For a Pentium CPU i586 with compiler version egcs-2.91.66, the best optimization options would be:

             CFLAGS=-O3 -march=pentium -mcpu=pentium -ffast-math -funroll-loops -fomit-frame-pointer -fforce-mem -fforce-addr -malign-double -fno-exceptions
             

Comparitive analysis chart with the above mentioned flags