * hence split of this vertex is required.
-	 */
-	SPLIT_VERT_NEED_SPLIT = (1 << 0),
-	/* Original vertex was already re-used by split logic. */
-	SPLIT_VERT_REUSED     = (1 << 1),
-};
-enum {
-	/* Edge is adjacent to any of vertex tagged for split.
-	 */
-	SPLIT_EDGE_NEED_SPLIT = (1 << 0),
-	/* Original edge was already re-used by split logic. */
-	SPLIT_EDGE_REUSED     = (1 << 1),
-};
-
-/* Tag vertices which normals are not equal to any adjacent loop
- * and hence split on that vertex is required.
- *
- * Returns truth if any of vertex needs to be split.
- */
-static bool split_faces_tag_verts(const Mesh *mesh, uchar *vert_flags)
+void BKE_mesh_calc_normals_split(Mesh *mesh)
 {
-	const int num_polys = mesh->totpoly;
-	const MVert *mvert = mesh->mvert;
-	const MLoop *mloop = mesh->mloop;
-	const MPoly *mpoly = mesh->mpoly;
-	float (*lnors)[3] = CustomData_get_layer(&mesh->ldata, CD_NORMAL);
-	bool has_split_verts = false;
-	for (int poly = 0; poly < num_polys; poly++) {
-		const MPoly *mp = &mpoly[poly];
-		for (int loop = 0; loop < mp->totloop; loop++) {
-			const MLoop *ml = &mloop[mp->loopstart + loop];
-			const MVert *mv = &mvert[ml->v];
-			float vn[3];
-			normal_short_to_float_v3(vn, mv->no);
-			if (len_squared_v3v3(vn, lnors[mp->loopstart + loop]) > FLT_EPSILON) {
-				vert_flags[ml->v] |= SPLIT_VERT_NEED_SPLIT;
-				has_split_verts = true;
-			}
-		}
-	}
-	return has_split_verts;
+	BKE_mesh_calc_normals_split_ex(mesh, NULL);
 }
 
-/* Count number of new vertices to be added.
- *
- * Note that one of the loop where split is required will re-use
- * it's vertex in order to avoid creation of loose vertices.
- */
-static int split_faces_count_new_verts(const Mesh *mesh, uchar *vert_flags)
-{
-	const int num_polys = mesh->totpoly;
-	const MLoop *mloop = mesh->mloop;
-	const MPoly *mpoly = mesh->mpoly;
-	int num_new_verts = 0;
-	for (int poly = 0; poly < num_polys; poly++) {
-		const MPoly *mp = &mpoly[poly];
-		for (int loop = 0; loop < mp->totloop; loop++) {
-			const MLoop *ml = &mloop[mp->loopstart + loop];
-			if (vert_flags[ml->v] & SPLIT_VERT_NEED_SPLIT) {
-				if (vert_flags[ml->v] & SPLIT_VERT_REUSED) {
-					++num_new_verts;
+/* Split faces helper functions. */
+
+typedef struct SplitFaceNewVert {
+	struct SplitFaceNewVert *next;
+	int new_index;
+	int orig_index;
+	float *vnor;
+} SplitFaceNewVert;
+
+typedef struct SplitFaceNewEdge {
+	struct SplitFaceNewEdge *next;
+	int new_index;
+	int orig_index;
+	int v1;
+	int v2;
+} SplitFaceNewEdge;
+
+/* Detect needed new vertices, and update accordingly loops' vertex indices.
