It turns out that although there was all kinds of code to manage
these members, there were no uses anywhere. It must have been a
legacy from a previous era. So remove the dead code so that it
doesn't confuse me again.
Remove some wasteful and excess scans of the Nexus of some links,
and remove dead code so we can see what we're doing. Also inline
some commonly used and trivial functions.
It turns out that it is possible for an otherwise constant net
mux to be non-constant if there is a force that can drive the
net. This can be detected as an l-value reference to an otherwise
constant selector.
The optimization that allowed a mux that had one input always
connected to high-Z produces code that is actually slower than
the original mux, so for now we are removing that optimization.
The bufif devices do not correctly propagate the Z value as
required by the standard. The MOS devices do propagate a Z
value, but they do not correctly handle the case where both
inputs are Z and the select is undefined/Z. The standard
specifies that this should be X, but we produce Z. The full
MUX and %blend operator also function this way, so Icarus
may not match the standard, but it is consistent. I would
also argue that the standard is incorrect. If both inputs
are Z then the output should be Z no matter what select is.
The NetBranch object is connected, but not like an object, so the
NetPins object base class is factored out from NetObj to handle the
connectivity, and the NetBranch class uses the NetPins to connect a
branch.
Also, account for the fact that nets with a discipline are by default
real-valued.
If the select of a MUXZ is constant 0 or 1, then we can elide the MUX
in place of a simple BUFZ. This should allow the dead code on the
unused side to be removed as well.
Constant propagation incorrectly elided an entire constant net node
if only the LSB of the driven vector was HiZ. This caused the entire
vector to look like HiZ. Also, the code generator for writing the
constant values missed bits.
Signed-off-by: Stephen Williams <steve@icarus.com>
syntax. Hook this support into existing $attribute
handling, and add number and void value types.
Add to the ivl_target API new functions for access
of complex attributes attached to gates.
Stephen Williams
Cary R
Larry Doolittle
steve