https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126426
--- Comment #5 from Jeffrey A. Law <law at gcc dot gnu.org> --- So I think I know what's going on here. I don't think we're handling vector modes correctly in ext-dce. Consider something like (subreg:V2HI (reg:DI N)). What bits are live within N? Right now ext-dce reports 0xffff and that in turn enables removal of an extension. That seems horribly wrong. 0xfffffff seems much more correct ;-) I need to go back to the initial review discussion which pushed us towards using GET_MODE_INNER in various places. Conceptually for destinations, GET_MODE_INNER is likely conservatively correct. But for source operands, it seems quite wrong.
