https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109040
--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
WORD_REGISTER_OPERATIONS means that subword register operations in RTL are
actually performed on the whole word register by the machine, e.g.. for
(insn 47 35 39 2 (set (reg:HI 175)
(subreg:HI (reg:SI 166) 0)) "pr109040.c":9:11 179 {*movhi_internal}
(expr_list:REG_DEAD (reg:SI 166)
(nil)))
you can infer the MSB of (reg:SI 175) instead of them being undefined. But I
don't think that it prevents any particular instruction form.
