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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/95405] Unnecessary stores with std::optional
Date: Sun, 06 Jun 2021 18:07:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95405

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
For a self-contained version, see below. Notice how the extra constructor in
_Optional_payload_base changes the generated code, or storing directly a
_Optional_payload_base instead of _Optional_payload in optional

struct _Optional_payload_base {
  long _M_value;
  bool _M_engaged =3D false;
  _Optional_payload_base() =3D default;
  ~_Optional_payload_base() =3D default;
  _Optional_payload_base(const _Optional_payload_base&) =3D default;
  _Optional_payload_base(_Optional_payload_base&&) =3D default;

  _Optional_payload_base(double,float);
};

struct _Optional_payload : _Optional_payload_base { };

struct optional
{
  _Optional_payload _M_payload;
};

optional foo();
long bar()
{
  auto r =3D foo();
  if (r._M_payload._M_engaged)
    return r._M_payload._M_value;
  else
    return 0L;
}=