<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 --- - Incorrect optimisation of next power of 2 code."
href="https://llvm.org/bugs/show_bug.cgi?id=26930">26930</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Incorrect optimisation of next power of 2 code.
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.7
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>maandree@member.fsf.org
</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>The following code is only executed correctly of compiled with -O0,
not if compiled with -O1, -O2, -O3, -Os, or -Oz.
#include <stddef.h>
int main(void)
{
size_t i, x, n = 7;
if ((n & (~n + 1)) != n) {
n |= n >> 1;
n |= n >> 2;
n |= n >> 4;
for (i = sizeof(n), x = 8; i; i >>= 1, x <<= 1)
n |= n >> x;
n += 1;
}
return n != 8;
}
If you run this code, the program's exit value should be 0 (n was
changed from 7 to 8), but if optimised it's exit value is 1 (n was
changed from 7 to something else than 8, 0 in my experience.)
The code shall change n to the next power of 2.</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>