https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106261
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
@item -dx
@opindex dx
Just generate RTL for a function instead of compiling it. Usually used
with @option{-fdump-rtl-expand}.
@end table
so possibly some side-effect of not doing things something we do later depends
on with -dx