+ * WARNING! Leaves mesh in invalid state. */
+static int split_faces_prepare_new_verts(
+        const Mesh *mesh, MLoopNorSpaceArray *lnors_spacearr, SplitFaceNewVert **new_verts, MemArena *memarena,
+        bool *r_need_vnors_recalc)
+{
+	/* Note: if lnors_spacearr is NULL, ther is no autosmooth handling, and we only split out flat polys. */
+	const int num_loops = mesh->totloop;
+	int num_verts = mesh->totvert;
+	MVert *mvert = mesh->mvert;
+	MLoop *mloop = mesh->mloop;
+
+	BLI_bitmap *verts_used = BLI_BITMAP_NEW(num_verts, __func__);
+
+	if (lnors_spacearr) {
+		BLI_bitmap *done_loops = BLI_BITMAP_NEW(num_loops, __func__);
+
+		MLoop *ml = mloop;
+		MLoopNorSpace **lnor_space = lnors_spacearr->lspacearr;
+		for (int loop_idx = 0; loop_idx < num_loops; loop_idx++, ml++, lnor_space++) {
+			if (!BLI_BITMAP_TEST(done_loops, loop_idx)) {
+				const int vert_idx = ml->v;
+				const bool vert_used = BLI_BITMAP_TEST_BOOL(verts_used, vert_idx);
+				/* If vert is already used by another smooth fan, we need a new vert for this one. */
+				const int new_vert_idx = vert_used ? num_verts++ : vert_idx;
+
+				if ((*lnor_space)->loops) {
+					for (LinkNode *lnode = (*lnor_space)->loops; lnode; lnode = lnode->next) {
+						const int ml_fan_idx = GET_INT_FROM_POINTER(lnode->link);
+						BLI_BITMAP_ENABLE(done_loops, ml_fan_idx);
+						if (vert_used) {
+							mloop[ml_fan_idx].v = new_vert_idx;
+						}
+					}
 				}
 				else {
-					vert_flags[ml->v] |= SPLIT_VERT_REUSED;
+					/* Single loop in this fan... */
+					BLI_BITMAP_ENABLE(done_loops, loop_idx);
+					if (vert_used) {
+						ml->v = new_vert_idx;
+					}
 				}
-			}
-		}
-	}
-	return num_new_verts;
-}
 
-/* Tag edges which are adjacent to at least one vertex tagged for split. */
-static void split_faces_tag_edges(Mesh *mesh,
-                                  const uchar *vert_flags,
-                                  uchar *edge_flags)
-{
-	const int num_polys = mesh->totpoly;
-	const MLoop *mloop = mesh->mloop;
-	const MPoly *mpoly = mesh->mpoly;
-	for (int poly = 0; poly < num_polys; poly++) {
-		const MPoly *mp = &mpoly[poly];
-		int loop_prev = mp->totloop - 1;
-		for (int loop = 0; loop < mp->totloop; loop++) {
-			const int poly_loop_prev = mp->loopstart + loop_prev;
-			const MLoop *ml = &mloop[mp->loopstart + loop];
-			const MLoop *ml_prev = &mloop[poly_loop_prev];
-			const int mv_flag = vert_flags[ml->v];
-			const int mv_prev_flag = vert_flags[ml_prev->v];
-			bool need_split = false;
-			if (mv_flag & SPLIT_VERT_NEED_SPLIT) {
-				if (mv_prev_flag & SPLIT_VERT_NEED_SPLIT) {
-					/* Create new edge between twp split vertices. */
-					need_split = true;
+				if (!vert_used) {
+					BLI_BITMAP_ENABLE(verts_used, vert_idx);
+					/* We need to update that vertex's normal here, we won't go over it again. */
+					/* This is important! *DO NOT* set vnor to final computed lnor, vnor should always be defined to
+					 * 'automatic normal' value computed from its polys, not some custom normal.
+					 * Fortunately, that's the loop normal space's 'lnor' reference vector. ;) */
+					normal_float_to_short_v3(mvert[vert_idx].no, (*lnor_space)->vec_lnor);
 				}
 				else {
-					/* Create new edge from existing vertex to a split one. */
-					need_split = true;
+					/* Add new vert to list. */
+					SplitFaceNewVert *new_vert = BLI_memarena_alloc(memarena, sizeof(*new_vert));
+					new_vert->orig_index = vert_idx;
+					new_vert->new_index = new_vert_idx;
+					new_vert->vnor = (*lnor_space)->vec_lnor;  /* See note above. */
+					new_vert->next = *new_verts;
+					*new_verts = new_vert;
 				}
 			}
-			else if (mv_prev_flag & SPLIT_VERT_NEED_SPLIT) {
-				/* Create new edge from split vertex to existing one. */
-				need_split = true;
+		}
+
+		MEM_freeN(done_loops);
+	}
+	else {
+		/* No loop normal spaces available, we only split out flat polys. */
+		const int num_polys = mesh->totpoly;
+		const MPoly *mpoly = mesh->mpoly;
+
+		/* We do that in two loops, to keep original edges/verts to smooth polys preferencially. */
+		const MPoly *mp = mpoly;
+		for (int i = 0; i < num_polys; i++, mp++) {
+			if (mp->flag & ME_SMOOTH) {
+				const MLoop *ml = &mloop[mp->loopstart];
