<html>
<head>
<base href="http://llvm.org/bugs/" />
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Port bitfield optimisations from AArch64 to ARM64"
href="http://llvm.org/bugs/show_bug.cgi?id=19424">19424</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Port bitfield optimisations from AArch64 to ARM64
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: AArch64
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>t.p.northover@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>ARM64 doesn't currently form enough (any?) BFI instructions, which means it
cannot pass test/CodeGen/bitfield-insert.ll.
AArch64 gets this via a DAG combine (search for "tryCombineToBFI"). However,
there's a wrinkle: transplanting the code directly results in failures in
ARM64's rev.ll test because the early formation of ARM64ISD::BFI nodes
interferes with the normal DAG combines.
However, the AArch64 code relies on (at least) two passes of DAG combine to
reach its optimal form. This is a flaw which means we can't simply disable it
during early combines.
So, the options are:
+ Rewrite it to be recursive and build the final BFI in just one pass.
+ Rewrite it as some C++ ISelDAG code (similarly recursive).
I *suspect* that the second option will give better results, and not be
significantly harder than the first.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>