[Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation")

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78113

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:cfb582f62791dfadc243d97d37f0b83ef77cf480

commit r12-4068-gcfb582f62791dfadc243d97d37f0b83ef77cf480
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 4 23:31:48 2021 +0100

    libstdc++: Optimize std::visit for the common case [PR 78113]

    GCC does not do a good job of optimizing the table of function pointers
    used for variant visitation. This avoids using the table for the common
    case of visiting a single variant with a small number of alternative
    types. Instead we use:

      switch(v.index())
      {
      case 0: return visitor(get<0>(v));
      case 1: return visitor(get<1>(v));
      ...
      }

    It's not quite that simple, because get<1>(v) is ill-formed if the
    variant only has one alternative, and similarly for each get<N>.  We
    need to ensure each case only applies the visitor if the index is in
    range for the actual type we're dealing with, and tell the compiler that
    the case is unreachable otherwise. We also need to invoke the visitor
    via the __gen_vtable_impl::__visit_invoke function, to handle the raw
    visitation cases used to implement std::variant assignments and
    comparisons.

    Because that gets quite verbose and repetitive, a macro is used to stamp
    out the cases.

    We also need to handle the valueless_by_exception case, but only for raw
    visitation, because std::visit already checks for it before calling
    __do_visit.

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/78113
            * include/std/variant (__do_visit): Use a switch when we have a
            single variant with a small number of alternatives.