Comment by robinsonb5

Interestingly, gcc-amigaos-gcc 6.5 uses dbra without having to jump through any of those contortions, as long as the optimisation level is set to at least -O1:

  _clear_screen:
        move.w #28672,3932160
        move.w #1,3932164
        move.l #3932162,a0
        move.w #-13570,d1
        move.w #1279,d0
  .L2:
        move.w d1,(a0)
        dbra d0,.L2
        rts