Re: aliasing warnings [patch]

[Zack Weinberg <zack at bitmover dot com>]

Mark Mitchell wrote:
> >>>>> "Joe" == Joe Buck <jbuck@synopsys.COM> writes:
>     Joe> What if, instead of immediately issuing a warning, instead
> 
>     Joe> you simply tag the tree for the pointer somehow?  The idea is
>     Joe> to mark pointers that are unsafe to dereference.  If the
>     Joe> pointer is later dereferenced, you could do a pedwarn (as the
>     Joe> program unquestionably breaks the strict ANSI/ISO rules, and
>     Joe> that's what -pedantic is for).  If it goes out of scope
>     Joe> without being dereferenced, no problem.  The only problematic
>     Joe> case is if it is on the heap, passed to a function, etc.  You
>     Joe> could optionally generate a warning for such cases.
> 
> I think your idea is probably a good one, but your statement is not
> 100% correct.  Consider:
> 
>   int i;
>   int *ip = &i;
>   double *dp = (double *) ip;
>   int *ip2 = (int*) dp;
> 
>   *ip2 = 3;
> 
> As I understand it, your proposal would warn about this construct.  I
> think that's fine; it's "suspicious", even though it happens to be
> valid.  So, the sentence I disagree with is "the program
> unquestionably breaks the strict ANSI/ISO rules".  I don't think
> that's true.

I had a very similar idea in the shower this morning, and will try to
implement it when I get a chance.  It won't warn about the construct
you have there.  It will, however, involve adding another pointer slot
to every tree node - this may be an unacceptable memory penalty.

zw