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, 43 insertions, 0 deletions

diff --git a/source/blender/functions/intern/generic_virtual_array.cc b/source/blender/functions/intern/generic_virtual_array.cc
index bd033a429de..9a83d8cd497 100644
--- a/source/blender/functions/intern/generic_virtual_array.cc
+++ b/source/blender/functions/intern/generic_virtual_array.cc
@@ -387,4 +387,47 @@ void GVMutableArray_GSpan::disable_not_applied_warning()
   show_not_saved_warning_ = false;
 }
 
+/* --------------------------------------------------------------------
+ * GVArray_For_SlicedGVArray.
+ */
+
+void GVArray_For_SlicedGVArray::get_impl(const int64_t index, void *r_value) const
+{
+  varray_.get(index + offset_, r_value);
+}
+
+void GVArray_For_SlicedGVArray::get_to_uninitialized_impl(const int64_t index, void *r_value) const
+{
+  varray_.get_to_uninitialized(index + offset_, r_value);
+}
+
+/* --------------------------------------------------------------------
+ * GVArray_Slice.
+ */
+
+GVArray_Slice::GVArray_Slice(const GVArray &varray, const IndexRange slice)
+{
+  if (varray.is_span()) {
+    /* Create a new virtual for the sliced span. */
+    const GSpan span = varray.get_internal_span();
+    const GSpan sliced_span = span.slice(slice.start(), slice.size());
+    varray_span_.emplace(sliced_span);
+    varray_ = &*varray_span_;
+  }
+  else if (varray.is_single()) {
+    /* Can just use the existing virtual array, because it's the same value for the indices in the
+     * slice anyway. */
+    varray_ = &varray;
+  }
+  else {
+    /* Generic version when none of the above method works.
+     * We don't necessarily want to materialize the input varray because there might be
+     * large distances between the required indices. Then we would materialize many elements that
+     * are not accessed later on.
+     */
+    varray_any_.emplace(varray, slice);
+    varray_ = &*varray_any_;
+  }
+}
+
 }  // namespace blender::fn