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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - RegScavenger (as used by BranchFolding) pessimises liveness"
href="https://llvm.org/bugs/show_bug.cgi?id=28142">28142</a>
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<th>Summary</th>
<td>RegScavenger (as used by BranchFolding) pessimises liveness
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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>PC
</td>
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<th>OS</th>
<td>All
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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>Common Code Generator Code
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>ahmed.bougacha@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>This came up in <a href="http://reviews.llvm.org/D21085">http://reviews.llvm.org/D21085</a>; on the original testcase:
Looks like BranchFolding, when merging tails, indiscriminately marks every
register defined but not killed (it might really be live, but not necessarily)
before the tail as live-in the new tail block. It queries the
RegisterScavenger, which walks the original block forward, using kill flags;
maybe it should walk the tail block backward instead?
That patch adds forward liveness computation to X86FixupBWInsts (using flags,
via LivePhysRegs) as a stopgap.
Instead, seems like we should:
- teach RegScavenger to compute liveness backwards (or replace it with
something else)
- fix the various users; in this case, BranchFolding
- remove the X86FixupBWInsts workaround, and probably more</pre>
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