<html>
<head>
<base href="https://bugs.llvm.org/">
</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 - Suboptimal code generation for __builtin_ctz(ll)"
href="https://bugs.llvm.org/show_bug.cgi?id=34843">34843</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Suboptimal code generation for __builtin_ctz(ll)
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>5.0
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gcp@sjeng.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Right now, when no specific arch target is set, the builtin
__builtin_ctz (and long, long long variants)
will generate a bsf instruction.
This is suboptimal for AMD machines, which can do a TZCNT much faster than they
can do a BSF. Due to the way TZCNT is encoded, it is equal to a REP BSF, so it
is in fact "backwards compatible" as long as the different behavior for a 0 is
fine. And it is, because __builtin_ctz has undefined behavior for 0 (which is
why it can use BSF in the first place).
On Intel hardware, either way is equally fast, so for a generic target it makes
sense to deal with the AMD case and encode the intrinsic as REP BSF/TZNCT.
At least GCC 4.8 and later are able to do this optimization and generate a REP
BSF for their generic target. Clang fails to do so. (It does generate TZCNT
with -march=znver1)
Example snippet:
<a href="https://godbolt.org/g/eXU6xf">https://godbolt.org/g/eXU6xf</a>
Of note in this snippet is also that newer GCC adds a XOR ESI, ESI before the
REP BSF. So there may be a false dependency issue in some CPUs.</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>