Geometry Nodes: multi threaded field evaluation

This adds a new `ParallelMultiFunction` which wraps another multi-function
and evaluates it with multiple threads. The speeds up field evaluation
quite a bit (the effect is most noticeable when the number of evaluations
and the field is large).

There are still other single-threaded performance bottlenecks in field
evaluation that will need to be solved separately. Most notably here
is the process of copying the computed data into the position attribute
in the Set Position node.

Differential Revision: https://developer.blender.org/D12457

1 files changed, 46 insertions, 0 deletions

diff --git a/source/blender/functions/FN_generic_virtual_array.hh b/source/blender/functions/FN_generic_virtual_array.hh
index f429243e2de..703118ba23e 100644
--- a/source/blender/functions/FN_generic_virtual_array.hh
+++ b/source/blender/functions/FN_generic_virtual_array.hh
@@ -911,4 +911,50 @@ template<typename T> class GVMutableArray_Typed {
   }
 };
 
+class GVArray_For_SlicedGVArray : public GVArray {
+ protected:
+  const GVArray &varray_;
+  int64_t offset_;
+
+ public:
+  GVArray_For_SlicedGVArray(const GVArray &varray, const IndexRange slice)
+      : GVArray(varray.type(), slice.size()), varray_(varray), offset_(slice.start())
+  {
+    BLI_assert(slice.one_after_last() <= varray.size());
+  }
+
+  /* TODO: Add #materialize method. */
+  void get_impl(const int64_t index, void *r_value) const override;
+  void get_to_uninitialized_impl(const int64_t index, void *r_value) const override;
+};
+
+/**
+ * Utility class to create the "best" sliced virtual array.
+ */
+class GVArray_Slice {
+ private:
+  const GVArray *varray_;
+  /* Of these optional virtual arrays, at most one is constructed at any time. */
+  std::optional<GVArray_For_GSpan> varray_span_;
+  std::optional<GVArray_For_SlicedGVArray> varray_any_;
+
+ public:
+  GVArray_Slice(const GVArray &varray, const IndexRange slice);
+
+  const GVArray &operator*()
+  {
+    return *varray_;
+  }
+
+  const GVArray *operator->()
+  {
+    return varray_;
+  }
+
+  operator const GVArray &()
+  {
+    return *varray_;
+  }
+};
+
 }  // namespace blender::fn