From Gentoo Linux Wiki

[edit] optimistic options

            -fno-thread-jumps (1.222)
            -fstrength-reduce (1.136)
                  -fforce-mem (1.862)
               -falign-labels (1.307)
                  -fno-inline (1.691)
                     -ftracer (1.051)
                     -fnew-ra (1.136)
         -fomit-frame-pointer (1.734)
              -fno-math-errno (1.179)
  -funsafe-math-optimizations (1.222)

[edit] pessimistic options

-fno-guess-branch-probability (-1.639)
           -fno-if-conversion (-2.023)
          -fno-if-conversion2 (-1.127)
          -fno-delayed-branch (-1.084)
     -foptimize-sibling-calls (-1.041)
                       -fgcse (-1.511)
             -fschedule-insns (-1.895)
       -fprefetch-loop-arrays (-1.255)
               -funroll-loops (-1.81)
                 -mfpmath=387 (-1.81)
                 -mfpmath=sse (-1.468)
             -mfpmath=sse,387 (-1.212)
    -momit-leaf-frame-pointer (-1.767)