/in/foo.cc: In function 'void pre()':
/in/foo.cc:17:35: warning: iteration 14 invokes undefined behavior [-Waggressive-loop-optimizations]
     for (int i=1;i<=15;++i) pw3[i]=pw3[i-1]*3;
                             ~~~~~~^~~~~~~~~~~
/in/foo.cc:17:19: note: within this loop
     for (int i=1;i<=15;++i) pw3[i]=pw3[i-1]*3;
                  ~^~~~