'[llvm-bugs] [Bug 46519] New: Copy elision is not performed for objects with non overlapping lifetime'

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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 46519] New: Copy elision is not performed for objects with non overlapping lifetime
From:       via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2020-06-30 15:01:14
Message-ID: bug-46519-206 () http ! bugs ! llvm ! org/
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Date: Tue, 30 Jun 2020 08:01:14 -0700
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https://bugs.llvm.org/show_bug.cgi?id=46519

            Bug ID: 46519
           Summary: Copy elision is not performed for objects with non
                    overlapping lifetimes
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs@nondot.org
          Reporter: antoshkka@gmail.com
                CC: blitzrakete@gmail.com, dgregor@apple.com,
                    erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org,
                    richard-llvm@metafoo.co.uk

Consider the example:

struct widget {
  widget();
  widget(widget&&);
};

// Inspired by https://wg21.link/p2025
widget test(int i) {
  {
      widget w1;
      if (i == 1) return w1;
  }
  widget w2;
  return w2;
}

Lifetimes of w1 and w2 do not overlap and it is possible to elide the move
constructor by constructing w1 and w2 at the same destination address.

Godbolt playground: https://godbolt.org/z/BZPgwi

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Date: Tue, 30 Jun 2020 08:01:14 -0700
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<html>
    <head>
      <base href="https://bugs.llvm.org/">
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - Copy elision is not performed for objects with non overlapping \
lifetimes"  href="https://bugs.llvm.org/show_bug.cgi?id=46519">46519</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Copy elision is not performed for objects with non overlapping \
lifetimes  </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>trunk
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>enhancement
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>C++
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs&#64;nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>antoshkka&#64;gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>blitzrakete&#64;gmail.com, dgregor&#64;apple.com, \
erik.pilkington&#64;gmail.com, llvm-bugs&#64;lists.llvm.org, \
richard-llvm&#64;metafoo.co.uk  </td>
        </tr></table>
      <p>
        <div>
        <pre>Consider the example:

struct widget {
  widget();
  widget(widget&amp;&amp;);
};

// Inspired by <a href="https://wg21.link/p2025">https://wg21.link/p2025</a>
widget test(int i) {
  {
      widget w1;
      if (i == 1) return w1;
  }
  widget w2;
  return w2;
}

Lifetimes of w1 and w2 do not overlap and it is possible to elide the move
constructor by constructing w1 and w2 at the same destination address.

Godbolt playground: <a \
href="https://godbolt.org/z/BZPgwi">https://godbolt.org/z/BZPgwi</a></pre>  </div>
      </p>

      <hr>
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      <ul>
          <li>You are on the CC list for the bug.</li>
      </ul>
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