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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Register Spills Caused by the Machine Reassociation Optimization"
href="https://llvm.org/bugs/show_bug.cgi?id=25016">25016</a>
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<th>Summary</th>
<td>Register Spills Caused by the Machine Reassociation Optimization
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
</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
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>haicheng@codeaurora.org
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mssimpso@codeaurora.org, spatel+llvm@rotateright.com
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=14976" name="attach_14976" title="a .ll test case that delays the addition of the first number">attachment 14976</a> <a href="attachment.cgi?id=14976&action=edit" title="a .ll test case that delays the addition of the first number">[details]</a></span>
a .ll test case that delays the addition of the first number
I observed some extra register spills when applying the reassociation pass on
some programs.
For example, code like this
…
X=a+b
…
Y=X+c
…
Z=Y+d
…
There are some other instructions between these float adds. The reassociation
pass first swaps a and c when checking the second add, and then swaps a and d
when checking the third add. The transformed code looks like
…
X=c+b
…
Y=X+d
…
Z=Y+a
a is pushed all the way down to the bottom and its live range is much larger
now.
A .ll test case is attached which computes a+b+c+d+e+f+g+h. The transformation
delays the computation of a to the end and it becomes b+c+d+e+f+g+h+a</pre>
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