TIP OF THE MONTH: The dangers of using "set undriven signal"

Tip of the Month: The dangers of using "set undriven signal" in Conformal EC

Say you have a design with floating inputs that cause non-equivalences
in Conformal EC and you hear "set undriven signal" fixed a similar problem
with another design. You ask yourself, "Is this a safe method to verify
my design? Is there a better way of doing this?"
 
set undriven signal is a Conformal EC command that globally ties all
floating signals to a defined value:
 
  set undriven signal [-Both | -Golden | -Revised]
 
One has to exercise caution with global constraints. Here are two
potential pitfalls when using this command:

1) The undriven signal was not intended. This can occur more easily in
VHDL than in Verilog. As an example:

  signal one : std_logic := '1';

Here the designer's obvious intent is to have a signal called 'one' tied
to a constant '1'. Yet, synthesis will tie it off to '0' since the
initial assignment (:= in VHDL) is ignored. Blindly using 'set undriven
signal 0 -golden' will cause this gross error to be covered up. The code
should have been changed to:

  signal one : std_logic;
  [begin]
     one

2) If just "set undriven signal 0" is used, Conformal EC will tie floating inputs
to zero in both the golden and revised (since -both is the default).
This could make the non-equivalences go away but it could also mask a
tie-off synthesis problem. If the synthesis tool didn't tie a floating
input to zero then the implied -both will tie RTL and gate floating
inputs to zero and report equivalence.

Here are recommendations when encountering undriven signals in an
RTL-to-gate run:
 

• Use "report floating signal" and verify that the floating signals are expected.
• Fix the RTL. Tie the signal off in the RTL. Floating signals in RTL are usually a bad design practice.
• If that's not possible, use "add tied signals 0 mysignal -golden" to explicitly tie individual signals or buses. This avoids a global setting and clearly shows the verification intent in the dofile.
• As a last resort, consider "set undriven signal 0 -golden". This will only tie floating inputs in the RTL.