BLI: add use_threading parameter to parallel_invoke

`parallel_invoke` allows executing functions on separate threads.
However, creating tasks in tbb has a measurable amount of overhead.
Therefore, it can be benefitial to disable parallelization when
the amount of work done per function is small.

See D15539 for some benchmark results.

Differential Revision: https://developer.blender.org/D15539

source/blender/blenkernel/intern/curves_geometry.cc

@@ -1165,6 +1165,7 @@ static CurvesGeometry copy_with_removed_points(const CurvesGeometry &curves,
   CurvesGeometry new_curves{new_point_count, new_curve_count};
 
   threading::parallel_invoke(
+      256 < new_point_count * new_curve_count,
       /* Initialize curve offsets. */
       [&]() { new_curves.offsets_for_write().copy_from(new_curve_offsets); },
       [&]() {
@@ -1237,6 +1238,7 @@ static CurvesGeometry copy_with_removed_curves(const CurvesGeometry &curves,
   CurvesGeometry new_curves{new_tot_points, new_tot_curves};
 
   threading::parallel_invoke(
+      256 < new_tot_points * new_tot_curves,
       /* Initialize curve offsets. */
       [&]() {
         MutableSpan<int> new_offsets = new_curves.offsets_for_write();

source/blender/blenlib/BLI_task.hh

@@ -105,6 +105,22 @@ template<typename... Functions> void parallel_invoke(Functions &&...functions)
 #endif
 }
 
+/**
+ * Same #parallel_invoke, but allows disabling threading dynamically. This is useful because when
+ * the individual functions do very little work, there is a lot of overhead from starting parallel
+ * tasks.
+ */
+template<typename... Functions>
+void parallel_invoke(const bool use_threading, Functions &&...functions)
+{
+  if (use_threading) {
+    parallel_invoke(std::forward<Functions>(functions)...);
+  }
+  else {
+    (functions(), ...);
+  }
+}
+
 /** See #BLI_task_isolate for a description of what isolating a task means. */
 template<typename Function> void isolate_task(const Function &function)
 {

source/blender/editors/sculpt_paint/curves_sculpt_density.cc

@@ -188,6 +188,7 @@ struct DensityAddOperationExecutor {
      * curves. */
     Array<bool> new_curve_skipped(new_positions_cu.size(), false);
     threading::parallel_invoke(
+        512 < already_added_curves + new_positions_cu.size(),
         /* Build kdtree from root points created by the current stroke. */
         [&]() {
           for (const int i : IndexRange(already_added_curves)) {
@@ -309,6 +310,7 @@ struct DensityAddOperationExecutor {
     };
 
     threading::parallel_invoke(
+        1024 < original_positions.size() + deformed_positions.size(),
         [&]() {
           self_->original_curve_roots_kdtree_ = roots_kdtree_from_positions(original_positions);
         },

source/blender/editors/sculpt_paint/curves_sculpt_ops.cc

@@ -734,6 +734,8 @@ static void select_grow_invoke_per_curve(Curves &curves_id,
   }
 
   threading::parallel_invoke(
+      1024 < curve_op_data.selected_point_indices.size() +
+                 curve_op_data.unselected_point_indices.size(),
       [&]() {
         /* Build KD-tree for the selected points. */
         KDTree_3d *kdtree = BLI_kdtree_3d_new(curve_op_data.selected_point_indices.size());

source/blender/nodes/geometry/nodes/node_geo_deform_curves_on_surface.cc

@@ -60,6 +60,7 @@ static void deform_curves(const CurvesGeometry &curves,
   Array<ReverseUVSampler::Result> surface_samples_old(curves_num);
   Array<ReverseUVSampler::Result> surface_samples_new(curves_num);
   threading::parallel_invoke(
+      1024 < curves_num,
       [&]() { reverse_uv_sampler_old.sample_many(curve_attachment_uvs, surface_samples_old); },
       [&]() { reverse_uv_sampler_new.sample_many(curve_attachment_uvs, surface_samples_new); });
 

source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_line.cc

@@ -183,6 +183,7 @@ Mesh *create_line_mesh(const float3 start, const float3 delta, const int count)
   MutableSpan<MEdge> edges{mesh->medge, mesh->totedge};
 
   threading::parallel_invoke(
+      1024 < count,
       [&]() {
         threading::parallel_for(verts.index_range(), 4096, [&](IndexRange range) {
           for (const int i : range) {

source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_uv_sphere.cc

@@ -307,6 +307,7 @@ static Mesh *create_uv_sphere_mesh(const float radius, const int segments, const
   MutableSpan<MPoly> polys{mesh->mpoly, mesh->totpoly};
 
   threading::parallel_invoke(
+      1024 < segments * rings,
       [&]() {
         MutableSpan vert_normals{(float3 *)BKE_mesh_vertex_normals_for_write(mesh), mesh->totvert};
         calculate_sphere_vertex_data(verts, vert_normals, radius, segments, rings);