On Thu, Sep 17, 2015 at 10:37:40AM -0600, Martin Sebor wrote:
> >>The patch currently issues a false positive for the test case
> >>below. I suspect the chain might need to be cleared after each
> >>condition that involves a side-effect.
> >>
> >> int foo (int a)
> >> {
> >> if (a) return 1; else if (++a) return 2; else if (a) return 3;
> >> return 0;
> >> }
> >
> >But the last branch here can never be reached, right? If a == 0, foo
> >returns 2, otherwise it just returns 1. So I think we should diagnose
> >this.
>
> It probably wasn't the best example. The general issue here is
> that the second condition has a side-effect that can change (in
> this case clearly does) the value of the expression.
>
> Here's a better example:
>
> int a;
>
> int bar (void) { a = 1; return 0; }
>
> int foo (void) {
> if (a) return 1;
> else if (foo ()) return 2;
> else if (a) return 3;
> return 0;
> }
>
> Since we don't know bar's side-effects we must assume they change
> the value of a and so we must avoid diagnosing the third if.
Ok, I'm convinced now. We have something similar in the codebase:
libsupc++/eh_catch.cc has
int count = header->handlerCount;
if (count < 0)
{
// This exception was rethrown. Decrement the (inverted) catch
// count and remove it from the chain when it reaches zero.
if (++count == 0)
globals->caughtExceptions = header->nextException;
}
else if (--count == 0)
{
// Handling for this exception is complete. Destroy the object.
globals->caughtExceptions = header->nextException;
_Unwind_DeleteException (&header->unwindHeader);
return;
}
else if (count < 0)
// A bug in the exception handling library or compiler.
std::terminate ();
Here all arms are reachable. I guess I need to kill the chain of conditions
when we find something with side-effects, exactly as you suggested.
Again, thanks for your comments.
Marek
