Fix T58251: Cycles ignores linked meshes when rendering

The idea is to share a mesh data-block as a result across all objects
which are sharing same original mesh and have no effective modifiers.
This mesh is owned by an original copy-on-written version of object data.

Tricky part is to make sure it is only initialized once, and currently a
silly mutex lock is used. In practice it only locks if the mesh is not
already there.

As an extra bonus, even viewport memory is also lower after this change.

Reviewers: brecht, mont29

Reviewed By: brecht, mont29

Differential Revision: https://developer.blender.org/D4954

1 files changed, 6 insertions, 0 deletions

diff --git a/source/blender/makesdna/DNA_mesh_types.h b/source/blender/makesdna/DNA_mesh_types.h
index f54c178f1d0..8a9a69ac178 100644
--- a/source/blender/makesdna/DNA_mesh_types.h
+++ b/source/blender/makesdna/DNA_mesh_types.h
@@ -75,6 +75,12 @@ struct MLoopTri_Store {
 
 /* not saved in file! */
 typedef struct Mesh_Runtime {
+  /* Evaluated mesh for objects which do not have effective modifiers. This mesh is sued as a
+   * result of modifier stack evaluation.
+   * Since modifier stack evaluation is threaded on object level we need some synchronization. */
+  struct Mesh *mesh_eval;
+  void *eval_mutex;
+
   struct EditMeshData *edit_data;
   void *batch_cache;