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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - uchar4 vector element-by-element LUT handled worse than 3.4."
href="http://llvm.org/bugs/show_bug.cgi?id=20006">20006</a>
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<th>Summary</th>
<td>uchar4 vector element-by-element LUT handled worse than 3.4.
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>simon.hosie@arm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12640" name="attach_12640" title="Function demonstrating regression.">attachment 12640</a> <a href="attachment.cgi?id=12640&action=edit" title="Function demonstrating regression.">[details]</a></span>
Function demonstrating regression.
Given a simple LUT loop which operates independently on each element of a
uchar4, trunk presses ahead with a bunch of inserts and extracts through a
temporary vector, along with a flurry of type conversion operations.
In contrast, Clang 3.4 appears to abandon the vector pretense at the outset and
takes the scalar values directly from the source pointer -- completing the work
in half the time.
GCC 4.8 appears to behave like Clang 3.4, but additionally packs the output
into a single 32-bit scalar register before writing.
This affects both amd64 and ARM. Simply -Ofast to reproduce on amd64, and
additionally -mfpu=neon for ARM to ensure that SIMD operations are available.</pre>
</div>
</p>
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