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Fix for pr1879226, large constant shift values 

Addresses pr1879226 on a couple of levels.  Analysis:

NetNet* PEBinary::elaborate_net_shift_() didn't flag dist==lwidth
as a case where all bits of the input are shifted away, which made
it create a real concatenator for this case.  It attempted to create
NetNet*tmp with zero width, which doesn't work; (lsb,msb) ends up
as (0,-1), which is later interpreted as a 2-bit wide net.

Added an assert to the NetNet constructor to catch any other lurking
attempts to create zero-width nets.

Added short-circuit code to handle the case where all bits of the input
are shifted away.  This case used to be "handled" by an assert failure.
This commit is contained in:
Larry Doolittle 2008-01-29 08:59:55 -08:00 committed by Stephen Williams
parent 76039cf595
commit 31afec57b1
2 changed files with 11 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
elab_net.cc
View File

@ -1110,12 +1110,6 @@ NetNet* PEBinary::elaborate_net_shift_(Design*des, NetScope*scope,
the left signal again. */
if (dist == 0) return lsig;
/* Another very special case: constant shift the entire
value away. The result is a const. */
if (dist > lwidth) {
assert(0);
}
/* The construction that I'm making will ultimately
connect its output to the osig here. This will be the
result that I return from this function. */
@ -1154,6 +1148,8 @@ NetNet* PEBinary::elaborate_net_shift_(Design*des, NetScope*scope,
zero->local_flag(true);
zero->set_line(*this);
/* Padding bits are zero in most cases, but copies of
* the sign bit in the case of a signed right shift */
if (op_ == 'R') {
NetPartSelect*sign_bit
= new NetPartSelect(lsig, lsig->vector_width()-1,
@ -1180,6 +1176,14 @@ NetNet* PEBinary::elaborate_net_shift_(Design*des, NetScope*scope,
connect(zero->pin(0), zero_c->pin(0));
}
/* If all data bits get shifted away, connect the zero or
* padding bits directly to output, and stop before building the
* concatenation. */
if (dist >= lwidth) {
connect(osig->pin(0), zero->pin(0));
return osig;
}
/* Make a concatenation operator that will join the
part-selected right expression at the pad values. */
NetConcat*cc = new NetConcat(scope, scope->local_symbol(),

1
netlist.cc
View File

@ -402,6 +402,7 @@ NetNet::NetNet(NetScope*s, perm_string n, Type t, unsigned npins)
local_flag_(false), eref_count_(0), lref_count_(0)
{
assert(s);
assert(npins>0);
verinum::V init_value = verinum::Vz;
Link::DIR dir = Link::PASSIVE;