Functions: use static names for multi-functions

Previously, the function names were stored in `std::string` and were often
created dynamically (especially when the function just output a constant).
This resulted in a lot of overhead.

Now the function name is just a `const char *` that should be statically
allocated. This is good enough for the majority of cases. If a multi-function
needs a more dynamic name, it can override the `MultiFunction::debug_name`
method.

In my test file with >400,000 simple math nodes, the execution time improves from
3s to 1s.

1 files changed, 10 insertions, 3 deletions

diff --git a/source/blender/functions/FN_multi_function_signature.hh b/source/blender/functions/FN_multi_function_signature.hh
index d05948cc645..2ccaa60fed6 100644
--- a/source/blender/functions/FN_multi_function_signature.hh
+++ b/source/blender/functions/FN_multi_function_signature.hh
@@ -30,7 +30,14 @@
 namespace blender::fn {
 
 struct MFSignature {
-  std::string function_name;
+  /**
+   * The name should be statically allocated so that it lives longer than this signature. This is
+   * used instead of an #std::string because of the overhead when many functions are created.
+   * If the name of the function has to be more dynamic for debugging purposes, override
+   * #MultiFunction::debug_name() instead. Then the dynamic name will only be computed when it is
+   * actually needed.
+   */
+  const char *function_name;
   Vector<std::string> param_names;
   Vector<MFParamType> param_types;
   Vector<int> param_data_indices;
@@ -51,9 +58,9 @@ class MFSignatureBuilder {
   int vector_array_count_ = 0;
 
  public:
-  MFSignatureBuilder(std::string function_name)
+  MFSignatureBuilder(const char *function_name)
   {
-    signature_.function_name = std::move(function_name);
+    signature_.function_name = function_name;
   }
 
   MFSignature build() const