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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/112480] optional<T>::reset emits inefficient code
 when T is trivially-destructible
Date: Mon, 13 Nov 2023 12:22:36 +0000
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X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.2.0
X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112480

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Good point, it looks like we get the same codegen improvement for ~T(){} ev=
en
at -O1 if we don't restrict it to trivially destructible types.

There seems to be no difference in codegen for _M_engaged=3Dfalse or
_M_engaged=3D_M_engaged (which isn't too surprising, since we know that eit=
her it
was already false, or we're setting it to false). Given that, I think I pre=
fer
explicitly setting to false.

So I'll test this:

--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -311,6 +311,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
        if (this->_M_engaged)
          _M_destroy();
+       else // This seems redundant but improves codegen, see PR 112480.
+         this->_M_engaged =3D false;
       }
     };=