+				for (int j = 0; j < mp->totloop; j++, ml++) {
+					/* Just mark the vertex as used/reserved, that way neighbor flat polys, if any,
+					 * will have to create their own. */
+					BLI_BITMAP_ENABLE(verts_used, ml->v);
+				}
 			}
-			if (need_split) {
-				edge_flags[ml_prev->e] |= SPLIT_EDGE_NEED_SPLIT;
+		}
+
+		mp = mpoly;
+		for (int i = 0; i < num_polys; i++, mp++) {
+			if (!(mp->flag & ME_SMOOTH)) {
+				MLoop *ml = &mloop[mp->loopstart];
+				for (int j = 0; j < mp->totloop; j++, ml++) {
+					const int vert_idx = ml->v;
+
+					if (BLI_BITMAP_TEST(verts_used, vert_idx)) {
+						/* Add new vert to list. */
+						const int new_vert_idx = num_verts++;
+						ml->v = new_vert_idx;
+
+						SplitFaceNewVert *new_vert = BLI_memarena_alloc(memarena, sizeof(*new_vert));
+						new_vert->orig_index = vert_idx;
+						new_vert->new_index = new_vert_idx;
+						new_vert->vnor = NULL;  /* See note below about normals. */
+						new_vert->next = *new_verts;
+						*new_verts = new_vert;
+					}
+					else {
+						BLI_BITMAP_ENABLE(verts_used, vert_idx);
+					}
+				}
+				/* Note: there is no way to get new normals for smooth vertices here (and we don't have direct access
+				 * to poly normals either for flat ones), so we'll have to recompute all vnors at the end... */
+				*r_need_vnors_recalc = true;
 			}
-			loop_prev = loop;
 		}
 	}
+
+	MEM_freeN(verts_used);
+
+	return num_verts - mesh->totvert;
 }
 
-/* Count number of new edges to be added.
- *
- * Note that one of the loop where split is required will re-use
- * it's edge in order to avoid creation of loose edges.
- */
-static int split_faces_count_new_edges(const Mesh *mesh, uchar *edge_flags)
+/* Detect needed new edges, and update accordingly loops' edge indices.
+ * WARNING! Leaves mesh in invalid state. */
+static int split_faces_prepare_new_edges(
+        const Mesh *mesh, SplitFaceNewEdge **new_edges, MemArena *memarena)
 {
 	const int num_polys = mesh->totpoly;
-	const MLoop *mloop = mesh->mloop;
+	int num_edges = mesh->totedge;
+	MEdge *medge = mesh->medge;
+	MLoop *mloop = mesh->mloop;
 	const MPoly *mpoly = mesh->mpoly;
-	int num_new_edges = 0;
-	for (int poly = 0; poly < num_polys; poly++) {
-		const MPoly *mp = &mpoly[poly];
-		for (int loop = 0; loop < mp->totloop; loop++) {
-			const MLoop *ml = &mloop[mp->loopstart + loop];
-			if (edge_flags[ml->e] & SPLIT_EDGE_NEED_SPLIT) {
-				if (edge_flags[ml->e] & SPLIT_EDGE_REUSED) {
-					++num_new_edges;
+
+	BLI_bitmap *edges_used = BLI_BITMAP_NEW(num_edges, __func__);
+	EdgeHash *edges_hash = BLI_edgehash_new_ex(__func__, num_edges);
+
+	const MPoly *mp = mpoly;
+	for (int poly_idx = 0; poly_idx < num_polys; poly_idx++, mp++) {
+		MLoop *ml_prev = &mloop[mp->loopstart + mp->totloop - 1];
+		MLoop *ml = &mloop[mp->loopstart];
+		for (int loop_idx = 0; loop_idx < mp->totloop; loop_idx++, ml++) {
+			void **eval;
+			if (!BLI_edgehash_ensure_p(edges_hash, ml_prev->v, ml->v, &eval)) {
+				/* That edge has not been encountered yet, define it. */
+				if (BLI_BITMAP_TEST(edges_used, ml_prev->e)) {
+					/* Original edge has already been used, we need to define a new one. */
+					const int edge_idx = num_edges++;
+					*eval = SET_INT_IN_POINTER(edge_idx);
+					ml_prev->e = edge_idx;
+
+					SplitFaceNewEdge *new_edge = BLI_memarena_alloc(memarena, sizeof(*new_edge));
+					new_edge->orig_index = ml_prev->e;
+					new_edge->new_index = edge_idx;
+					new_edge->v1 = ml_prev->v;
+					new_edge->v2 = ml->v;
+					new_edge->next = *new_edges;
+					*new_edges = new_edge;
 				}
 				else {
-					edge_flags[ml->e] |= SPLIT_EDGE_REUSED;
+					/* We can re-use original edge. */
+					const int edge_idx = ml_prev->e;
+					medge[edge_idx].v1 = ml_prev->v;
+					medge[edge_idx].v2 = ml->v;
+					*eval = SET_INT_IN_POINTER(edge_idx);
+					BLI_BITMAP_ENABLE(edges_used, edge_idx);
 				}
 			}
+			else {
+				/* Edge already known, just update loop's edge index. */
+				ml_prev->e = GET_INT_FROM_POINTER(*eval);
+			}
+
+			ml_prev = ml;
 		}
 	}
-	return num_new_edges;
+
+	MEM_freeN(edges_used);
+	BLI_edgehash_free(edges_hash, NULL);
+
+	return num_edges - mesh->totedge;
 }
 
-/* Perform actual split of vertices.
