[Bug c/77886] New: -Wimplicit-fallthrough: breaks duff's device

["marc.mutz at kdab dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77886

            Bug ID: 77886
           Summary: -Wimplicit-fallthrough: breaks duff's device
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marc.mutz at kdab dot com
  Target Milestone: ---

The recent addition (or activation) of -Wimplicit-fallthrough breaks Duff's
Device:

    int n = (count + 7) / 8;
    switch (count & 0x07)
    {
    case 0: do { *dest++ = value;
    case 7:      *dest++ = value;
    case 6:      *dest++ = value;
    case 5:      *dest++ = value;
    case 4:      *dest++ = value;
    case 3:      *dest++ = value;
    case 2:      *dest++ = value;
    case 1:      *dest++ = value;
    } while (--n > 0);
    }

adding all those pesky // fall through comments makes the code unreadable, the
more so as GCC doesn"t accept it as a trailing comment after each line but
insists on having it on a line of its own.