Add support for module input port default values (issue #489).

Module.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998-2019 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 1998-2021 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -104,6 +104,11 @@ perm_string Module::get_port_name(unsigned idx) const
       return ports[idx]->name;
 }
 
+PExpr* Module::get_port_default_value(unsigned idx) const
+{
+      assert(idx < ports.size());
+      return ports[idx] ? ports[idx]->default_value : 0;
+}
 
 
 PGate* Module::get_gate(perm_string name)

Module.h

@@ -1,7 +1,7 @@
 #ifndef IVL_Module_H
 #define IVL_Module_H
 /*
- * Copyright (c) 1998-2019 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 1998-2021 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -60,11 +60,13 @@ class Module : public PScopeExtra, public PNamedItem {
 	   objects. Each port has a name and an ordered list of
 	   wires. The name is the means that the outside uses to
 	   access the port, the wires are the internal connections to
-	   the port. */
+	   the port. In SystemVerilog, input ports may also have a
+	   default value. */
     public:
       struct port_t {
 	    perm_string name;
 	    vector<PEIdent*> expr;
+	    PExpr*default_value;
       };
 
     public:
@@ -148,6 +150,8 @@ class Module : public PScopeExtra, public PNamedItem {
       // Return port name ("" for undeclared port)
       perm_string get_port_name(unsigned idx) const;
 
+      PExpr* get_port_default_value(unsigned idx) const;
+
       PGate* get_gate(perm_string name);
 
       const list<PGate*>& get_gates() const;

elaborate.cc

@@ -1323,9 +1323,15 @@ void PGModule::elaborate_mod_(Design*des, Module*rmod, NetScope*scope) const
 
 	    perm_string port_name = rmod->get_port_name(idx);
 
-	      // Skip unconnected module ports. This happens when a
-	      // null parameter is passed in.
+	      // If the port is unconnected, substitute the default
+	      // value. The parser ensures that a default value only
+	      // exists for input ports.
+	    if (pins[idx] == 0)
+		  pins[idx] = rmod->get_port_default_value(idx);
 
+	      // Skip unconnected module ports. This happens when a
+	      // null parameter is passed in and there is no default 
+	      // value.
 	    if (pins[idx] == 0) {
 
 		  if (pins_fromwc[idx]) {

parse.y

@@ -4634,6 +4634,22 @@ port_declaration
 	delete[]$4;
 	$$ = ptmp;
       }
+  | attribute_list_opt K_input net_type_opt data_type_or_implicit IDENTIFIER '=' expression
+      { if (!gn_system_verilog()) {
+	      yyerror("error: Default port values require SystemVerilog.");
+	}
+	Module::port_t*ptmp;
+	perm_string name = lex_strings.make($5);
+	data_type_t*use_type = $4;
+	ptmp = pform_module_port_reference(name, @2.text, @2.first_line);
+	ptmp->default_value = $7;
+	pform_module_define_port(@2, name, NetNet::PINPUT, $3, use_type, $1);
+	port_declaration_context.port_type = NetNet::PINPUT;
+	port_declaration_context.port_net_type = $3;
+	port_declaration_context.data_type = $4;
+	delete[]$5;
+	$$ = ptmp;
+      }
   | attribute_list_opt K_inout net_type_opt data_type_or_implicit IDENTIFIER dimensions_opt
       { Module::port_t*ptmp;
 	perm_string name = lex_strings.make($5);
@@ -5972,6 +5988,7 @@ port_reference
 	  Module::port_t*ptmp = new Module::port_t;
 	  ptmp->name = perm_string();
 	  ptmp->expr.push_back(wtmp);
+	  ptmp->default_value = 0;
 
 	  delete[]$1;
 	  $$ = ptmp;
@@ -5995,6 +6012,7 @@ port_reference
 	  Module::port_t*ptmp = new Module::port_t;
 	  ptmp->name = perm_string();
 	  ptmp->expr.push_back(tmp);
+	  ptmp->default_value = 0;
 	  delete[]$1;
 	  $$ = ptmp;
 	}
@@ -6006,6 +6024,7 @@ port_reference
 	  FILE_NAME(wtmp, @1);
 	  ptmp->name = lex_strings.make($1);
 	  ptmp->expr.push_back(wtmp);
+	  ptmp->default_value = 0;
 	  delete[]$1;
 	  $$ = ptmp;
 	}

pform.cc

@@ -1394,6 +1394,7 @@ Module::port_t* pform_module_port_reference(perm_string name,
       FILE_NAME(tmp, file, lineno);
       ptmp->name = name;
       ptmp->expr.push_back(tmp);
+      ptmp->default_value = 0;
 
       return ptmp;
 }