Avoid creating Nexus objects until they are really needed.

By not creating Nexus objects until necessary, we avoid creating a
lot of spurious objects. In fact, it is true that almost every
link that is created and connected to another link will create a
spurious Nexus object without this patch.

design_dump.cc

@@ -215,7 +215,10 @@ void NetNet::dump_net(ostream&o, unsigned ind) const
 
       o << " (";
       for (unsigned idx = pin_count() ;  idx > 0 ;  idx -= 1)
-	    o << pin(idx-1).nexus()->get_init();
+	    if (const Nexus*tmp = pin(idx-1).nexus())
+		  o << tmp->get_init();
+	    else
+		  o << ".";
       o << ")" << endl;
 
       for (unsigned idx = 0 ;  idx < pin_count() ;  idx += 1) {

ivl_target.h

@@ -29,6 +29,9 @@
 #define _END_DECL
 #endif
 
+#ifndef __GNUC__
+# define __attribute__(x)
+#endif
 
 _BEGIN_DECL
 
@@ -173,7 +176,7 @@ typedef struct ivl_process_s  *ivl_process_t;
 typedef struct ivl_scope_s    *ivl_scope_t;
 typedef struct ivl_signal_s   *ivl_signal_t;
 typedef struct ivl_switch_s   *ivl_switch_t;
-typedef struct ivl_memory_s   *ivl_memory_t; /* DEPRECATED */
+typedef struct ivl_memory_s   *ivl_memory_t; //XXXX __attribute__((deprecated));
 typedef struct ivl_statement_s*ivl_statement_t;
 
 /*
@@ -1414,7 +1417,7 @@ extern ivl_signal_t ivl_lval_sig(ivl_lval_t net);
  * width. The compiler will insure this is so.
  */
 
-extern const char*     ivl_nexus_name(ivl_nexus_t net);
+extern const char*     ivl_nexus_name(ivl_nexus_t net) __attribute__((deprecated));
 extern unsigned        ivl_nexus_ptrs(ivl_nexus_t net);
 extern ivl_nexus_ptr_t ivl_nexus_ptr(ivl_nexus_t net, unsigned idx);
 

net_link.cc

@@ -33,11 +33,16 @@
 void connect(Nexus*l, Link&r)
 {
       assert(l);
-      assert(r.nexus_);
 
       if (l == r.nexus_)
 	    return;
 
+	// Special case: The "r" link is connected to nothing. The
+	// connect becomes trivially easy.
+      if (r.nexus_ == 0) {
+	    l->relink(&r);
+	    return;
+      }
 
       Nexus*tmp = r.nexus_;
       while (Link*cur = tmp->list_) {
@@ -56,30 +61,36 @@ void connect(Nexus*l, Link&r)
 void connect(Link&l, Link&r)
 {
       assert(&l != &r);
-      if (r.is_linked() && !l.is_linked())
-	    connect(r.nexus_, l);
-      else
+      if (l.nexus_ != 0) {
 	    connect(l.nexus_, r);
+      } else if (r.nexus_ != 0) {
+	    connect(r.nexus_, l);
+      } else {
+	    Nexus*tmp = new Nexus;
+	    connect(tmp, l);
+	    connect(tmp, r);
+      }
 }
 
 Link::Link()
 : dir_(PASSIVE), drive0_(STRONG), drive1_(STRONG), init_(verinum::Vx),
   next_(0), nexus_(0)
 {
-      (new Nexus()) -> relink(this);
 }
 
 Link::~Link()
 {
-      assert(nexus_);
-      Nexus*tmp = nexus_;
-      nexus_->unlink(this);
-      if (tmp->list_ == 0)
-	    delete tmp;
+      if (Nexus*tmp = nexus_) {
+	    nexus_->unlink(this);
+	    if (tmp->list_ == 0)
+		  delete tmp;
+      }
 }
 
 Nexus* Link::nexus()
 {
+      if (nexus_ == 0)
+	    (new Nexus()) ->relink(this);
       return nexus_;
 }
 
@@ -158,7 +169,6 @@ void Link::unlink()
 	    return;
 
       nexus_->unlink(this);
-      (new Nexus()) -> relink(this);
 }
 
 bool Link::is_equal(const Link&that) const
@@ -168,6 +178,8 @@ bool Link::is_equal(const Link&that) const
 
 bool Link::is_linked() const
 {
+      if (nexus_ == 0)
+	    return false;
       if (next_)
 	    return true;
       if (nexus_->first_nlink() != this)
@@ -178,6 +190,8 @@ bool Link::is_linked() const
 
 bool Link::is_linked(const Link&that) const
 {
+      if (nexus_ == 0)
+	    return false;
       return nexus_ == that.nexus_;
 }
 
