iverilog/link_const.cc

/*
 * Copyright (c) 2000-2013 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
 *    General Public License as published by the Free Software
 *    Foundation; either version 2 of the License, or (at your option)
 *    any later version.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU General Public License for more details.
 *
 *    You should have received a copy of the GNU General Public License
 *    along with this program; if not, write to the Free Software
 *    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */

# include "config.h"

# include  "netlist.h"
# include  "netmisc.h"
# include  "ivl_assert.h"

/*
 * Scan the link for drivers. If there are only constant drivers, then
 * the nexus has a known constant value.
 */
bool Nexus::drivers_constant() const
{
      if (driven_ == VAR)
	    return false;
      if (driven_ != NO_GUESS)
	    return true;

      unsigned constant_drivers = 0;
      for (const Link*cur = first_nlink() ; cur  ;  cur = cur->next_nlink()) {

	      /* A target of a procedural assign or force statement
		 can't be treated as constant. */
	    const NetNet*sig = dynamic_cast<const NetNet*>(cur->get_obj());
	    if (sig && (sig->peek_lref() > 0)) {
		  driven_ = VAR;
		  return false;
	    }

	    Link::DIR cur_dir = cur->get_dir();
	    if (cur_dir == Link::INPUT)
		  continue;

	      /* If this is an input or inout port of a root module,
		 then this is probably not a constant value. I
		 certainly don't know what the value is, anyhow. This
		 can happen in cases like this:

		 module main(sig);
		     input sig;
		 endmodule

		 If main is a root module (it has no parent) then sig
		 is not constant because it connects to an unspecified
		 outside world. */

	    if (cur_dir == Link::PASSIVE) {
		  if (sig == 0 || sig->scope()->parent() != 0)
			continue;

		  if (sig->port_type() == NetNet::NOT_A_PORT)
			continue;

		  if (sig->port_type() == NetNet::POUTPUT)
			continue;

		  driven_ = VAR;
		  return false;
	    }

	      /* If there is an implicit pullup/pulldown on a net,
		 count it as a constant driver. */
	    if (sig)
		  switch (sig->type()) {
		      case NetNet::SUPPLY0:
		      case NetNet::TRI0:
			constant_drivers += 1;
			driven_ = V0;
			continue;
		      case NetNet::SUPPLY1:
		      case NetNet::TRI1:
			constant_drivers += 1;
			driven_ = V1;
			continue;
		      default:
			break;
		  }

	    if (! dynamic_cast<const NetConst*>(cur->get_obj())) {
		  driven_ = VAR;
		  return false;
	    }

	    constant_drivers += 1;
      }

	/* If there is more than one constant driver for this nexus, we
	   would need to resolve the constant value, taking into account
	   the drive strengths. This is a lot of work for something that
	   will rarely occur, so for now leave the resolution to be done
	   at run time. */
      if (constant_drivers > 1) {
	    driven_ = VAR;
	    return false;
      }

      return true;
}

verinum::V Nexus::driven_value() const
{
      switch (driven_) {
	  case V0:
	    return verinum::V0;
	  case V1:
	    return verinum::V1;
	  case Vx:
	    return verinum::Vx;
	  case Vz:
	    return verinum::Vz;
	  case VAR:
	    assert(0);
	    break;
	  case NO_GUESS:
	    break;
      }

      const Link*cur = list_;

      verinum::V val = verinum::Vz;

      for (cur = first_nlink() ; cur  ;  cur = cur->next_nlink()) {

	    const NetConst*obj;
	    const NetNet*sig;
	    if ((obj = dynamic_cast<const NetConst*>(cur->get_obj()))) {
		    // Multiple drivers are not currently supported.
		  ivl_assert(*obj, val == verinum::Vz);
		  val = obj->value(cur->get_pin());

	    } else if ((sig = dynamic_cast<const NetNet*>(cur->get_obj()))) {

		    // If we find an implicit pullup or pulldown on a
		    // net, this is a good guess for the driven value,
		    // but keep looking for other drivers.
		  if ((sig->type() == NetNet::SUPPLY0) ||
		      (sig->type() == NetNet::TRI0)) {
			  // Multiple drivers are not currently supported.
			ivl_assert(*obj, val == verinum::Vz);
			val = verinum::V0;
		  }
		  if ((sig->type() == NetNet::SUPPLY1) ||
		      (sig->type() == NetNet::TRI1)) {
			  // Multiple drivers are not currently supported.
			ivl_assert(*obj, val == verinum::Vz);
			val = verinum::V1;
		  }
	    }
      }

	/* Cache the result. */
      switch (val) {
	  case verinum::V0:
	    driven_ = V0;
	    break;
	  case verinum::V1:
	    driven_ = V1;
	    break;
	  case verinum::Vx:
	    driven_ = Vx;
	    break;
	  case verinum::Vz:
	    driven_ = Vz;
	    break;
      }

      return val;
}


verinum Nexus::driven_vector() const
{
      const Link*cur = list_;

      verinum val;
      unsigned width = 0;

      for (cur = first_nlink() ; cur  ;  cur = cur->next_nlink()) {

	    const NetConst*obj;
	    const NetNet*sig;
	    if ((obj = dynamic_cast<const NetConst*>(cur->get_obj()))) {
		    // Multiple drivers are not currently supported.
		  ivl_assert(*obj, val.len() == 0);
		  ivl_assert(*obj, cur->get_pin() == 0);
		  val = obj->value();
		  width = val.len();

	    } else if ((sig = dynamic_cast<const NetNet*>(cur->get_obj()))) {

		  width = sig->vector_width();

		    // If we find an implicit pullup or pulldown on a
		    // net, this is a good guess for the driven value,
		    // but keep looking for other drivers.
		  if ((sig->type() == NetNet::SUPPLY0) ||
		      (sig->type() == NetNet::TRI0)) {
			  // Multiple drivers are not currently supported.
			ivl_assert(*obj, val.len() == 0);
			val = verinum(verinum::V0, width);
		  }
		  if ((sig->type() == NetNet::SUPPLY1) ||
		      (sig->type() == NetNet::TRI1)) {
			  // Multiple drivers are not currently supported.
			ivl_assert(*obj, val.len() == 0);
			val = verinum(verinum::V1, width);
		  }
	    }
      }

	// If we have a width but not a value, this must be an undriven net.
      if (val.len() != width)
	    val = verinum(verinum::Vz, width);

      return val;
}