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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - ARM NEON vectorization miscompiles code in 5.0"
href="https://bugs.llvm.org/show_bug.cgi?id=33921">33921</a>
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<th>Summary</th>
<td>ARM NEON vectorization miscompiles code in 5.0
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>5.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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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>Backend: ARM
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>martin@martin.st
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Starting with SVN r306803, some C code that wasn't vectorized before now is
vectorized. This seems to break e.g. some VP9 inverse transforms in libavcodec,
in code like this:
<a href="https://git.libav.org/?p=libav.git;a=blob;f=libavcodec/vp9dsp.c;h=7f863943284b8970744c81b0b7d749e5c9cf8bdf;hb=2b1324bd167553f49736e4eaa94f96da9982925e#l947">https://git.libav.org/?p=libav.git;a=blob;f=libavcodec/vp9dsp.c;h=7f863943284b8970744c81b0b7d749e5c9cf8bdf;hb=2b1324bd167553f49736e4eaa94f96da9982925e#l947</a>
with the idct8_1d and iadst8_1d functions. If this file is built with
-fno-tree-vectorize, the issue disappears.
This regressed when upgrading from SVN r305659 to the 5.0 branch point:
<a href="https://fate.libav.org/armv7-win32-clang-5.0">https://fate.libav.org/armv7-win32-clang-5.0</a>
Bisecting this leads to SVN r306803. The same issue can also be reproduced when
targeting linux, not only windows. It should be reproducable by building libav
and running "make fate-checkasm-vp9dsp", which should pass unless this is
miscompiled.
I will dig into it further, hopefully soon, to fill in with a better
reproducable testcase of the issue.</pre>
</div>
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