Functions: make multi functions smaller and cheaper to construct in many cases

Previously, the signature of a `MultiFunction` was always embedded into the function.
There are two issues with that. First, `MFSignature` is relatively large, because it contains
multiple strings and vectors. Secondly, constructing it can add overhead that should not
be necessary, because often the same signature can be reused.

The solution is to only keep a pointer to a signature in `MultiFunction` that is set during
construction. Child classes are responsible for making sure that the signature lives
long enough. In most cases, the signature is either embedded into the child class or
it is allocated statically (and is only created once).

3 files changed, 21 insertions, 5 deletions

diff --git a/source/blender/functions/intern/multi_function.cc b/source/blender/functions/intern/multi_function.cc
index a41c6bdf3cd..bb657f321ec 100644
--- a/source/blender/functions/intern/multi_function.cc
+++ b/source/blender/functions/intern/multi_function.cc
@@ -22,7 +22,14 @@ class DummyMultiFunction : public MultiFunction {
  public:
   DummyMultiFunction()
   {
-    this->get_builder("Dummy");
+    static MFSignature signature = create_signature();
+    this->set_signature(&signature);
+  }
+
+  static MFSignature create_signature()
+  {
+    MFSignatureBuilder signature{"Dummy"};
+    return signature.build();
   }
 
   void call(IndexMask UNUSED(mask),
diff --git a/source/blender/functions/intern/multi_function_builder.cc b/source/blender/functions/intern/multi_function_builder.cc
index c9e8b88ba03..3567f4f9167 100644
--- a/source/blender/functions/intern/multi_function_builder.cc
+++ b/source/blender/functions/intern/multi_function_builder.cc
@@ -23,10 +23,12 @@ namespace blender::fn {
 CustomMF_GenericConstant::CustomMF_GenericConstant(const CPPType &type, const void *value)
     : type_(type), value_(value)
 {
-  MFSignatureBuilder signature = this->get_builder("Constant " + type.name());
+  MFSignatureBuilder signature{"Constant " + type.name()};
   std::stringstream ss;
   type.debug_print(value, ss);
   signature.single_output(ss.str(), type);
+  signature_ = signature.build();
+  this->set_signature(&signature_);
 }
 
 void CustomMF_GenericConstant::call(IndexMask mask,
@@ -73,8 +75,10 @@ static std::string gspan_to_string(GSpan array)
 CustomMF_GenericConstantArray::CustomMF_GenericConstantArray(GSpan array) : array_(array)
 {
   const CPPType &type = array.type();
-  MFSignatureBuilder signature = this->get_builder("Constant " + type.name() + " Vector");
+  MFSignatureBuilder signature{"Constant " + type.name() + " Vector"};
   signature.vector_output(gspan_to_string(array), type);
+  signature_ = signature.build();
+  this->set_signature(&signature_);
 }
 
 void CustomMF_GenericConstantArray::call(IndexMask mask,
@@ -92,13 +96,15 @@ CustomMF_DefaultOutput::CustomMF_DefaultOutput(StringRef name,
                                                Span<MFDataType> output_types)
     : output_amount_(output_types.size())
 {
-  MFSignatureBuilder signature = this->get_builder(name);
+  MFSignatureBuilder signature{name};
   for (MFDataType data_type : input_types) {
     signature.input("Input", data_type);
   }
   for (MFDataType data_type : output_types) {
     signature.output("Output", data_type);
   }
+  signature_ = signature.build();
+  this->set_signature(&signature_);
 }
 void CustomMF_DefaultOutput::call(IndexMask mask, MFParams params, MFContext UNUSED(context)) const
 {
diff --git a/source/blender/functions/intern/multi_function_network_evaluation.cc b/source/blender/functions/intern/multi_function_network_evaluation.cc
index 98500a350ae..daa0a42963a 100644
--- a/source/blender/functions/intern/multi_function_network_evaluation.cc
+++ b/source/blender/functions/intern/multi_function_network_evaluation.cc
@@ -115,7 +115,7 @@ MFNetworkEvaluator::MFNetworkEvaluator(Vector<const MFOutputSocket *> inputs,
     : inputs_(std::move(inputs)), outputs_(std::move(outputs))
 {
   BLI_assert(outputs_.size() > 0);
-  MFSignatureBuilder signature = this->get_builder("Function Tree");
+  MFSignatureBuilder signature{"Function Tree"};
 
   for (const MFOutputSocket *socket : inputs_) {
     BLI_assert(socket->node().is_dummy());
@@ -144,6 +144,9 @@ MFNetworkEvaluator::MFNetworkEvaluator(Vector<const MFOutputSocket *> inputs,
         break;
     }
   }
+
+  signature_ = signature.build();
+  this->set_signature(&signature_);
 }
 
 void MFNetworkEvaluator::call(IndexMask mask, MFParams params, MFContext context) const