https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67609
--- Comment #17 from Vladimir Makarov <vmakarov at gcc dot gnu.org> --- (In reply to Jeffrey A. Law from comment #16) > reload has traditionally removed subregs of hardregs and passes after reload > have depended on that behaviour. Doing something similar in lra is > obviously necessary. In fact, subregs of multi-word hard regs isn't ever > supposed to appear in the except during allocation & reloading. > > I'm not sure why final has another call to cleanup_subreg_operands. While > git blame blames me, I was just refactoring existing code code back in '98. > > Does that shed any light on what the right behaviour for LRA ought to be? LRA before the patch did the same as reload. This case shows that still some subregs should stay after reload otherwise we have wrong transformations (as making insn setting high part of the register a dead insn). Of course, some optimizations can not deal with subregs of multi-regs. The patch avoids keeping such subregs. I also found x86 reg-stack pass can not deal with any subregisters of stack fp regs. In any case I spent a lot of time for this small patch which works for at least 5 tested targets. I guess what we need more is to make final pass (at least for x86-64) to deal with the rest of subregs in a right way.
