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

>All that optional short-circuiting allows you to do is stop early

According to 5.1.4 

  "However, if the final result of an expression can be determined
early, the entire expression need not be evaluated."

The commonsense reading of that sentence says that

    "if (f(x) || 1 || f(y))"

can be optimized to 

    "if (1)"

Evaluating f(x) is not required.  

-- Brad

-----Original Message-----
From: owner-sv-bc@eda-stds.org [mailto:owner-sv-bc@eda-stds.org] On
Behalf Of Steven Sharp
Sent: Tuesday, August 15, 2006 3:09 PM
To: doug_warmke@mentor.com; sv-bc@eda-stds.org;
Karen.Pieper@synopsys.COM
Subject: RE: [sv-bc] [Fwd: Issues with IEEE 1364-2005]


>From: "Karen Pieper" <Karen.Pieper@synopsys.com>

>I'm concerned about forcing side effects.
>
>For synthesis, users like the best quality of results, timing, area, 
>etc.  In the case that someone codes:
>
>if (f(x) || 1 || f(y))
>
>being able to evaluate it as if (1) and just build the then clause can 
>reduce the critical path and the area in the design.

I am surprised that synthesis would allow such a construct if the
function has side-effects.  The behavior of the synthesized design could
be completely different from what was simulated.  I would think users
would consider that a bug in the tool.

At any rate, the LRM does not allow what you have described.  You cannot
avoid evaluating f(x).  The LRM specifies that the expression evaluation
order follows the associativity rules.  That would be left-to-right in
this case.  So it requires you to evaluate f(x) before the 1.  All that
optional short-circuiting allows you to do is stop early, not
arbitrarily re-order.  So it is optional to evaluate f(y), but not f(x).

If there are no side-effects in f(), then it doesn't matter, and you can
evaluate any way you like.  The results are equivalent.  But if there
are side-effects, then the results may not be equivalent.  Then the
correct behavior is the behavior you would get from evaluating
left-to-right, possibly short-circuiting.  Since synthesis would know
whether there were any side-effects, I would think that you would error
out in the presence of possible short-circuiting of those side-effects.


>  The problem also
>shows up in a case like:
>
>If (a || b || .... || f(x))
>
>in the event that a is a critical path signal, then there are a lot of 
>ors that the signal will have to go through to prove that the side 
>effects of f(x) can happen, putting that side effect now on the 
>critical path.

I assume that you are saying that you could do the side-effects of f(x)
unconditionally, by assuming no short-circuiting.  That would avoid
making the side-effects conditional on the ORs of the previous
conditions.
In this case, I agree that the current LRM allows this.

But the resulting logic may not match what the user expected when they
wrote this code, and which they verified by simulation.  Users may like
better timing and area, but not if the circuit doesn't work.

Steven Sharp
sharp@cadence.com