@@ -489,6 +503,7 @@ unsigned NexusSet::count() const
 
 void NexusSet::add(Nexus*that)
 {
+      assert(that);
       if (nitems_ == 0) {
 	    assert(items_ == 0);
 	    items_ = (Nexus**)malloc(sizeof(Nexus*));
@@ -518,6 +533,7 @@ void NexusSet::add(const NexusSet&that)
 
 void NexusSet::rem(Nexus*that)
 {
+      assert(that);
       if (nitems_ == 0)
 	    return;
 

netlist.h

@@ -290,6 +290,9 @@ class Link {
       verinum::V init_   : 2;
 
     private:
+	// The Nexus uses these to maintain a single linked list of
+	// Link objects. If this link is not connected to anything,
+	// then these pointers are nil.
       Link *next_;
       Nexus*nexus_;
 

t-dll-api.cc

@@ -1354,7 +1354,9 @@ extern "C" const char* ivl_nexus_name(ivl_nexus_t net)
 {
       assert(net);
       if (net->name_ == 0) {
-	    net->name_ = api_strings.add(net->nexus_->name());
+	    char tmp[2 * sizeof(net) + 5];
+	    snprintf(tmp, sizeof tmp, "n%p", net);
+	    net->name_ = api_strings.add(tmp);
       }
       return net->name_;
 }

t-dll.cc

@@ -1429,16 +1429,15 @@ void dll_target::udp(const NetUDP*net)
       obj->npins_ = net->pin_count();
       obj->pins_ = new ivl_nexus_t[obj->npins_];
       for (unsigned idx = 0 ;  idx < obj->npins_ ;  idx += 1) {
-	    const Nexus*nex = net->pin(idx).nexus();
-
 	      /* Skip unconnected input pins. These will take on HiZ
 		 values by the code generators. */
-	    if (nex->t_cookie() == 0) {
+	    if (! net->pin(idx).is_linked()) {
 		  obj->pins_[idx] = 0;
 		  continue;
 	    }
 
-	    assert(nex->t_cookie());
+	    const Nexus*nex = net->pin(idx).nexus();
+	    ivl_assert(*net, nex && nex->t_cookie());
 	    obj->pins_[idx] = nex->t_cookie();
 	    nexus_log_add(obj->pins_[idx], obj, idx);
       }
@@ -2566,8 +2565,19 @@ void dll_target::signal(const NetNet*net)
       for (unsigned idx = 0 ;  idx < obj->array_words ;  idx += 1) {
 
 	    const Nexus*nex = net->pin(idx).nexus();
-	    ivl_assert(*net, nex);
-	    if (nex->t_cookie()) {
+	    if (nex == 0) {
+		    // Special case: This pin is connected to
+		    // nothing. This can happen, for example, if the
+		    // variable is only used in behavioral
+		    // code. Create a stub nexus.
+		  ivl_nexus_t tmp = nexus_sig_make(obj, idx);
+		  tmp->nexus_ = nex;
+		  tmp->name_ = 0;
+		  if (obj->array_words > 1)
+			obj->pins[idx] = tmp;
+		  else
+			obj->pin = tmp;
+	    } else if (nex->t_cookie()) {
 		  if (obj->array_words > 1) {
 			obj->pins[idx] = nex->t_cookie();
 			nexus_sig_add(obj->pins[idx], obj, idx);
@@ -2585,7 +2595,6 @@ void dll_target::signal(const NetNet*net)
 		  else
 			obj->pin = tmp;
 	    }
-	    ivl_assert(*net, net->pin(idx).nexus()->t_cookie());
       }
 }
 

tgt-vvp/draw_net_input.c

@@ -466,8 +466,7 @@ static char* draw_net_input_drive(ivl_nexus_t nex, ivl_nexus_ptr_t nptr)
 
       }
 
-      fprintf(stderr, "internal error: no input to nexus %s\n",
-	      ivl_nexus_name(nex));
+      fprintf(stderr, "internal error: no input to nexus.\n");
       assert(0);
       return strdup("C<z>");
 }