Re: [PATCH] C undefined behavior fix

Tim Hollebeek
2002-01-10T04:13:04Z

> Nope, that's not enough, it's not that simple. Yes your
> example of a-a is of course straightforward but what about
>
> b = (a & 1) | (b & 1);
>
> if a is volatile and b is known to be odd, can the read of a be eliminated?
> The answer should be no (and I think the standard guarantees this), but the
> reasoning is completely different from thinking about the fact that a may
> change unexpectedly, since obviously no matter what value comes from
> reading a, b will be set to 1 if b is known to be odd.

But presumably the transformation to:

a; b = 1;

is ok. where 'a;' represents the operation "read the value a into a
register and then let that register die".