diff --git a/vhdlpp/entity.h b/vhdlpp/entity.h
index 4faefe441..01a5e15fb 100644
--- a/vhdlpp/entity.h
+++ b/vhdlpp/entity.h
@@ -45,6 +45,10 @@ class InterfacePort : public LineInfo {
           : mode(PORT_NONE), type(typ), expr(NULL)
       {}
 
+      InterfacePort(const VType*typ, port_mode_t mod)
+          : mode(mod), type(typ), expr(NULL)
+      {}
+
 	// Port direction from the source code.
       port_mode_t mode;
 	// Name of the port from the source code
diff --git a/vhdlpp/expression.h b/vhdlpp/expression.h
index ee609ae60..50c2e8e2c 100644
--- a/vhdlpp/expression.h
+++ b/vhdlpp/expression.h
@@ -903,4 +903,8 @@ class ExpTime : public Expression {
         timeunit_t unit_;
 };
 
+// Elaborates an expression used as an argument in a procedure/function call.
+int elaborate_argument(Expression*expr, const SubprogramHeader*subp,
+                       int idx, Entity*ent, ScopeBase*scope);
+
 #endif /* IVL_expression_H */
diff --git a/vhdlpp/expression_elaborate.cc b/vhdlpp/expression_elaborate.cc
index 0c57544b3..79fa6c04c 100644
--- a/vhdlpp/expression_elaborate.cc
+++ b/vhdlpp/expression_elaborate.cc
@@ -782,14 +782,8 @@ int ExpFunc::elaborate_expr(Entity*ent, ScopeBase*scope, const VType*)
       def_ = prog;
 
 	// Elaborate arguments
-      for (size_t idx = 0 ; idx < argv_.size() ; idx += 1) {
-	    const VType*tmp = argv_[idx]->probe_type(ent, scope);
-	    const VType*param_type = prog ? prog->peek_param_type(idx) : NULL;
-
-	    if(!tmp && param_type)
-	        tmp = param_type;
-
-	    errors += argv_[idx]->elaborate_expr(ent, scope, tmp);
+      for (size_t idx = 0; idx < argv_.size(); ++idx) {
+	    errors += elaborate_argument(argv_[idx], prog, idx, ent, scope);
       }
 
 	// SystemVerilog functions work only with defined size data types, therefore
@@ -1058,3 +1052,28 @@ int ExpTime::elaborate_expr(Entity*, ScopeBase*, const VType*)
       set_type(&primitive_INTEGER);
       return 0;
 }
+
+int elaborate_argument(Expression*expr, const SubprogramHeader*subp,
+                       int idx, Entity*ent, ScopeBase*scope)
+{
+    const VType*type = expr->probe_type(ent, scope);
+
+    if(subp) {
+        const InterfacePort*param = subp->peek_param(idx);
+
+        // Enable reg_flag for variables that might be modified in subprograms
+        if(param->mode == PORT_OUT || param->mode == PORT_INOUT) {
+            if(const ExpName*e = dynamic_cast<const ExpName*>(expr)) {
+                if(Signal*sig = scope->find_signal(e->peek_name()))
+                    sig->count_ref_sequ();
+                else if(Variable*var = scope->find_variable(e->peek_name()))
+                    var->count_ref_sequ();
+            }
+        }
+
+        if(!type)
+            type = param->type;
+    }
+
+    return expr->elaborate_expr(ent, scope, type);
+}
diff --git a/vhdlpp/sequential_elaborate.cc b/vhdlpp/sequential_elaborate.cc
index 6967fc574..3f90f0f9f 100644
--- a/vhdlpp/sequential_elaborate.cc
+++ b/vhdlpp/sequential_elaborate.cc
@@ -173,13 +173,8 @@ int ProcedureCall::elaborate(Entity*ent, ScopeBase*scope)
       if(param_list_) {
 	    for(list<named_expr_t*>::iterator cur = param_list_->begin()
 		 ; cur != param_list_->end() ; ++cur) {
-                const VType*tmp = (*cur)->expr()->probe_type(ent, scope);
-                const VType*param_type = def_ ? def_->peek_param_type(idx) : NULL;
-
-                if(!tmp && param_type)
-                    tmp = param_type;
-
-                errors += (*cur)->expr()->elaborate_expr(ent, scope, tmp);
+		errors += elaborate_argument((*cur)->expr(), def_, idx, ent, scope);
+		++idx;
             }
       }
 
diff --git a/vhdlpp/subprogram.cc b/vhdlpp/subprogram.cc
index 9f6c58aee..59862f97d 100644
--- a/vhdlpp/subprogram.cc
+++ b/vhdlpp/subprogram.cc
@@ -138,7 +138,7 @@ const InterfacePort*SubprogramHeader::find_param(perm_string nam) const
       return NULL;
 }
 
-const VType*SubprogramHeader::peek_param_type(int idx) const
+const InterfacePort*SubprogramHeader::peek_param(int idx) const
 {
       if(!ports_ || idx < 0 || (size_t)idx >= ports_->size())
         return NULL;
@@ -146,7 +146,17 @@ const VType*SubprogramHeader::peek_param_type(int idx) const
       std::list<InterfacePort*>::const_iterator p = ports_->begin();
       std::advance(p, idx);
 
-      return (*p)->type;
+      return *p;
+}
+
+const VType*SubprogramHeader::peek_param_type(int idx) const
+{
+      const InterfacePort*port = peek_param(idx);
+
+      if(port)
+        return port->type;
+
+      return NULL;
 }
 
 void SubprogramHeader::set_parent(const ScopeBase*par)
diff --git a/vhdlpp/subprogram.h b/vhdlpp/subprogram.h
index 0aa5dbdbc..6972ae0be 100644
--- a/vhdlpp/subprogram.h
+++ b/vhdlpp/subprogram.h
@@ -71,6 +71,7 @@ class SubprogramHeader : public LineInfo {
       bool compare_specification(SubprogramHeader*that) const;
 
       const InterfacePort*find_param(perm_string nam) const;
+      const InterfacePort*peek_param(int idx) const;
       const VType*peek_param_type(int idx) const;
       const VType*peek_return_type() const { return return_type_; }