[v3 PATCH, RFC] Rewrite variant. Also PR libstdc++/85517

[Ville Voutilainen]

On Wed, 6 Feb 2019 at 15:12, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:

> And, to emphasize, the most important reason for this was to be able
> to write straightforward
> code for the special member functions, with the hope that it wouldn't
> have a negative codegen
> effect. Our Microsoft friends described the general technique as "has
> crazy-good codegen",
> but I have no idea what their starting point was; our starting point
> probably wasn't bad
> to begin with.

However, the codegen should be somewhat improved; this patch removes a
bag of run-time ifs from the implementation.

An amended patch attached. This gets rid of all __erased* stuff,
including hash, swap, constructors, relops.
I consider variant to no longer be in the state of sin after this.
Since this is touching just a C++17 facility with no
impact elsewhere, we could consider landing it in GCC 9 as a late
change. Failing that, it certainly seems safe enough
to put into GCC 9.2.

2019-03-04  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Rewrite variant.
    Also PR libstdc++/85517
    * include/std/variant (__do_visit): New.
    (__variant_cast): Likewise.
    (__variant_cookie): Likewise.
    (__erased_*): Remove.
    (_Variant_storage::_S_vtable): Likewise.
    (_Variant_storage::__M_reset_impl): Adjust to use __do_visit.
    (_Variant_storage::__M_reset): Adjust.
    (_Copy_ctor_base(const _Copy_ctor_base&)): Adjust to use __do_visit.
    (_Move_ctor_base(_Move_ctor_base&&)): Likewise.
    (_Move_ctor_base::__M_destructive_copy): New.
    (_Copy_assign_base::operator=): Adjust to use __do_visit.
    (_Copy_assign_alias): Adjust to check both copy assignment
    and copy construction for triviality.
    (_Move_assign_base::operator=): Adjust to use __do_visit.
    (_Multi_array): Add support for visitors that accept and return
    a __variant_cookie.
    (__gen_vtable_impl::_S_apply_all_alts): Likewise.
    (__gen_vtable_impl::_S_apply_single_alt): Likewise.
    (__gen_vtable_impl::__element_by_index_or_cookie): New. Generate
    a __variant_cookie temporary for a variant that is valueless and..
    (__gen_vtable_impl::__visit_invoke): ..adjust here.
    (__gen_vtable::_Array_type): Conditionally make space for
    the __variant_cookie visitor case.
    (relops): Adjust to use __do_visit.
    (variant): Add __variant_cast as a friend.
    (variant::emplace): Use _M_reset() instead of self-destruction.
    (visit): Reimplement in terms of __do_visit.
    * testsuite/20_util/variant/compile.cc: Adjust.
    * testsuite/20_util/variant/run.cc: Likewise.
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