From Gentoo Linux Wiki

[edit] optimistic options

-fno-guess-branch-probability (1.299)
                       -fgcse (2.313)
       -frerun-cse-after-loop (1.13)
            -fstrict-aliasing (1.806)
             -freorder-blocks (1.637)
           -fsched-interblock (1.933)
                 -fsched-spec (1.299)
                -ffloat-store (1.975)
       -minline-all-stringops (1.511)
              -fno-math-errno (1.595)
           -fno-trapping-math (1.215)

[edit] pessimistic options

     -foptimize-sibling-calls (-1.066)
    -fexpensive-optimizations (-1.235)
            -fstrength-reduce (-1.447)
                -falign-jumps (-1.066)
            -freduce-all-givs (-1.785)
               -funroll-loops (-1.278)
           -funroll-all-loops (-1.658)
                 -mfpmath=387 (-1.911)
                 -mfpmath=sse (-1.911)
             -mfpmath=sse,387 (-1.278)
         -fomit-frame-pointer (-1.109)