https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105023
--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
@findex BLKmode
@item BLKmode
``Block'' mode represents values that are aggregates to which none of
the other modes apply. In RTL, only memory references can have this mode,
and only if they appear in string-move or vector instructions. On machines
which have no such instructions, @code{BLKmode} will not appear in RTL@.