- *
- * NOTE: Will leave edges in inconsistent state.
- */
-static void split_faces_split_verts(Mesh *mesh,
-                                    const int num_new_verts,
-                                    uchar *vert_flags)
+/* Perform actual split of vertices. */
+static void split_faces_split_new_verts(
+        Mesh *mesh, SplitFaceNewVert *new_verts, const int num_new_verts)
 {
 	const int num_verts = mesh->totvert - num_new_verts;
-	const int num_polys = mesh->totpoly;
 	MVert *mvert = mesh->mvert;
-	MLoop *mloop = mesh->mloop;
-	MPoly *mpoly = mesh->mpoly;
-	const float (*lnors)[3] = CustomData_get_layer(&mesh->ldata, CD_NORMAL);
-	int num_added_verts = 0;
-	/* Clear reused flag, we need it again. */
-	for (int i = 0; i < num_verts; ++i) {
-		vert_flags[i] &= ~SPLIT_VERT_REUSED;
-	}
-	for (int poly = 0; poly < num_polys; poly++) {
-		MPoly *mp = &mpoly[poly];
-		/* First we split all vertices to get proper flag whether they are
-		 * split or not for all of them before handling edges.
-		 */
-		for (int loop = 0; loop < mp->totloop; loop++) {
-			int poly_loop = mp->loopstart + loop;
-			MLoop *ml = &mloop[poly_loop];
-			if (vert_flags[ml->v] & SPLIT_VERT_NEED_SPLIT) {
-				if ((vert_flags[ml->v] & SPLIT_VERT_REUSED) == 0) {
-					/* Ignore first split on vertex, re-use it instead. */
-					vert_flags[ml->v] |= SPLIT_VERT_REUSED;
-					normal_float_to_short_v3(mvert[ml->v].no, lnors[poly_loop]);
-					continue;
-				}
-				/* Create new vertex. */
-				int new_vert = num_verts + num_added_verts;
-				CustomData_copy_data(&mesh->vdata, &mesh->vdata,
-				                     ml->v, new_vert, 1);
-				normal_float_to_short_v3(mvert[new_vert].no,
-				                         lnors[poly_loop]);
-				ml->v = new_vert;
-				num_added_verts++;
-			}
+
+	/* Remember new_verts is a single linklist, so its items are in reversed order... */
+	MVert *new_mv = &mvert[mesh->totvert - 1];
+	for (int i = mesh->totvert - 1; i >= num_verts ; i--, new_mv--, new_verts = new_verts->next) {
+		BLI_assert(new_verts->new_index == i);
+		CustomData_copy_data(&mesh->vdata, &mesh->vdata, new_verts->orig_index, i, 1);
+		if (new_verts->vnor) {
+			normal_float_to_short_v3(new_mv->no, new_verts->vnor);
 		}
 	}
 }
 
-/* Perform actual split of edges.
- *
- * NOTE: Will correct all edges.
