Attributes: Improve custom data initialization options

When allocating new `CustomData` layers, often we do redundant
initialization of arrays. For example, it's common that values are
allocated, set to their default value, and then set to some other
value. This is wasteful, and it negates the benefits of optimizations
to the allocator like D15082. There are two reasons for this. The
first is array-of-structs storage that makes it annoying to initialize
values manually, and the second is confusing options in the Custom Data
API. This patch addresses the latter.

The `CustomData` "alloc type" options are rearranged. Now, besides
the options that use existing layers, there are two remaining:
* `CD_SET_DEFAULT` sets the default value.
  * Usually zeroes, but for colors this is white (how it was before).
  * Should be used when you add the layer but don't set all values.
* `CD_CONSTRUCT` refers to the "default construct" C++ term.
  * Only necessary or defined for non-trivial types like vertex groups.
  * Doesn't do anything for trivial types like `int` or `float3`.
  * Should be used every other time, when all values will be set.

The attribute API's `AttributeInit` types are updated as well.
To update code, replace `CD_CALLOC` with `CD_SET_DEFAULT` and
`CD_DEFAULT` with `CD_CONSTRUCT`. This doesn't cause any functional
changes yet. Follow-up commits will change to avoid initializing
new layers where the correctness is clear.

Differential Revision: https://developer.blender.org/D15617

2 files changed, 7 insertions, 5 deletions

diff --git a/source/blender/editors/object/object_facemap_ops.c b/source/blender/editors/object/object_facemap_ops.c
index dddf5e40e87..4364375a4e3 100644
--- a/source/blender/editors/object/object_facemap_ops.c
+++ b/source/blender/editors/object/object_facemap_ops.c
@@ -53,7 +53,7 @@ void ED_object_facemap_face_add(Object *ob, bFaceMap *fmap, int facenum)
 
     /* if there's is no facemap layer then create one */
     if ((facemap = CustomData_get_layer(&me->pdata, CD_FACEMAP)) == NULL) {
-      facemap = CustomData_add_layer(&me->pdata, CD_FACEMAP, CD_DEFAULT, NULL, me->totpoly);
+      facemap = CustomData_add_layer(&me->pdata, CD_FACEMAP, CD_SET_DEFAULT, NULL, me->totpoly);
     }
 
     facemap[facenum] = fmap_nr;
diff --git a/source/blender/editors/object/object_modifier.cc b/source/blender/editors/object/object_modifier.cc
index f6e97f81cb6..085ef59ac21 100644
--- a/source/blender/editors/object/object_modifier.cc
+++ b/source/blender/editors/object/object_modifier.cc
@@ -586,9 +586,11 @@ bool ED_object_modifier_convert_psys_to_mesh(ReportList *UNUSED(reports),
   me->totvert = verts_num;
   me->totedge = edges_num;
 
-  me->mvert = (MVert *)CustomData_add_layer(&me->vdata, CD_MVERT, CD_CALLOC, nullptr, verts_num);
-  me->medge = (MEdge *)CustomData_add_layer(&me->edata, CD_MEDGE, CD_CALLOC, nullptr, edges_num);
-  me->mface = (MFace *)CustomData_add_layer(&me->fdata, CD_MFACE, CD_CALLOC, nullptr, 0);
+  me->mvert = (MVert *)CustomData_add_layer(
+      &me->vdata, CD_MVERT, CD_SET_DEFAULT, nullptr, verts_num);
+  me->medge = (MEdge *)CustomData_add_layer(
+      &me->edata, CD_MEDGE, CD_SET_DEFAULT, nullptr, edges_num);
+  me->mface = (MFace *)CustomData_add_layer(&me->fdata, CD_MFACE, CD_SET_DEFAULT, nullptr, 0);
 
   MVert *mvert = me->mvert;
   MEdge *medge = me->medge;
@@ -2642,7 +2644,7 @@ static Object *modifier_skin_armature_create(Depsgraph *depsgraph, Main *bmain,
   MVert *mvert = me_eval_deform->mvert;
 
   /* add vertex weights to original mesh */
-  CustomData_add_layer(&me->vdata, CD_MDEFORMVERT, CD_CALLOC, nullptr, me->totvert);
+  CustomData_add_layer(&me->vdata, CD_MDEFORMVERT, CD_SET_DEFAULT, nullptr, me->totvert);
 
   ViewLayer *view_layer = DEG_get_input_view_layer(depsgraph);
   Object *arm_ob = BKE_object_add(bmain, view_layer, OB_ARMATURE, nullptr);