<html>
    <head>
      <base href="https://llvm.org/bugs/" />
    </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 --- - ARM64: Backend should know about implicit masking of variable shift distances"
   href="https://llvm.org/bugs/show_bug.cgi?id=27582">27582</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>ARM64: Backend should know about implicit masking of variable shift distances
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>libraries
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>3.8
          </td>
        </tr>

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

        <tr>
          <th>OS</th>
          <td>Windows NT
          </td>
        </tr>

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

        <tr>
          <th>Severity</th>
          <td>normal
          </td>
        </tr>

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

        <tr>
          <th>Component</th>
          <td>Backend: AArch64
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>fabiang@radgametools.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs@lists.llvm.org
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr></table>
      <p>
        <div>
        <pre>Created <span class=""><a href="attachment.cgi?id=16281" name="attach_16281" title="Repro C file">attachment 16281</a> <a href="attachment.cgi?id=16281&action=edit" title="Repro C file">[details]</a></span>
Repro C file

ARM64 ASRV, LSLV, LSRV, RORV are specified in the ARMv8 ARM as interpreting the
shift distance "MOD datasize", that is, modulo 32 for 32-bit shifts and modulo
64 for 64-bit shifts.

Since C/C++ shifts larger than the register width are UB, it is desirable that
the (well-defined, portable) alternatives "x >> (y & 31)" (32-bit x) and "x >>
(y & 63)" (64-bit x) not generate an extra "and", but as of Clang-3.8 they do.

Trivial reproducer is attached: "clang -O2 -c -S -target arm64-apple-darwin
test.c".

The expected result is for f to generate "lsr w0, w0, w1; ret" and g "lsr x0,
x0, x1; ret", but with Clang-3.8 explicit ANDs are present.</pre>
        </div>
      </p>
      <hr>
      <span>You are receiving this mail because:</span>
      
      <ul>
          <li>You are on the CC list for the bug.</li>
      </ul>
    </body>
</html>