Allow variables to implicitly convert to unresolved nets.

SystemVerilog allows variables to be either variables or unresolved
nets, depending on how they are used. If they are assigned by
procedural code, then they are variables. If they are assigned
by a continuous assignment, they are unresolved nets. Note that
they cannot be both, and when they are unresolved nets they can
only be assigned once.

compiler.h

@@ -151,6 +151,16 @@ extern bool gn_io_range_error_flag;
    re-evaluated. */
 extern bool gn_strict_ca_eval_flag;
 
+/* If variables can be converted to uwires by a continuous assignment
+   (assuming no procedural assign, then return true. This will be true
+   for SystemVerilog */
+static inline bool gn_var_can_be_uwire(void)
+{
+      if (generation_flag == GN_VER2009)
+	    return true;
+      return false;
+}
+
 /* The bits of these GN_KEYWORDS_* constants define non-intersecting
    sets of keywords. The compiler enables groups of keywords by setting
    lexor_keyword_mask with the OR of the bits for the keywords to be

elab_net.cc

@@ -414,6 +414,22 @@ NetNet* PEIdent::elaborate_lnet_common_(Design*des, NetScope*scope,
 
       assert(sig);
 
+	/* If this is SystemVerilog and the variable is not yet
+	   assigned by anything, then convert it to an unresolved
+	   wire. */
+      if (gn_var_can_be_uwire()
+	  && (sig->type() == NetNet::REG)
+	  && (sig->peek_eref() == 0) ) {
+	    sig->type(NetNet::UNRESOLVED_WIRE);
+      }
+
+      if (sig->type() == NetNet::UNRESOLVED_WIRE && sig->pin(0).is_linked()) {
+	    cerr << get_fileline() << ": error: Unresolved net " << sig->name()
+		 << " cannot have multiple drivers." << endl;
+	    des->errors += 1;
+	    return 0;
+      }
+
 	/* Don't allow registers as assign l-values. */
       if (sig->type() == NetNet::REG) {
 	    cerr << get_fileline() << ": error: reg " << sig->name()

netlist.cc

@@ -78,7 +78,7 @@ ostream& operator<< (ostream&o, NetNet::Type t)
 	  case NetNet::WIRE:
 	    o << "wire";
 	    break;
-	  case NetNet::UWIRE:
+	  case NetNet::UNRESOLVED_WIRE:
 	    o << "uwire";
       }
       return o;

netlist.h

@@ -553,7 +553,7 @@ class NetNet  : public NetObj {
     public:
       enum Type { NONE, IMPLICIT, IMPLICIT_REG, INTEGER, WIRE, TRI, TRI1,
 		  SUPPLY0, SUPPLY1, WAND, TRIAND, TRI0, WOR, TRIOR, REG,
-		  UWIRE };
+		  UNRESOLVED_WIRE };
 
       enum PortType { NOT_A_PORT, PIMPLICIT, PINPUT, POUTPUT, PINOUT };
 

parse.y

@@ -2708,12 +2708,12 @@ net_type
 	| K_supply1 { $$ = NetNet::SUPPLY1; }
 	| K_wor     { $$ = NetNet::WOR; }
 	| K_trior   { $$ = NetNet::TRIOR; }
-	| K_wone    { $$ = NetNet::UWIRE;
+	| K_wone    { $$ = NetNet::UNRESOLVED_WIRE;
 		      cerr << @1.text << ":" << @1.first_line << ": warning: "
 		              "'wone' is deprecated, please use 'uwire' "
 		              "instead." << endl;
 		    }
-	| K_uwire   { $$ = NetNet::UWIRE; }
+	| K_uwire   { $$ = NetNet::UNRESOLVED_WIRE; }
 	;
 
 var_type

t-dll.cc

@@ -2386,7 +2386,7 @@ void dll_target::signal(const NetNet*net)
 
 	      /* We will convert this to a TRI after we check that there
 		 is only one driver. */
-	  case NetNet::UWIRE:
+	  case NetNet::UNRESOLVED_WIRE:
 	    obj->type_ = IVL_SIT_UWIRE;
 	    break;