<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 --- - Compiler missing corner case of single bit test optimization"
href="http://llvm.org/bugs/show_bug.cgi?id=21993">21993</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Compiler missing corner case of single bit test optimization
</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>Windows NT
</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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>douglas_yung@playstation.sony.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>In r222871, an optimization was re-added to optimize sequences of the form:
(X & Y) == 0 ? X : X ^ Y --> X & ~Y
(X & Y) != 0 ? X ^ Y : X --> X & ~Y
(X & Y) == 0 ? X ^ Y : X --> X | Y
(X & Y) != 0 ? X : X ^ Y --> X | Y
It seems to work for most cases, but it seems if Y=0x80000000, the optimization
is not applied when it could be.
Consider the following code:
=====
unsigned int test0a(unsigned int x) {
return (x & CONSTANT) == 0 ? x : (x ^ CONSTANT);
}
-- or --
unsigned int test0b(unsigned int x) {
if((x & CONSTANT) == 0)
return x;
else
return (x ^ CONSTANT);
}
=====
Both represent the first pattern. If they are compiled targeting x64 linux with
-O1 and Y=0x8, clang (r224566) will generate the following code for both
functions:
andl $-9, %edi
movl %edi, %eax
which is what we expect.
However, if we use the same compiler and instead use Y=0x80000000, the compiler
generates the following for the functions:
movl %edi, %eax
xorl $-2147483648, %eax # imm = 0xFFFFFFFF80000000
testl %edi, %edi
cmovnsl %edi, %eax
The code is still correct, however, it could be better. The optimization
re-added with r222871 does not seem to be detecting this case as valid for
optimization when it should.</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>