Geometry Nodes: refactor multi-threading in field evaluation

Previously, there was a fixed grain size for all multi-functions. That was
not sufficient because some functions could benefit a lot from smaller
grain sizes.

This refactors adds a new `MultiFunction::call_auto` method which has the
same effect as just calling `MultiFunction::call` but additionally figures
out how to execute the specific multi-function efficiently. It determines
a good grain size and decides whether the mask indices should be shifted
or not.

Most multi-function evaluations benefit from this, but medium sized work
loads (1000 - 50000 elements) benefit from it the most. Especially when
expensive multi-functions (e.g. noise) is involved. This is because for
smaller work loads, threading is rarely used and for larger work loads
threading worked fine before already.

With this patch, multi-functions can specify execution hints, that allow
the caller to execute it most efficiently. These execution hints still
have to be added to more functions.

Some performance measurements of a field evaluation involving noise and
math nodes, ordered by the number of elements being evaluated:
```
1,000,000: 133 ms   -> 120 ms
  100,000:  30 ms   ->  18 ms
   10,000:  20 ms   ->   2.7 ms
    1,000:   4 ms   ->   0.5 ms
      100:   0.5 ms ->   0.4 ms
```

1 files changed, 44 insertions, 0 deletions

diff --git a/source/blender/functions/intern/multi_function_params.cc b/source/blender/functions/intern/multi_function_params.cc
new file mode 100644
index 00000000000..376c5b2deb7
--- /dev/null
+++ b/source/blender/functions/intern/multi_function_params.cc
@@ -0,0 +1,44 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#include "FN_multi_function_params.hh"
+
+namespace blender::fn {
+
+GMutableSpan MFParams::ensure_dummy_single_output(int data_index)
+{
+  /* Lock because we are actually modifying #builder_ and it may be used by multiple threads. */
+  std::lock_guard lock{builder_->mutex_};
+
+  for (const std::pair<int, GMutableSpan> &items : builder_->dummy_output_spans_) {
+    if (items.first == data_index) {
+      return items.second;
+    }
+  }
+
+  const CPPType &type = builder_->mutable_spans_[data_index].type();
+  void *buffer = builder_->scope_.linear_allocator().allocate(
+      builder_->min_array_size_ * type.size(), type.alignment());
+  if (!type.is_trivially_destructible()) {
+    builder_->scope_.add_destruct_call(
+        [&type, buffer, mask = builder_->mask_]() { type.destruct_indices(buffer, mask); });
+  }
+  const GMutableSpan span{type, buffer, builder_->min_array_size_};
+  builder_->dummy_output_spans_.append({data_index, span});
+  return span;
+}
+
+}  // namespace blender::fn