http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59564
Richard Biener rguenth at gcc dot gnu.org changed:
What|Removed |Added
Keywords||diagnostic
Status|UNCONFIRMED |NEW
Last reconfirmed||2013-12-20
Component|c |tree-optimization
Version|unknown |4.9.0
Ever confirmed|0 |1
Known to fail||4.8.2, 4.9.0
--- Comment #2 from Richard Biener rguenth at gcc dot gnu.org ---
DOM optimizes this to
if (n = 0)
{
n = 0;
arr[0] = 0;
}
else
arr[n] = 0;
after which value-range propagation warns for the arr[n] on the else
branch because there n = 1 and thus the access is out-of-bounds.
Fixing the obfuscation by doing
if (n 0) n = 0;
instead of
if (n = 0) n = 0;
fixes the warning.
The warning is basically that n is not constrained to be 1 which is
of course kind-of-useless. We don't warn for a plain arr[n] either.
But as you give VRP a half-useful range you make it emit the warning.
Common to these kind of bogus warnings is that we have duplicated the
memory access and warn about the bad half of it. But we have no way
of tracking whether there are now two pieces instead of just one ...
(apart from maybe using source location info and making sure that VRP
computes the same warning for equal source location accesses).
