Question

I seem to remember being able to print out (or locate) the specific switches that each -O<num> option turns on. Can you remind?

Thanks!

Answer 1

The list of new features on gcc 4.3 shows a way to do it, via an extension to the --help command line option:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled 

Note, however that I never tried that, only read about it. The documentation about this command line option is at http://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html#Overall-Options

If you ever read the list of new features on gcc 4.3, perhaps this was what you were recalling.