[Bug c++/86173] New: Default construction of a union (in std::optional)

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86173

            Bug ID: 86173
           Summary: Default construction of a union (in std::optional)
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

Default construction of std::optional<A> always starts with a memset of the
whole optional to 0, while it doesn't with clang using the same libstdc++.

#include <optional>
struct AA {
  double a[1024];
#ifndef TRIVIAL
  AA(); AA(AA const&); AA& operator=(AA const&); ~AA();
#endif
};
typedef std::optional<AA> O;
// O fff(){ return {}; }
O fff(){ O o; return o; }

The .original dump has

*<retval> = {.D.34926={._M_payload={.D.34026={._M_empty={}}, ._M_engaged=0}}}

which looks good, it says it is initializing the small _M_empty part of the
union, but the gimple dump has

*<retval> = {};

which eagerly zeroes everything.