RE: [sv-bc] [Fwd: Issues with IEEE 1364-2005]

>From: "Brad Pierce" <Brad.Pierce@synopsys.com>

>What's the justification for only tightening these two particular
>Verilog operators, && and ||?

I think there is some justification for it.  However, I agree that
it would be better to tighten them all up.

Not only should && and || be guaranteed to short-circuit, all the
other binary operators should be guaranteed not to short-circuit.
(As always, an implementation is free to do actual evaluations in
any order it likes, as long as it doesn't change visible behavior.
So this is really only required when there are side-effects.)


>There would still be nondeterminism for all of the other binary
>operators in the presence of side-effecting operands, because the
>evaluation order of their operands is not specified either.

I think the LRM tried to specify this, but I have to concede that
it did a poor job.  I agree that this should be fixed.  Evaluation
of operands of all binary operators should be guaranteed to be done
left-operand-before-right.


>In for a penny, in for a pound.

Absolutely.

This does still leave the justification of why && and || should be
short-circuiting when the other operators are not.

Part of this is based on the fact that C (and Vera) work this way.
But there are underlying reasons that they work this way.  There are
common idioms that use && and || and rely on short-circuiting to
work properly.  Examples have been given of these.  I don't know of
any such common idioms for the other operators.

The logical connectives tend to be used in software-ish situations,
where avoiding run-time errors is an issue.  The bitwise logical
operators tend to be used in hardware-ish situations, where this is
not an issue.  The special values where short-circuiting can be done
for && and || are common (one expects about 50% of the time).  The
special values where short-circuiting can be done for other operators
like + are rare corner cases.  So from an simulation optimization
perspective, you have allowed short-circuiting where it is most
profitable.


Steven Sharp
sharp@cadence.com