Using an appropriate -m option to generate specific code for your
CPU may be worthwhile, though it may mean the executable won't run on
other versions of the CPU that don't support the same instruction set.
See Hardware Models and Configurations. For instance on an x86 system the
compiler might have
been built--as shown by g77 -v--for the target
i386-pc-linux-gnu, i.e. an i386 CPU. In that case to
generate code best optimized for a Pentium you could use the option
-march=pentium.
For recent CPUs that don't have explicit support in the released version
of gcc, it might still be possible to get improvements
with certain -m options.
-fomit-frame-pointer can help performance on x86 systems and
others. It will, however, inhibit debugging on the systems on which it
is not turned on anyway by -O.