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, 28 insertions, 0 deletions

diff --git a/source/blender/functions/FN_multi_function.hh b/source/blender/functions/FN_multi_function.hh
index 3059fe59ca7..af60e54808e 100644
--- a/source/blender/functions/FN_multi_function.hh
+++ b/source/blender/functions/FN_multi_function.hh
@@ -60,6 +60,7 @@ class MultiFunction {
   {
   }
 
+  void call_auto(IndexMask mask, MFParams params, MFContext context) const;
   virtual void call(IndexMask mask, MFParams params, MFContext context) const = 0;
 
   virtual uint64_t hash() const
@@ -110,6 +111,31 @@ class MultiFunction {
     return *signature_ref_;
   }
 
+  /**
+   * Information about how the multi-function behaves that help a caller to execute it efficiently.
+   */
+  struct ExecutionHints {
+    /**
+     * Suggested minimum workload under which multi-threading does not really help.
+     * This should be lowered when the multi-function is doing something computationally expensive.
+     */
+    int64_t min_grain_size = 10000;
+    /**
+     * Indicates that the multi-function will allocate an array large enough to hold all indices
+     * passed in as mask. This tells the caller that it would be preferable to pass in smaller
+     * indices. Also maybe the full mask should be split up into smaller segments to decrease peak
+     * memory usage.
+     */
+    bool allocates_array = false;
+    /**
+     * Tells the caller that every execution takes about the same time. This helps making a more
+     * educated guess about a good grain size.
+     */
+    bool uniform_execution_time = true;
+  };
+
+  ExecutionHints execution_hints() const;
+
  protected:
   /* Make the function use the given signature. This should be called once in the constructor of
    * child classes. No copy of the signature is made, so the caller has to make sure that the
@@ -121,6 +147,8 @@ class MultiFunction {
     BLI_assert(signature != nullptr);
     signature_ref_ = signature;
   }
+
+  virtual ExecutionHints get_execution_hints() const;
 };
 
 inline MFParamsBuilder::MFParamsBuilder(const MultiFunction &fn, int64_t mask_size)