when the size of an edgehash is known or can be guessed,

pass in the argument to reserve the size.

source/blender/blenkernel/intern/cdderivedmesh.c

@@ -2405,7 +2405,7 @@ DerivedMesh *CDDM_merge_verts(DerivedMesh *dm, const int *vtargetmap, const int
 	STACK_DECLARE(mpoly);
 	STACK_DECLARE(oldp);
 
-	EdgeHash *ehash = BLI_edgehash_new(__func__);
+	EdgeHash *ehash = BLI_edgehash_new_ex(__func__, totedge);
 
 	int i, j, c;
 	
@@ -2607,10 +2607,12 @@ void CDDM_calc_edges_tessface(DerivedMesh *dm)
 	EdgeHashIterator *ehi;
 	MFace *mf = cddm->mface;
 	MEdge *med;
-	EdgeHash *eh = BLI_edgehash_new(__func__);
-	int i, *index, numEdges, maxFaces = dm->numTessFaceData;
+	EdgeHash *eh;
+	int i, *index, numEdges, numFaces = dm->numTessFaceData;
 
-	for (i = 0; i < maxFaces; i++, mf++) {
+	eh = BLI_edgehash_new_ex(__func__, BLI_EDGEHASH_SIZE_GUESS_FROM_POLYS(numFaces));
+
+	for (i = 0; i < numFaces; i++, mf++) {
 		if (!BLI_edgehash_haskey(eh, mf->v1, mf->v2))
 			BLI_edgehash_insert(eh, mf->v1, mf->v2, NULL);
 		if (!BLI_edgehash_haskey(eh, mf->v2, mf->v3))
@@ -2668,21 +2670,27 @@ void CDDM_calc_edges(DerivedMesh *dm)
 	MPoly *mp = cddm->mpoly;
 	MLoop *ml;
 	MEdge *med, *origmed;
-	EdgeHash *eh = BLI_edgehash_new(__func__);
+	EdgeHash *eh;
+	unsigned int eh_reserve;
 	int v1, v2;
 	int *eindex;
-	int i, j, *index, numEdges = cddm->dm.numEdgeData, maxFaces = dm->numPolyData;
+	int i, j, *index;
+	const int numFaces = dm->numPolyData;
+	const int numLoops = dm->numLoopData;
+	int numEdges = dm->numEdgeData;
 
 	eindex = DM_get_edge_data_layer(dm, CD_ORIGINDEX);
-
 	med = cddm->medge;
+
+	eh_reserve = max_ii(med ? numEdges : 0, BLI_EDGEHASH_SIZE_GUESS_FROM_LOOPS(numLoops));
+	eh = BLI_edgehash_new_ex(__func__, eh_reserve);
 	if (med) {
 		for (i = 0; i < numEdges; i++, med++) {
 			BLI_edgehash_insert(eh, med->v1, med->v2, SET_INT_IN_POINTER(i + 1));
 		}
 	}
 
-	for (i = 0; i < maxFaces; i++, mp++) {
+	for (i = 0; i < numFaces; i++, mp++) {
 		ml = cddm->mloop + mp->loopstart;
 		for (j = 0; j < mp->totloop; j++, ml++) {
 			v1 = ml->v;
@@ -2732,7 +2740,7 @@ void CDDM_calc_edges(DerivedMesh *dm)
 	cddm->medge = CustomData_get_layer(&dm->edgeData, CD_MEDGE);
 
 	mp = cddm->mpoly;
-	for (i = 0; i < maxFaces; i++, mp++) {
+	for (i = 0; i < numFaces; i++, mp++) {
 		ml = cddm->mloop + mp->loopstart;
 		for (j = 0; j < mp->totloop; j++, ml++) {
 			v1 = ml->v;

source/blender/blenkernel/intern/mesh.c

@@ -177,8 +177,8 @@ static int customdata_compare(CustomData *c1, CustomData *c2, Mesh *m1, Mesh *m2
 		if (l1->type == CD_MEDGE) {
 			MEdge *e1 = l1->data;
 			MEdge *e2 = l2->data;
-			EdgeHash *eh = BLI_edgehash_new(__func__);
 			int etot = m1->totedge;
+			EdgeHash *eh = BLI_edgehash_new_ex(__func__, etot);
 		
