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Thorough constant propagation for or and nor gates.

This commit is contained in:
steve 2000-11-18 05:13:27 +00:00
parent ff5846100c
commit 2750977e4a
1 changed files with 137 additions and 3 deletions
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140
cprop.cc
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@ -17,7 +17,7 @@
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#if !defined(WINNT) && !defined(macintosh)
#ident "$Id: cprop.cc,v 1.19 2000/11/18 04:10:37 steve Exp $"
#ident "$Id: cprop.cc,v 1.20 2000/11/18 05:13:27 steve Exp $"
#endif
# include "netlist.h"
@ -224,13 +224,142 @@ void cprop_functor::lpm_logic(Design*des, NetLogic*obj)
count += 1;
return;
case NetLogic::NOR:
case NetLogic::OR: {
unsigned top = obj->pin_count();
unsigned idx = 1;
/* Eliminate all the 0 inputs. They have no effect
on the output of an OR gate. */
while (idx < top) {
if (! link_drivers_constant(obj->pin(idx))) {
idx += 1;
continue;
}
if (driven_value(obj->pin(idx)) == verinum::V0) {
obj->pin(idx).unlink();
top -= 1;
if (idx < top) {
connect(obj->pin(idx), obj->pin(top));
obj->pin(top).unlink();
}
continue;
}
if (driven_value(obj->pin(idx)) != verinum::V1) {
idx += 1;
continue;
}
/* Oops! We just stumbled on a driven-1 input
to the OR gate. That means we can replace
the whole bloody thing with a constant
driver and exit now. */
NetConst*tmp;
switch (obj->type()) {
case NetLogic::OR:
tmp = new NetConst(obj->name(), verinum::V1);
break;
case NetLogic::NOR:
tmp = new NetConst(obj->name(), verinum::V0);
break;
default:
assert(0);
}
des->add_node(tmp);
tmp->pin(0).drive0(obj->pin(0).drive0());
tmp->pin(0).drive1(obj->pin(0).drive1());
connect(obj->pin(0), tmp->pin(0));
delete obj;
count += 1;
return;
}
/* If all the inputs were eliminated, then replace
the gate with a constant 0 and I am done. */
if (top == 1) {
NetConst*tmp;
switch (obj->type()) {
case NetLogic::OR:
tmp = new NetConst(obj->name(), verinum::V0);
break;
case NetLogic::NOR:
tmp = new NetConst(obj->name(), verinum::V1);
break;
default:
assert(0);
}
des->add_node(tmp);
tmp->pin(0).drive0(obj->pin(0).drive0());
tmp->pin(0).drive1(obj->pin(0).drive1());
connect(obj->pin(0), tmp->pin(0));
delete obj;
count += 1;
return;
}
/* If we are down to only one input, then replace
the OR with a BUF and exit now. */
if (top == 2) {
NetLogic*tmp;
switch (obj->type()) {
case NetLogic::OR:
tmp = new NetLogic(obj->scope(),
obj->name(), 2,
NetLogic::BUF);
break;
case NetLogic::NOR:
tmp = new NetLogic(obj->scope(),
obj->name(), 2,
NetLogic::NOT);
break;
default:
assert(0);
}
des->add_node(tmp);
tmp->pin(0).drive0(obj->pin(0).drive0());
tmp->pin(0).drive1(obj->pin(0).drive1());
connect(obj->pin(0), tmp->pin(0));
connect(obj->pin(1), tmp->pin(1));
delete obj;
count += 1;
return;
}
/* Finally, this cleans up the gate by creating a
new [N]OR gate that has the right number of
inputs, connected in the right place. */
if (top < obj->pin_count()) {
NetLogic*tmp = new NetLogic(obj->scope(),
obj->name(), top,
obj->type());
des->add_node(tmp);
tmp->pin(0).drive0(obj->pin(0).drive0());
tmp->pin(0).drive1(obj->pin(0).drive1());
for (unsigned idx = 0 ; idx < top ; idx += 1)
connect(tmp->pin(idx), obj->pin(idx));
delete obj;
count += 1;
return;
}
break;
}
case NetLogic::XOR: {
unsigned top = obj->pin_count();
unsigned idx = 1;
/* Eliminate all the 0 inputs. They have no effect
on the output of an XOR gate. Don't remove the
last input, though. */
on the output of an XOR gate. */
while (idx < top) {
if (! link_drivers_constant(obj->pin(idx))) {
idx += 1;
@ -372,7 +501,9 @@ void cprop_functor::lpm_logic(Design*des, NetLogic*obj)
count += 1;
return;
}
break;
}
default:
break;
}
@ -531,6 +662,9 @@ void cprop(Design*des)
/*
* $Log: cprop.cc,v $
* Revision 1.20 2000/11/18 05:13:27 steve
* Thorough constant propagation for or and nor gates.
*
* Revision 1.19 2000/11/18 04:10:37 steve
* Handle constant propagation through XOR gates,
* including reducing the gate to a constant,