What is the difference between 'set ft=' and 'setfiletype'?

Question

I am using vim 8.1. When I need to change the default filetype for .s extension to gas (which by default is asm), I first look for the default command to set filetype in /usr/share/vim/vim81/filetype.vim. It contains this line:

au BufNewFile,BufRead *.asm,*.[sS],*.[aA],*.mac,*.lst   call dist#ft#FTasm()

Then, I tried to overwrite that in my vimrc with this:

autocmd BufNewFile,BufRead *.s setfiletype gas

But, it doesn't work. Filetype still asm when I open any *.s file.

Then, when I use this:

autocmd BufNewFile,BufRead *.s set ft=gas

It works.

So, what is the difference between set ft=<filetype> and setfiletype <filetype>?

Answer 1

From the :setfiletype help:

:setf[iletype] [FALLBACK] {filetype}                    :setf :setfiletype
                     Set the 'filetype' option to {filetype}, but only if
                     not done yet in a sequence of (nested) autocommands.
                     This is short for:
                             :if !did_filetype()
                             :  setlocal filetype={filetype}
                             :endif
                     This command is used in a filetype.vim file to avoid
                     setting the 'filetype' option twice, causing different
                     settings and syntax files to be loaded.

                     When the optional FALLBACK argument is present, a
                     later :setfiletype command will override the
                     'filetype'.  This is to used for filetype detections
                     that are just a guess.  did_filetype() will return
                     false after this command.

So :setfiletype does not override existing file type, when :set ft does.