<html>
<head>
<base href="https://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 --- - [InstCombine] SimplifyDemandedBits makes suboptimal transform"
href="https://llvm.org/bugs/show_bug.cgi?id=28556">28556</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[InstCombine] SimplifyDemandedBits makes suboptimal transform
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>define i5 @or_3_or_1(i3 %a) {
%or1 = or i3 %a, 3
%z = zext i3 %or1 to i5
%or2 = or i5 %z, 1
ret i5 %or2
}
The 2nd 'or' is covered by the 1st, but SimplifyDemandedBits calls
ShrinkDemandedConstant and causes this:
$ ./opt -instcombine -S sandwiches.ll
define i5 @or_3_or_1(i3 %a) {
%or1 = or i3 %a, 2 <--- shrink constant causing later 'or' to be needed
%z = zext i3 %or1 to i5
%or2 = or i5 %z, 1
ret i5 %or2
}
I don't know if this is working as intended or if this is a bug.</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>