"Jean Delvare" <[email protected]> said:
> > > > No, there is a third case: the pointer can be NULL, but the compiler
> > > > happened to move the dereference down to after the check.
> > > Wow. Great point. I completely missed that possibility. In fact I didn't
> > > know that the compiler could possibly alter the order of the
> > > instructions. For one thing, I thought it was simply not allowed to. For
> > > another, I didn't know that it had been made so aware that it could
> > > actually figure out how to do this kind of things. What a mess. Let's
> > > just hope that the gcc folks know their business :)
> > The compiler is most definitely /not/ allowed to change the results the
> > code gives.
> I think that Andrew's point was that the compiler could change the order
> of the instructions *when this doesn't change the result*, not just in
> the general case, of course. In our example, The instructions:
>
> v = p->field;
> if (!p) return;
>
> can be seen as equivalent to
>
> if (!p) return;
> v = p->field;
They are not. If p == NULL, the first gives an exception (SIGSEGV), the
second one doesn't. Just as you can't "optimize" by switching:
x = b / a;
if (a == 0) return;
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Hi!
> "Jean Delvare" <[email protected]> said:
> > > > > No, there is a third case: the pointer can be NULL, but the compiler
> > > > > happened to move the dereference down to after the check.
>
> > > > Wow. Great point. I completely missed that possibility. In fact I didn't
> > > > know that the compiler could possibly alter the order of the
> > > > instructions. For one thing, I thought it was simply not allowed to. For
> > > > another, I didn't know that it had been made so aware that it could
> > > > actually figure out how to do this kind of things. What a mess. Let's
> > > > just hope that the gcc folks know their business :)
>
> > > The compiler is most definitely /not/ allowed to change the results the
> > > code gives.
>
> > I think that Andrew's point was that the compiler could change the order
> > of the instructions *when this doesn't change the result*, not just in
> > the general case, of course. In our example, The instructions:
> >
> > v = p->field;
> > if (!p) return;
> >
> > can be seen as equivalent to
> >
> > if (!p) return;
> > v = p->field;
>
> They are not. If p == NULL, the first gives an exception (SIGSEGV), the
> second one doesn't. Just as you can't "optimize" by switching:
>
> x = b / a;
> if (a == 0) return;
Dereferencing NULL pointer is undefined. It *may* give SIGSEGV. That's
what enables optimization above. You can't rely on dereferencing NULL
to always give SIGSEGV. Sorry.
Pavel
--
Boycott Kodak -- for their patent abuse against Java.