From Gentoo Linux Wiki

[edit] optimistic options

            -fcse-skip-blocks (1.243)
    -fexpensive-optimizations (1.007)
                     -ftracer (1.203)
   -maccumulate-outgoing-args (1.085)
              -fno-math-errno (1.833)
  -funsafe-math-optimizations (2.029)
           -fno-trapping-math (1.793)
           -ffinite-math-only (1.715)
          -fno-signaling-nans (1.439)

[edit] pessimistic options

           -fno-loop-optimize (-1.746)
                       -fgcse (-1.078)
            -fstrength-reduce (-1.196)
                    -fregmove (-1.314)
 -fdelete-null-pointer-checks (-1.038)
                -ffloat-store (-1.864)
                  -fno-inline (-1.707)
                     -fnew-ra (-1.904)
               -funroll-loops (-1.392)
               -mno-push-args (-1.078)
                 -mfpmath=387 (-1.432)
                 -mfpmath=sse (-1.314)
             -mfpmath=sse,387 (-1.078)
    -momit-leaf-frame-pointer (-1.274)