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).