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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang v10 optimizations causing Valgrind false-positives"
href="https://bugs.llvm.org/show_bug.cgi?id=49372">49372</a>
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<th>Summary</th>
<td>clang v10 optimizations causing Valgrind false-positives
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>10.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>eyalsoha@gmail.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Starting in clang version 10 (not in version 9), one of the loop optimizations
is generating code that is correct but can lead to a false-positive error
report in Valgrind.
I think that clang could generate code that would perform just as well as the
code that it's already creating but it would be more Valgrind-friendly and
allow Valgrind's memchecker to handle it better.
This code:
401697: 66 0f 6e d2 movd %edx,%xmm2
40169b: 66 0f 60 d2 punpcklbw %xmm2,%xmm2
40169f: 66 0f 61 da punpcklwd %xmm2,%xmm3
which was generated to use two different xmm registers could just as well be
this:
4016a3: 66 0f 6e da movd %edx,%xmm3
4016a7: 66 0f 60 db punpcklbw %xmm3,%xmm3
4016ab: 66 0f 61 db punpcklwd %xmm3,%xmm3
Which only uses one xmm register. Doing that fixes the spurious error report
from Valgrind and I think that it would have no negative impact on performance.
More information and a test case are available on the Valgrind bug report:
<a href="https://bugs.kde.org/show_bug.cgi?id=432801#c12">https://bugs.kde.org/show_bug.cgi?id=432801#c12</a></pre>
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