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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW " title="NEW --- - Incorrect VMX/Altivec Code Generation - Mis-using splat instructions" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D24216&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=-6-84vKebcCEOQxA7A4NE1OAxy7Z1R1cEV-yBw1BRjM&s=gkQ372JdsPBEfXptHcFPnPtW2XmXXmAhtmwr5eYK6Fk&e=">24216</a>
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<th>Summary</th>
<td>Incorrect VMX/Altivec Code Generation - Mis-using splat instructions
</td>
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<th>Product</th>
<td>tools
</td>
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<th>Version</th>
<td>trunk
</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>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>llc
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>tjkenney@us.ibm.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=14629" name="attach_14629" title="tarball w/ .cl source, .ll source, .s output & Makefile">attachment 14629</a> <a href="attachment.cgi?id=14629&action=edit" title="tarball w/ .cl source, .ll source, .s output & Makefile">[details]</a></span>
tarball w/ .cl source, .ll source, .s output & Makefile
This bug causes the compiler to emit splat instructions which 'splat' the wrong
bytes from the source register. The issue is that the function
PPC::isSplatShuffleMask() in PPCISelLowering.cpp does not ensure that the
'splat' pattern found is requesting bytes that are aligned on an 'EltSize'
boundary. Therefore, the compiler emits splat instructions when it shouldn't;
the operation should be performed by a vperm or some other sequence of
instructions instead. Attached is an OpenCL-C function & the generated IR that
trigger this bug.
A simple fix would be to add this check in isSplatShuffleMask() with the
following line:
if (N->getMaskElt(0) % EltSize) return false;
This causes the VECTOR_SHUFFLE node to be lowered to a vperm inst instead. Is
there a better solution? Would a shift followed by a splat be preferred over a
vperm?</pre>
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