You can also get the same result by declaring the function's type:

  * (declaim (ftype (function (fixnum fixnum) fixnum) fx-add))
  (FX-ADD)
  * (defun fx-add (x y)
      (declare (optimize (speed 3) (safety 0) (debug 0)))
      (+ x y))
  FX-ADD
  * (disassemble 'fx-add)
  ; disassembly for FX-ADD
  ; Size: 6 bytes. Origin: #x552C81A6                           ; FX-ADD
  ; 6:       4801FA           ADD RDX, RDI
  ; 9:       C9               LEAVE
  ; A:       F8               CLC
  ; B:       C3               RET
  NIL