As far as I can see the loop in the function f() always terminates without the loop counter overflowing, but GCC cannot tell that it does.
$ g++-4.3-20070907 -v Using built-in specs. Target: x86_64-unknown-linux-gnu Configured with: ../gcc-4.3-20070907/configure --program-suffix=-4.3-20070907 --enable-language=c,c++ --prefix=/home/jack/opt --with-mpfr=/home/jack/opt Thread model: posix gcc version 4.3.0 20070907 (experimental) (GCC) $ cat no_loop_opt.c void f(unsigned int in) { unsigned int rnd_to_2 = (in - (in % 2)); unsigned int i; for(i = 0; i != rnd_to_2; i += 2) ; } $ gcc-4.3-20070907 -O2 -Wall -Wextra -Wunsafe-loop-optimizations -c no_loop_opt.c no_loop_opt.c: In function f: no_loop_opt.c:7: warning: cannot optimize loop, the loop counter may overflow -- Summary: Missed optimization: cannot determine loop termination Product: gcc Version: 4.3.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: lloyd at randombit dot net GCC build triplet: x86_64-unknown-linux-gnu GCC host triplet: x86_64-unknown-linux-gnu GCC target triplet: x86_64-unknown-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34114