Virtualize Pins until needed

Reduces resource usage when compiling large memories.
Normal usage patterns still create large nexus arrays
in t-dll.cc:dll_target::signal().

This patch is extensively tested; it shouldn't break anything.

The existing debug "optimizer" flag is (ab)used to control
message printing when large (>1000 element) arrays are
devirtualized or nexus-ized.

The new global variable disable_virtual_pins is available
to force allocation of Pin arrays when they are declared,
but no user interface is provided.

See extensive discussion on iverilog-devel, March 4-5 2009,
title "pr2023076: large memories".
(cherry picked from commit 5bc41e1a17)

compiler.h

@@ -89,6 +89,9 @@ extern bool debug_elab_pexpr;
 extern bool debug_synth2;
 extern bool debug_optimizer;
 
+/* Possibly temporary flag to control virtualization of pin arrays */
+extern bool disable_virtual_pins;
+
 /* Path to a directory useful for finding subcomponents. */
 extern const char*basedir;
 

design_dump.cc

@@ -208,8 +208,12 @@ void NetNet::dump_net(ostream&o, unsigned ind) const
 	    o << " scope=" << scope_path(scope());
       o << " #(" << rise_time() << "," << fall_time() << ","
 	<<  decay_time() << ") vector_width=" << vector_width()
-	<< " pin_count=" << pin_count()
+	<< " pin_count=" << pin_count();
-	<< " init=";
+      if (pins_are_virtual()) {
+	    o << " pins_are_virtual" << endl;
+	    return;
+      }
+      o << " init=";
       for (unsigned idx = pin_count() ;  idx > 0 ;  idx -= 1)
 	    o << pin(idx-1).get_init();
 

main.cc

@@ -128,6 +128,11 @@ bool debug_elab_pexpr = false;
 bool debug_synth2 = false;
 bool debug_optimizer = false;
 
+/*
+ * Miscellaneous flags.
+ */
+bool disable_virtual_pins = false;  // XXX needs a way for user to set
+
 /*
  * Verbose messages enabled.
  */

netlist.cc

@@ -172,26 +172,45 @@ Link* find_next_output(Link*lnk)
       return 0;
 }
 
-NetPins::NetPins(unsigned npins)
+void NetPins::devirtualize_pins(void)
-: npins_(npins)
 {
+      if (pins_) return;
+      if (debug_optimizer && npins_ > 1000) cerr << "debug: devirtualizing " << npins_ << " pins." << endl;
+
       pins_ = new Link[npins_];
       pins_[0].pin_zero_ = true;
       pins_[0].node_ = this;
+      pins_[0].dir_  = default_dir_;
+      pins_[0].init_ = default_init_;
 
       for (unsigned idx = 1 ;  idx < npins_ ;  idx += 1) {
 	    pins_[idx].pin_zero_ = false;
 	    pins_[idx].pin_      = idx;
+	    pins_[idx].dir_      = default_dir_;
+	    pins_[idx].init_     = default_init_;
       }
 }
 
+bool NetPins::pins_are_virtual(void) const
+{
+      return pins_ == NULL;
+}
+
+NetPins::NetPins(unsigned npins)
+: npins_(npins)
+{
+      pins_ = NULL;  // Wait until someone asks.
+      if (disable_virtual_pins) devirtualize_pins();  // Ask.  Bummer.
+}
+
 NetPins::~NetPins()
 {
-      delete[]pins_;
+      if (pins_) delete[]pins_;
 }
 
 Link& NetPins::pin(unsigned idx)
 {
+      if (!pins_) devirtualize_pins();
       if (idx >= npins_) {
 	    cerr << get_fileline() << ": internal error: pin("<<idx<<")"
 		 << " out of bounds("<<npins_<<")" << endl;
@@ -207,11 +226,22 @@ Link& NetPins::pin(unsigned idx)
 
 const Link& NetPins::pin(unsigned idx) const
 {
+      assert(pins_);
       assert(idx < npins_);
       assert(idx == 0? pins_[0].pin_zero_ : pins_[idx].pin_==idx);
       return pins_[idx];
 }
 
+void NetPins::set_default_dir(Link::DIR d)
+{
+       default_dir_ = d;
+}
+
+void NetPins::set_default_init(verinum::V val)
+{
+       default_init_ = val;
+}
+
 bool NetPins::is_linked(void)
 {
       bool linked_flag = false;
@@ -479,7 +509,11 @@ NetNet::NetNet(NetScope*s, perm_string n, Type t, unsigned npins)
 
 void NetNet::initialize_value_and_dir(verinum::V init_value, Link::DIR dir)
 {
-      if (1) {
+      if (pins_are_virtual()) {
+	    if (0) cerr << "NetNet setting Link default value and dir" << endl;
+	    set_default_init(init_value);
+	    set_default_dir(dir);
+      } else {
 	    for (unsigned idx = 0 ;  idx < pin_count() ;  idx += 1) {
 		  pin(idx).set_dir(dir);
 		  pin(idx).set_init(init_value);

netlist.h

@@ -193,11 +193,17 @@ class NetPins : public LineInfo {
       const Link&pin(unsigned idx) const;
 
       void dump_node_pins(ostream&, unsigned, const char**pin_names =0) const;
+      void set_default_dir(Link::DIR d);
+      void set_default_init(verinum::V val);
       bool is_linked();
+      bool pins_are_virtual(void) const;
 
     private:
       Link*pins_;
+      void devirtualize_pins(void);
       const unsigned npins_;
+      Link::DIR default_dir_;
+      verinum::V default_init_;
 };
 
 /* =========

t-dll.cc

@@ -2560,12 +2560,16 @@ void dll_target::signal(const NetNet*net)
       obj->array_base = net->array_first();
       obj->array_words = net->array_count();
       obj->array_addr_swapped = net->array_addr_swapped() ? 1 : 0;
+
+      assert(obj->array_words == net->pin_count());
+      if (debug_optimizer && obj->array_words > 1000) cerr << "debug: "
+	    "t-dll creating nexus array " << obj->array_words << " long" << endl;
       if (obj->array_words > 1)
 	    obj->pins = new ivl_nexus_t[obj->array_words];
 
       for (unsigned idx = 0 ;  idx < obj->array_words ;  idx += 1) {
 
-	    const Nexus*nex = net->pin(idx).nexus();
+	    const Nexus*nex = net->pins_are_virtual() ? 0 : net->pin(idx).nexus();
 	    if (nex == 0) {
 		    // Special case: This pin is connected to
 		    // nothing. This can happen, for example, if the
@@ -2597,6 +2601,7 @@ void dll_target::signal(const NetNet*net)
 			obj->pin = tmp;
 	    }
       }
+      if (debug_optimizer && obj->array_words > 1000) cerr << "debug: t-dll done with big nexus array" << endl;
 }
 
 bool dll_target::signal_paths(const NetNet*net)