Load 'morphology' symbol during registration (#1393)

This way if someone define again this symbol (in hoc file for example),
neuron will use the right one during allocation of the module.

This is how it is currently done in neuron.

This behaviour has been spotted with nmodl benchmark.

src/codegen/codegen_neuron_cpp_visitor.cpp

@@ -932,6 +932,10 @@ void CodegenNeuronCppVisitor::print_mechanism_global_var_structure(bool print_in
         // throw std::runtime_error("Not implemented, global vectorize something else.");
     }
 
+    if (info.diam_used) {
+        printer->fmt_line("Symbol* _morphology_sym;");
+    }
+
     printer->pop_block(";");
 
     print_global_var_struct_assertions();
@@ -1225,6 +1229,11 @@ void CodegenNeuronCppVisitor::print_mechanism_register() {
         printer->add_line("_nrn_thread_reg(mech_type, 0, thread_mem_cleanup);");
     }
 
+    if (info.diam_used) {
+        printer->fmt_line("{}._morphology_sym = hoc_lookup(\"morphology\");",
+                          global_struct_instance());
+    }
+
     printer->pop_block();
 }
 
@@ -1609,9 +1618,8 @@ void CodegenNeuronCppVisitor::print_nrn_alloc() {
         for (size_t i = 0; i < codegen_int_variables.size(); ++i) {
             auto var_info = codegen_int_variables[i];
             if (var_info.symbol->get_name() == naming::DIAM_VARIABLE) {
-                printer->add_line("Symbol * morphology_sym = hoc_lookup(\"morphology\");");
-                printer->fmt_line("Prop * morphology_prop = need_memb(morphology_sym);");
-
+                printer->fmt_line("Prop * morphology_prop = need_memb({}._morphology_sym);",
+                                  global_struct_instance());
                 printer->fmt_line(
                     "_ppvar[{}] = _nrn_mechanism_get_param_handle(morphology_prop, 0);", i);
             }