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Implement CA - ternary short circuit for constant T/F expressions. 

This patch only applies to the ternary operator when used in a
continuous assignment. It adds short circuiting when the expression
is a constant true or false. It does not optimize the special case of
a constant 'bx expression with constant T/F expressions.
This commit is contained in:
Cary R 2008-03-19 16:18:44 -07:00 committed by Stephen Williams
parent 77722a62c2
commit 48990d8e2d
1 changed files with 32 additions and 2 deletions
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34
elab_net.cc
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@ -2886,6 +2886,30 @@ NetNet* PEString::elaborate_net(Design*des, NetScope*scope,
return net;
}
/* Common processing for the case when a single argument is always
* selected in a ternary operator. */
static void process_single_ternary(Design*des, const PExpr*base,
unsigned width, NetNet*&sig)
{
/* We must have a type for the signal. */
if (sig->data_type() == IVL_VT_NO_TYPE) {
cerr << base->get_fileline() << ": internal error: constant "
<< "selected ternary clause has NO TYPE." << endl;
des->errors += 1;
sig = 0;
}
/* Use the signal width if one is not provided.
* Pad or crop as needed. */
if (width == 0) width = sig->vector_width();
if (sig->vector_width() < width) sig = pad_to_width(des, sig, width);
if (width < sig->vector_width()) sig = crop_to_width(des, sig, width);
sig->set_line(*base);
}
/*
* Elaborate the ternary operator in a netlist by creating a LPM_MUX
* with width matching the result, size == 2 and 1 select input. These
@ -2918,18 +2942,24 @@ NetNet* PETernary::elaborate_net(Design*des, NetScope*scope,
case C_0:
fal_sig = fal_->elaborate_net(des, scope, width, 0, 0, 0);
if (fal_sig == 0) return 0;
tru_sig = tru_->elaborate_net(des, scope, width, 0, 0, 0);
process_single_ternary(des, this, width, fal_sig);
return fal_sig;
break;
case C_1:
tru_sig = tru_->elaborate_net(des, scope, width, 0, 0, 0);
if (tru_sig == 0) return 0;
fal_sig = fal_->elaborate_net(des, scope, width, 0, 0, 0);
process_single_ternary(des, this, width, tru_sig);
return tru_sig;
break;
default:
tru_sig = tru_->elaborate_net(des, scope, width, 0, 0, 0);
fal_sig = fal_->elaborate_net(des, scope, width, 0, 0, 0);
/* We should really see if these are constant values and
* mix them as needed so we can omit the MUX? below, but
* since this is a very uncommon case, I'm going to pass
* on this for now. */
break;
}
expr_sig = expr->synthesize(des);