- */
-static void split_faces_split_edges(Mesh *mesh,
-                                    const int num_new_edges,
-                                    uchar *edge_flags)
+/* Perform actual split of edges. */
+static void split_faces_split_new_edges(
+        Mesh *mesh, SplitFaceNewEdge *new_edges, const int num_new_edges)
 {
 	const int num_edges = mesh->totedge - num_new_edges;
-	const int num_polys = mesh->totpoly;
 	MEdge *medge = mesh->medge;
-	MLoop *mloop = mesh->mloop;
-	MPoly *mpoly = mesh->mpoly;
-	int num_added_edges = 0;
-	/* Clear reused flag, we need it again. */
-	for (int i = 0; i < num_edges; ++i) {
-		edge_flags[i] &= ~SPLIT_EDGE_REUSED;
-	}
-	for (int poly = 0; poly < num_polys; poly++) {
-		MPoly *mp = &mpoly[poly];
-		for (int loop = 0, loop_prev = mp->totloop - 1; loop < mp->totloop; loop++) {
-			const int poly_loop_prev = mp->loopstart + loop_prev;
-			const MLoop *ml = &mloop[mp->loopstart + loop];
-			MLoop *ml_prev = &mloop[poly_loop_prev];
-			MEdge *me_prev = &medge[ml_prev->e];
-			if (edge_flags[ml_prev->e] & SPLIT_EDGE_NEED_SPLIT) {
-				if ((edge_flags[ml_prev->e] & SPLIT_EDGE_REUSED) == 0) {
-					edge_flags[ml_prev->e] |= SPLIT_EDGE_REUSED;
-					me_prev->v1 = ml_prev->v;
-					me_prev->v2 = ml->v;
-				}
-				else {
-					const int index = num_edges + num_added_edges;
-					CustomData_copy_data(&mesh->edata, &mesh->edata,
-					                     ml_prev->e, index, 1);
-					MEdge *me_new = &medge[index];
-					me_new->v1 = ml_prev->v;
-					me_new->v2 = ml->v;
-					ml_prev->e = index;
-					num_added_edges++;
-				}
-			}
-			loop_prev = loop;
-		}
+
+	/* Remember new_edges is a single linklist, so its items are in reversed order... */
+	MEdge *new_med = &medge[mesh->totedge - 1];
+	for (int i = mesh->totedge - 1; i >= num_edges ; i--, new_med--, new_edges = new_edges->next) {
+		BLI_assert(new_edges->new_index == i);
+		CustomData_copy_data(&mesh->edata, &mesh->edata, new_edges->orig_index, i, 1);
+		new_med->v1 = new_edges->v1;
+		new_med->v2 = new_edges->v2;
 	}
 }
 
-/* Split faces based on the edge angle.
+/* Split faces based on the edge angle and loop normals.
  * Matches behavior of face splitting in render engines.
  *
  * NOTE: Will leave CD_NORMAL loop data layer which is
@@ -2340,48 +2340,72 @@ static void split_faces_split_edges(Mesh *mesh,
  */
 void BKE_mesh_split_faces(Mesh *mesh)
 {
-	const int num_verts = mesh->totvert;
-	const int num_edges = mesh->totedge;
 	const int num_polys = mesh->totpoly;
-	if ((mesh->flag & ME_AUTOSMOOTH) == 0) {
-		return;
-	}
+
 	if (num_polys == 0) {
 		return;
 	}
 	BKE_mesh_tessface_clear(mesh);
-	/* Compute loop normals if needed. */
-	if (!CustomData_has_layer(&mesh->ldata, CD_NORMAL)) {
-		BKE_mesh_calc_normals_split(mesh);
-	}
-	/* Runtime flags. */
-	uchar *vert_flags = MEM_callocN(sizeof(*vert_flags) * num_verts,
-	                                "split faces vert flags");
-	/* Tag vertces and check whether anything is tagged. */
-	if (!split_faces_tag_verts(mesh, vert_flags)) {
-		/* No new vertices to be split added, can do early exit. */
-		MEM_freeN(vert_flags);
-		return;
+
+	MLoopNorSpaceArray *lnors_spacearr = NULL;
+	MemArena *memarena;
+	bool need_vnors_recalc = false;
+
+	if (mesh->flag & ME_AUTOSMOOTH) {
+		lnors_spacearr = MEM_callocN(sizeof(*lnors_spacearr), __func__);
+		/* Compute loop normals and loop normal spaces (a.k.a. smooth fans of faces around vertices). */
+		BKE_mesh_calc_normals_split_ex(mesh, lnors_spacearr);
