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/*
* Copyright (c) 2000 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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#if !defined(WINNT) && !defined(macintosh)
#ident "$Id: link_const.cc,v 1.8 2001/02/16 03:27:07 steve Exp $"
#endif

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

/*
* Scan the link for drivers. If there are only constant drivers, then
* the nexus has a known constant value. If there is a supply net,
* then the nexus again has a known constant value.
*/
bool link_drivers_constant(const Link&lnk)
{
      const Nexus*nex = lnk.nexus();
      bool flag = true;

      for (const Link*cur = nex->first_nlink()
; cur ; cur = cur->next_nlink()) {
const NetNet*sig;

if (cur == &lnk)
continue;
if (cur->get_dir() == Link::INPUT)
continue;

/* If this is an input or input port of a root module,
then the is probably not a constant value. I
certainly don't know what the value is, anyhow. */

if (sig = dynamic_cast<const NetNet*>(cur->get_obj())) do {
if (sig->port_type() == NetNet::NOT_A_PORT)
break;

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

if (sig->scope()->parent())
break;

flag = false;
} while(0);


/* If the link is PASSIVE then it doesn't count as a
driver if its initial value is Vz. This is pertinant
because REGs are PASSIVE but can receive values from
procedural assignments. */
if ((cur->get_dir() == Link::PASSIVE)
&& (cur->get_init() == verinum::Vz))
continue;

if (! dynamic_cast<const NetConst*>(cur->get_obj()))
flag = false;

/* If there is a supply net, then this nexus will have a
constant value independent of any drivers. */
if (const NetNet*sig = dynamic_cast<const NetNet*>(cur->get_obj()))
switch (sig->type()) {
case NetNet::SUPPLY0:
case NetNet::SUPPLY1:
return true;
default:
break;
}
      }

      return flag;
}

verinum::V driven_value(const Link&lnk)
{
      verinum::V val = lnk.get_init();

      const Nexus*nex = lnk.nexus();
      for (const Link*cur = nex->first_nlink()
; cur ; cur = cur->next_nlink()) {

const NetConst*obj;
const NetNet*sig;
if (obj = dynamic_cast<const NetConst*>(cur->get_obj())) {
val = obj->value(cur->get_pin());

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

if (sig->type() == NetNet::SUPPLY0)
return verinum::V0;

if (sig->type() == NetNet::SUPPLY1)
return verinum::V1;
}
      }

      return val;
}

/*
* $Log: link_const.cc,v $
* Revision 1.8 2001/02/16 03:27:07 steve
* links to root inputs are not constant.
*
* Revision 1.7 2000/11/20 01:41:12 steve
* Whoops, return the calculated constant value rom driven_value.
*
* Revision 1.6 2000/11/20 00:58:40 steve
* Add support for supply nets (PR#17)
*
* Revision 1.5 2000/07/14 06:12:57 steve
* Move inital value handling from NetNet to Nexus
* objects. This allows better propogation of inital
* values.
*
* Clean up constant propagation a bit to account
* for regs that are not really values.
*
* Revision 1.4 2000/06/25 19:59:42 steve
* Redesign Links to include the Nexus class that
* carries properties of the connected set of links.
*
* Revision 1.3 2000/05/14 17:55:04 steve
* Support initialization of FF Q value.
*
* Revision 1.2 2000/05/07 04:37:56 steve
* Carry strength values from Verilog source to the
* pform and netlist for gates.
*
* Change vvm constants to use the driver_t to drive
* a constant value. This works better if there are
* multiple drivers on a signal.
*
* Revision 1.1 2000/04/20 00:28:03 steve
* Catch some simple identity compareoptimizations.
*
*/
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