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Reduce mux slices that are constant throughout range.

This commit is contained in:
steve 2005-08-28 22:00:39 +00:00
parent 44fbf60885
commit fd29ba3398
1 changed files with 113 additions and 1 deletions
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114
cprop.cc
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@ -17,7 +17,7 @@
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#ifdef HAVE_CVS_IDENT
#ident "$Id: cprop.cc,v 1.47.2.2 2005/08/28 19:51:02 steve Exp $"
#ident "$Id: cprop.cc,v 1.47.2.3 2005/08/28 22:00:39 steve Exp $"
#endif
# include "config.h"
@ -47,6 +47,7 @@ struct cprop_functor : public functor_t {
virtual void lpm_ff(Design*des, NetFF*obj);
virtual void lpm_logic(Design*des, NetLogic*obj);
virtual void lpm_mux(Design*des, NetMux*obj);
virtual void lpm_mux_large(Design*des, NetMux*obj);
};
void cprop_functor::signal(Design*des, NetNet*obj)
@ -847,6 +848,10 @@ void cprop_functor::lpm_logic(Design*des, NetLogic*obj)
*/
void cprop_functor::lpm_mux(Design*des, NetMux*obj)
{
if (obj->size() > 2) {
lpm_mux_large(des, obj);
return;
}
if (obj->size() != 2)
return;
if (obj->sel_width() != 1)
@ -1094,6 +1099,110 @@ void cprop_functor::lpm_mux(Design*des, NetMux*obj)
count += 1;
}
void cprop_functor::lpm_mux_large(Design*des, NetMux*obj)
{
NetScope*scope = obj->scope();
unsigned width = obj->width();
unsigned size = obj->size();
/* This test looks for bit slices that are constant
throughout. If we find any, we can reduce the width of the
MUX to eliminate the fixed value. */
/* After the following for look, this array of bools will
contain "true" for each bit slice that is constant and
identical, and "false" otherwise. The reduce_width will
count the number of true entries in the flags array. */
bool*flags = new bool[width];
unsigned reduce_width = 0;
for (unsigned bit = 0 ; bit < width ; bit += 1) {
flags[bit] = true;
/* If not even the first selection is constant, then the
slice cannot be reduced. */
if (! obj->pin_Data(bit, 0).nexus()->drivers_constant()) {
flags[bit] = false;
continue;
}
/* If any of the remaining selections in non-consant, or
constant with a different value, then this slice
cannot be reduced. */
verinum::V val = obj->pin_Data(bit, 0).nexus()->driven_value();
for (unsigned idx = 1; flags[bit] && idx < size ; idx += 1) {
if (!obj->pin_Data(bit,idx).nexus()->drivers_constant()) {
flags[bit] = false;
break;
}
if (val != obj->pin_Data(bit,idx).nexus()->driven_value()) {
flags[bit] = false;
break;
}
}
if (! flags[bit]) {
/* This bit slice is too complex. Go on. */
continue;
}
reduce_width += 1;
}
/* If no slices can be reduced, then we are finished. */
if (reduce_width == 0) {
delete[]flags;
return;
}
/* Handle the very special case that all the slices can be
reduced. We don't need a MUX at all! */
if (reduce_width == width) {
for (unsigned idx = 0 ; idx < width ; idx += 1)
connect(obj->pin_Result(idx), obj->pin_Data(idx,0));
delete obj;
count += 1;
delete[]flags;
return;
}
/* Create a reduced mux with the same name and size, but fewer
slices. Connect all the slices that we are keeping. */
NetMux*tmp = new NetMux(scope, obj->name(),
width-reduce_width, size, obj->sel_width());
for (unsigned idx = 0 ; idx < obj->sel_width() ; idx += 1)
connect(obj->pin_Sel(idx), tmp->pin_Sel(idx));
unsigned dst_bit = 0;
for (unsigned bit = 0 ; bit < width ; bit += 1) {
if (flags[bit]) {
connect(obj->pin_Result(bit), obj->pin_Data(bit,0));
continue;
}
connect(obj->pin_Result(bit), tmp->pin_Result(dst_bit));
for (unsigned idx = 0 ; idx < size ; idx += 1)
connect(obj->pin_Data(bit,idx), tmp->pin_Data(dst_bit,idx));
dst_bit += 1;
}
/* Add the new node. Delete the old node. Signal that we
change the design and may use a rescan. */
des->add_node(tmp);
delete obj;
delete[]flags;
count += 1;
}
/*
* This functor looks to see if the constant is connected to nothing
* but signals. If that is the case, delete the dangling constant and
@ -1212,6 +1321,9 @@ void cprop(Design*des)
/*
* $Log: cprop.cc,v $
* Revision 1.47.2.3 2005/08/28 22:00:39 steve
* Reduce mux slices that are constant throughout range.
*
* Revision 1.47.2.2 2005/08/28 19:51:02 steve
* More thorough constant propagation through MUX devices.
*