[sv-bc] RE: true/false

AI: File new mantis item to request clarification of Short-circuiting
      of &&, ||, ->, ?: (11.3.5)

Comment: what was the issue here?

When we were discussing the issue of how X/Z and partially-X/Z values were treated as conditions of if-statements, the question was raised about how they were treated for && and ||. In particular, the question was what values of the first operand would cause short-circuiting of evaluation of the second operand, and whether this was the same as the treatment as true/false by an if-statement.

My answer was that it cannot be. Although && and || are most often used in conditions, where X is treated as false, they can also be used in situations where an X is treated differently from 0. For example, you can write

  cond = b && c;
  $display("%b", cond);

If b is X, you know the result is not a definite 1. But you don't know whether the result is 0 or X until you evaluate c. If c is 0, then the result is 0. But if c is nonzero (including X), then the result is X. So you cannot short-circuit the && if the first operand is X, even though that is treated as false by an if-statement. You can only short-circuit on a definite false.

The LRM text in 11.4.7 says "For &&, if the first operand value is logically false then the second operand shall not be evaluated." If "logically false" is interpreted as meaning "considered false by an if-statement", then this is wrong. If it is interpreted as meaning "definitely logically false, not uncertain", then it is fine.

I think the text for || works either way, since it has to be true to short-circuit.

And then we listed -> and ?: because those are specified to short-circuit, and might also have issues.

Thanks,
Shalom

Shalom Bresticker
Intel LAD DA, Jerusalem, Israel
+972 2 589 6582 (office)
+972 54 721 1033 (cell)
http://www.linkedin.com/in/shalombresticker

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