Re: [PATCH] only use 48-bit lba when necessary

[Chuck Ebbert]

>>   The operands of & can be evaluated in any order, while && requires
>> left-to-right and does not evaluate the right operand if the left one
>> is false.  Only the simplest cases could possibly generate the same
>> code.
>
> The code must execute AS IF the right operand is only evaluated if the left
> operand is true.
>
> If an optimizer can prove that evaluating an operand has no side effects
> (which a halfway-decent optimizer can usually do for simple expressions),
> then it is free to evaluate it in any way that will produce the same
> result.

  No, that's not quite right.  Take this code for example:

   struct foo *bar;

   if (bar && bar->baz == 6) /* something */;

If bar were zero, then evaluating the right side of the && would cause
a fault.  (This is not a side effect.)

  So the AS IF part if your statement is right but you have to consider
more than just side effects.

 --
 Chuck