From Gentoo Linux Wiki

[edit] optimistic options

                       -fgcse (2.524)
                  -fforce-mem (1.573)
 -fdelete-null-pointer-checks (1.621)
             -freorder-blocks (1.335)
           -finline-functions (1.288)
               -mno-push-args (1.24)
              -fno-math-errno (1.478)
  -funsafe-math-optimizations (1.145)
           -fno-trapping-math (1.145)

[edit] pessimistic options

-fno-guess-branch-probability (-2.088)
     -foptimize-sibling-calls (-1.328)
             -frerun-loop-opt (-1.042)
             -fschedule-insns (-2.231)
                -ffloat-store (-1.137)
                     -fnew-ra (-1.47)
           -funroll-all-loops (-1.613)
       -minline-all-stringops (-1.185)
                 -mfpmath=387 (-1.803)
                 -mfpmath=sse (-1.851)
         -fomit-frame-pointer (-1.518)