[Bug c++/51731] code generation bug in negative indices in arrays on 64-bit targets

[m.hekkelman at cmbi dot ru.nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51731

--- Comment #6 from M.L. Hekkelman <m.hekkelman at cmbi dot ru.nl> 2012-01-03 07:44:55 UTC ---
(In reply to comment #5)
> (In reply to comment #4)
> > Following your logic, if I rewrite the code from:
> > 
> >   return data.e[-1];
> > 
> > to
> > 
> >   int* ep = data.e;
> >   return ep[-1];
> > 
> > It  would  be  valid,  right? And you still believe the bug report is 
> > invalid?
> 
> Of course that wouldn't be valid.
> I meant that if you had say array
> type e[10];
> somewhere and
>   type *ep = data.e + 9;
> you can then access
>   ep[0] + ep[-1] + ep[-9]
> In C++03, please read [expr.add]/5 :
> "If both the pointer operand and the result point to elements of the same array
> object, or one past the last element of the array object, the evaluation shall
> not produce an overflow; otherwise, the behavior is undefined."

I stand corrected.

Still, it would be nice if gcc would then issue a warning instead of only
giving incorrect results when -O3 is specified.