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, 4 insertions, 0 deletions

diff --git a/source/blender/blenloader/BLO_undofile.h b/source/blender/blenloader/BLO_undofile.h
index 0388b3f3520..5f1142cc20e 100644
--- a/source/blender/blenloader/BLO_undofile.h
+++ b/source/blender/blenloader/BLO_undofile.h
@@ -34,6 +34,10 @@ typedef struct {
   unsigned int size;
   /** When true, this chunk doesn't own the memory, it's shared with a previous #MemFileChunk */
   bool is_identical;
+  /** When true, this chunk is also identical to the one in the next step (used by undo code to
+   * detect unchanged IDs).
+   * Defined when writing the next step (i.e. last undo step has those always false). */
+  bool is_identical_future;
 } MemFileChunk;
 
 typedef struct MemFile {