------- Comment #3 from lloyd at randombit dot net 2007-11-16 01:49 ------- Here's another example, which I think may represent a different case (and which I found much more surprising than the first):
$ cat no_loop_opt2.c void g(unsigned int n) { unsigned int k; for(k = 0; k <= n; ++k) ; } (motoko ~)$ g++-4.1.2 -O2 -Wunsafe-loop-optimizations -c no_loop_opt2.c no_loop_opt2.c: In function 'void g(unsigned int)': no_loop_opt2.c:5: warning: cannot optimize possibly infinite loops no_loop_opt2.c:5: warning: cannot optimize possibly infinite loops no_loop_opt2.c:5: warning: cannot optimize possibly infinite loops no_loop_opt2.c:5: warning: cannot optimize possibly infinite loops (same output with 4.2.0) However if <= is changed to <, no problem. Version info: $ g++-4.1.2 -v Using built-in specs. Target: x86_64-pc-linux-gnu Configured with: /var/tmp/portage/sys-devel/gcc-4.1.2/work/gcc-4.1.2/configure --prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.1.2 --includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.1.2/include --datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.1.2 --mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.1.2/man --infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.1.2/info --with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.1.2/include/g++-v4 --host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable-altivec --enable-nls --without-included-gettext --with-system-zlib --disable-checking --disable-werror --enable-secureplt --disable-libunwind-exceptions --enable-multilib --enable-libmudflap --disable-libssp --disable-libgcj --enable-languages=c,c++,objc,obj-c++,fortran --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu Thread model: posix gcc version 4.1.2 (Gentoo 4.1.2) $ g++-4.2.0 -v Using built-in specs. Target: x86_64-unknown-linux-gnu Configured with: ../gcc-4.2.0/configure --enable-languages=c,c++,objc,obj-c++ --program-suffix=-4.2.0 Thread model: posix gcc version 4.2.0 -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34114