 			for (j = 0; j < etot; j++, e1++) {
 				BLI_edgehash_insert(eh, e1->v1, e1->v2, e1);

source/blender/blenkernel/intern/mesh_validate.c

@@ -921,7 +921,8 @@ void BKE_mesh_calc_edges(Mesh *mesh, bool update, const bool select)
 	EdgeHashIterator *ehi;
 	MPoly *mp;
 	MEdge *med, *med_orig;
-	EdgeHash *eh = BLI_edgehash_new(__func__);
+	EdgeHash *eh;
+	unsigned int eh_reserve;
 	int i, totedge, totpoly = mesh->totpoly;
 	int med_index;
 	/* select for newly created meshes which are selected [#25595] */
@@ -930,6 +931,9 @@ void BKE_mesh_calc_edges(Mesh *mesh, bool update, const bool select)
 	if (mesh->totedge == 0)
 		update = false;
 
+	eh_reserve = max_ii(update ? mesh->totedge : 0, BLI_EDGEHASH_SIZE_GUESS_FROM_POLYS(totpoly));
+	eh = BLI_edgehash_new_ex(__func__, eh_reserve);
+
 	if (update) {
 		/* assume existing edges are valid
 		 * useful when adding more faces and generating edges from them */

source/blender/blenkernel/intern/particle_system.c

@@ -2484,7 +2484,7 @@ static EdgeHash *sph_springhash_build(ParticleSystem *psys)
 	ParticleSpring *spring;
 	int i = 0;
 
-	springhash = BLI_edgehash_new(__func__);
+	springhash = BLI_edgehash_new_ex(__func__, psys->tot_fluidsprings);
 
 	for (i=0, spring=psys->fluid_springs; i<psys->tot_fluidsprings; i++, spring++)
 		BLI_edgehash_insert(springhash, spring->particle_index[0], spring->particle_index[1], SET_INT_IN_POINTER(i+1));

source/blender/blenkernel/intern/subsurf_ccg.c

@@ -334,7 +334,7 @@ static int ss_sync_from_uv(CCGSubSurf *ss, CCGSubSurf *origss, DerivedMesh *dm,
 	}
 
 	/* create edges */
-	ehash = BLI_edgehash_new(__func__);
+	ehash = BLI_edgehash_new_ex(__func__, BLI_EDGEHASH_SIZE_GUESS_FROM_POLYS(totface));
 
 	for (i = 0; i < totface; i++) {
 		MPoly *mp = &((MPoly *) mpoly)[i];

source/blender/blenlib/BLI_edgehash.h

@@ -62,4 +62,8 @@ void                BLI_edgehashIterator_setValue(EdgeHashIterator *ehi, void *v
 void                BLI_edgehashIterator_step(EdgeHashIterator *ehi);
 bool                BLI_edgehashIterator_isDone(EdgeHashIterator *ehi);
 
+#define BLI_EDGEHASH_SIZE_GUESS_FROM_LOOPS(totloop)  ((totloop) / 2)
+#define BLI_EDGEHASH_SIZE_GUESS_FROM_POLYS(totpoly)  ((totpoly) * 2)
+
+
 #endif

source/blender/editors/armature/meshlaplacian.c

@@ -281,7 +281,7 @@ static void laplacian_system_construct_end(LaplacianSystem *sys)
 
 	sys->varea = MEM_callocN(sizeof(float) * totvert, "LaplacianSystemVarea");
 
-	sys->edgehash = BLI_edgehash_new(__func__);
+	sys->edgehash = BLI_edgehash_new_ex(__func__, BLI_EDGEHASH_SIZE_GUESS_FROM_POLYS(sys->totface));
 	for (a = 0, face = sys->faces; a < sys->totface; a++, face++) {
 		laplacian_increase_edge_count(sys->edgehash, (*face)[0], (*face)[1]);
 		laplacian_increase_edge_count(sys->edgehash, (*face)[1], (*face)[2]);