http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265
--- Comment #29 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-30 06:45:54 UTC --- If you want another testcase which doesn't warn and is optimized based on the assumption that undefined behavior doesn't occur, then say: http://blog.regehr.org/archives/918#comment-8646 contains: int a[4]; __attribute__((noinline, noclone)) int foo (int x) { int i, r = 0, n = x & 31; for (i = 0; i < n; i++) r += a[i]; return r; } int main () { int x = 255; __asm volatile ("" : "+r" (x)); return foo (x); } But in both the testcases warning is questionable, in your testcase, if p.r[0] is non-zero and any of p.r[1] through p.r[8] is zero, then no undefined behaviour is triggered and the program is valid, and gcc doesn't break it in any way. Similarly with the my testcase above and foo being called with x where the low 5 bits are 0 to 4, again, valid in that case (ignore main and the value 255 being hidden from the compiler there). What would you like to warn about? That if the loop is invoked with variables and data that result in undefined behaviour that the behaviour will be undefined? The difference between where we know is there the compiler knows that whenever you enter the loop construct, you will hit the undefined behavior (unless say one of the called functions in the body throws or exits), so the level of false positives is low.
