source/blender/blenkernel/intern/mesh.cc

@@ -1492,7 +1492,7 @@ std::optional<blender::Bounds<blender::float3>> Mesh::bounds_min_max() const
 
 void Mesh::bounds_set_eager(const blender::Bounds<float3> &bounds)
 {
-  this->runtime->bounds_cache.ensure([&](blender::Bounds<float3> &r_data) { r_data = bounds; });
+  this->runtime->bounds_cache.update([&](blender::Bounds<float3> &r_data) { r_data = bounds; });
 }
 
 void BKE_mesh_transform(Mesh *me, const float mat[4][4], bool do_keys)

source/blender/blenkernel/intern/pbvh.cc

@@ -1316,7 +1316,7 @@ static void pbvh_faces_update_normals(PBVH *pbvh, Span<PBVHNode *> nodes, Mesh &
   VectorSet<int> verts_to_update;
   threading::parallel_invoke(
       [&]() {
-        mesh.runtime->face_normals_cache.ensure([&](Vector<float3> &r_data) {
+        mesh.runtime->face_normals_cache.update([&](Vector<float3> &r_data) {
           threading::parallel_for(faces_to_update.index_range(), 512, [&](const IndexRange range) {
             for (const int i : faces_to_update.as_span().slice(range)) {
               r_data[i] = mesh::face_normal_calc(positions, corner_verts.slice(faces[i]));
@@ -1340,7 +1340,7 @@ static void pbvh_faces_update_normals(PBVH *pbvh, Span<PBVHNode *> nodes, Mesh &
       });
 
   const Span<float3> face_normals = mesh.face_normals();
-  mesh.runtime->vert_normals_cache.ensure([&](Vector<float3> &r_data) {
+  mesh.runtime->vert_normals_cache.update([&](Vector<float3> &r_data) {
     threading::parallel_for(verts_to_update.index_range(), 1024, [&](const IndexRange range) {
       for (const int vert : verts_to_update.as_span().slice(range)) {
         float3 normal(0.0f);

source/blender/blenlib/BLI_shared_cache.hh

@@ -30,6 +30,8 @@ template<typename T> class SharedCache {
   struct CacheData {
     CacheMutex mutex;
     T data;
+    CacheData() = default;
+    CacheData(const T &data) : data(data) {}
   };
   std::shared_ptr<CacheData> cache_;
 
@@ -60,6 +62,23 @@ template<typename T> class SharedCache {
     cache_->mutex.ensure([&]() { compute_cache(this->cache_->data); });
   }
 
+  /**
+   * Represents a combination of "tag dirty" and "update cache for new data." Existing cached
+   * values are kept available (copied from shared data if necessary). This can be helpful when
+   * the recalculation is only expected to make a small change to the cached data, since using
+   * #tag_dirty() and #ensure() separately may require rebuilding the cache from scratch.
+   */
+  void update(FunctionRef<void(T &data)> compute_cache)
+  {
+    if (cache_.unique()) {
+      cache_->mutex.tag_dirty();
+    }
+    else {
+      cache_ = std::make_shared<CacheData>(cache_->data);
+    }
+    cache_->mutex.ensure([&]() { compute_cache(this->cache_->data); });
+  }
+
   /** Retrieve the cached data. */
   const T &data() const
   {

source/blender/editors/sculpt_paint/sculpt.cc

@@ -5421,9 +5421,12 @@ void SCULPT_flush_update_step(bContext *C, SculptUpdateType update_flags)
 
   if (update_flags & SCULPT_UPDATE_COORDS && !ss->shapekey_active) {
     if (BKE_pbvh_type(ss->pbvh) == PBVH_FACES) {
-      /* When sculpting and changing the positions of a mesh, tag them as changed and update. */
+      /* Updating mesh positions without marking caches dirty is generally not good, but since
-      BKE_mesh_tag_positions_changed(mesh);
+       * sculpt mode has special requirements and is expected to have sole ownership of the mesh it
-      /* Update the mesh's bounds eagerly since the PBVH already has that information. */
+       * modifies, it's generally okay.
+       *
+       * Vertex and face normals are updated later in #BKE_pbvh_update_normals. However, we update
+       * the mesh's bounds eagerly here since they are trivial to access from the PBVH. */
       Bounds<float3> bounds;
       BKE_pbvh_bounding_box(ob->sculpt->pbvh, bounds.min, bounds.max);
       mesh->bounds_set_eager(bounds);

source/blender/makesdna/DNA_mesh_types.h

@@ -313,7 +313,10 @@ typedef struct Mesh {
    */
   std::optional<blender::Bounds<blender::float3>> bounds_min_max() const;
 
-  /** Set cached mesh bounds to a known-correct value to avoid their lazy calculation later on. */
+  /**
+   * After positions are changed, set cached mesh bounds to a known-correct value to avoid their
+   * lazy calculation later on.
+   */
   void bounds_set_eager(const blender::Bounds<blender::float3> &bounds);
 
   /**