Add experimental global undo speedup.

The feature is hidden behind an experimental option, you'll have to
enable it in the preferences to try it.

This feature is not yet considered fully stable, crashes may happen, as
well as .blend file corruptions (very unlikely, but still possible).

In a nutshell, the ideas behind this code are to:
* Detect unchanged IDs across an undo step.
* Reuse as much as possible existing IDs memory, even when its content
  did change.
* Re-use existing depsgraphs instead of building new ones from scratch.
* Store accumulated recalc flags, to avoid needless re-compute of things
  that did not change, when the ID itself is detected as modified.

See T60695 and D6580 for more technical details.

1 files changed, 3 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_undo_system.h b/source/blender/blenkernel/BKE_undo_system.h
index c503215be1f..4870b19fe1d 100644
--- a/source/blender/blenkernel/BKE_undo_system.h
+++ b/source/blender/blenkernel/BKE_undo_system.h
@@ -83,6 +83,9 @@ typedef struct UndoStep {
   bool skip;
   /** Some situations require the global state to be stored, edge cases when exiting modes. */
   bool use_memfile_step;
+  /** When this is true, undo/memfile read code is allowed to re-use old data-blocks for unchanged
+   * IDs, and existing depsgraphes. This has to be forbidden in some cases (like renamed IDs). */
+  bool use_old_bmain_data;
   /** For use by undo systems that accumulate changes (text editor, painting). */
   bool is_applied;
   /* Over alloc 'type->struct_size'. */