smex-init.el

(when (locate-library "smex")
  (smex-initialize)
  (global-set-key (kbd "M-x") 'smex)
  (global-set-key (kbd "M-X") 'smex-major-mode-commands)
  (global-set-key (kbd "C-c M-x") 'smex-update)
  ;; This is your old M-x.
  (global-set-key (kbd "C-c C-c M-x") 'execute-extended-command))