[Bug rtl-optimization/53260] New: cse doesn't record REG_EQUAL notes for function invariants

["rsandifo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53260

             Bug #: 53260
           Summary: cse doesn't record REG_EQUAL notes for function
                    invariants
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rsandifo@gcc.gnu.org


CSE adds REG_EQUAL notes for constant equivalences that it finds:

      /* If this is a single SET, we are setting a register, and we have an
     equivalent constant, we want to add a REG_NOTE.   We don't want
     to write a REG_EQUAL note for a constant pseudo since verifying that
     that pseudo hasn't been eliminated is a pain.  Such a note also
     won't help anything.

It might be worth extending this to function_invariant_p,
so that reload has the option of eliminating registers that
are equivalent to known frame addresses.  A hacked-up patch
to do this showed a surprising number of differences,
but I haven't had chance to see whether they're good or bad.