+		/* Stealing memarena from loop normals space array. */
+		memarena = lnors_spacearr->mem;
+	}
+	else {
+		/* We still have to split out flat faces... */
+		memarena = BLI_memarena_new(BLI_MEMARENA_STD_BUFSIZE, __func__);
+	}
+
+	SplitFaceNewVert *new_verts = NULL;
+	SplitFaceNewEdge *new_edges = NULL;
+
+	/* Detect loop normal spaces (a.k.a. smooth fans) that will need a new vert. */
+	const int num_new_verts = split_faces_prepare_new_verts(mesh, lnors_spacearr, &new_verts, memarena, &need_vnors_recalc);
+
+	if (num_new_verts > 0) {
+		/* Reminder: beyond this point, there is no way out, mesh is in invalid state (due to early-reassignment of
+		 * loops' vertex and edge indices to new, to-be-created split ones). */
+
+		const int num_new_edges = split_faces_prepare_new_edges(mesh, &new_edges, memarena);
+		BLI_assert(num_new_edges > 0);
+
+		/* Reallocate all vert and edge related data. */
+		mesh->totvert += num_new_verts;
+		mesh->totedge += num_new_edges;
+		CustomData_realloc(&mesh->vdata, mesh->totvert);
+		CustomData_realloc(&mesh->edata, mesh->totedge);
+		/* Update pointers to a newly allocated memory. */
+		BKE_mesh_update_customdata_pointers(mesh, false);
+
+		/* Perform actual split of vertices and edges. */
+		split_faces_split_new_verts(mesh, new_verts, num_new_verts);
+		split_faces_split_new_edges(mesh, new_edges, num_new_edges);
+	}
+
+	/* Note: after this point mesh is expected to be valid again. */
+
+	/* CD_NORMAL is expected to be temporary only. */
+	CustomData_free_layers(&mesh->ldata, CD_NORMAL, mesh->totloop);
+
+	if (lnors_spacearr) {
+		/* Also frees new_verts/edges temp data, since we used its memarena to allocate them. */
+		BKE_lnor_spacearr_free(lnors_spacearr);
+		MEM_freeN(lnors_spacearr);
+	}
+	else {
+		BLI_memarena_free(memarena);
+	}
+
+	if (need_vnors_recalc) {
+		BKE_mesh_calc_normals(mesh);
 	}
-	/* Flush vertex flags to edges. */
-	uchar *edge_flags = MEM_callocN(sizeof(*edge_flags) * num_edges,
-	                                "split faces edge flags");
-	split_faces_tag_edges(mesh, vert_flags, edge_flags);
-	/* Count amount of new geometry. */
-	int num_new_verts = split_faces_count_new_verts(mesh, vert_flags);
-	int num_new_edges =  split_faces_count_new_edges(mesh, edge_flags);
-	/* Reallocate all vert and edge related data. */
-	mesh->totvert += num_new_verts;
-	mesh->totedge += num_new_edges;
-	CustomData_realloc(&mesh->vdata, mesh->totvert);
-	CustomData_realloc(&mesh->edata, mesh->totedge);
-	/* Update pointers to a newly allocated memory. */
-	BKE_mesh_update_customdata_pointers(mesh, false);
-	/* Perform actual split of vertices and adjacent edges. */
-	split_faces_split_verts(mesh, num_new_verts, vert_flags);
-	split_faces_split_edges(mesh, num_new_edges, edge_flags);
-	MEM_freeN(vert_flags);
-	MEM_freeN(edge_flags);
 #ifdef VALIDATE_MESH
 	BKE_mesh_validate(mesh, true, true);
 #endif
diff --git a/source/blender/blenkernel/intern/mesh_evaluate.c b/source/blender/blenkernel/intern/mesh_evaluate.c
index f9eba118383..003b7b784d5 100644
--- a/source/blender/blenkernel/intern/mesh_evaluate.c
+++ b/source/blender/blenkernel/intern/mesh_evaluate.c
@@ -1152,7 +1152,6 @@ void BKE_mesh_normals_loop_split(
         const bool use_split_normals, float split_angle,
         MLoopNorSpaceArray *r_lnors_spacearr, short (*clnors_data)[2], int *r_loop_to_poly)
 {
-
 	/* For now this is not supported. If we do not use split normals, we do not generate anything fancy! */
 	BLI_assert(use_split_normals || !(r_lnors_spacearr));