<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 --- - Detecting byteswap sequence"
href="http://llvm.org/bugs/show_bug.cgi?id=20935">20935</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Detecting byteswap sequence
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>bisqwit@iki.fi
</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>This is just silly. Clang optimizes the first function into single opcode
(bswapl, but not the other. For GCC, it's the other way around.
unsigned byteswap_clang(unsigned result)
{
result = ((result & 0xFF00FF00u) >> 8) | ((result & 0x00FF00FFu) << 8);
result = ((result & 0xFFFF0000u) >>16) | ((result & 0x0000FFFFu) <<16);
return result;
}
unsigned byteswap_gcc(unsigned result)
{
result = ((result & 0xFFFF0000u) >>16) | ((result & 0x0000FFFFu) <<16);
result = ((result & 0xFF00FF00u) >> 8) | ((result & 0x00FF00FFu) << 8);
return result;
}
unsigned byteswap(unsigned v)
{
#ifdef __clang__
return byteswap_clang(v);
#else
return byteswap_gcc(v);
#endif
}
Clang output:
byteswap_clang: # @byteswap_clang
bswapl %edi
movl %edi, %eax
retq
byteswap_gcc: # @byteswap_gcc
roll $16, %edi
movl %edi, %eax
shrl $8, %eax
andl $16711935, %eax # imm = 0xFF00FF
shll $8, %edi
andl $-16711936, %edi # imm = 0xFFFFFFFFFF00FF00
orl %eax, %edi
movl %edi, %eax
retq
byteswap: # @byteswap
bswapl %edi
movl %edi, %eax
retq
GCC output:
byteswap_clang:
movl %edi, %eax
andl $-16711936, %eax
shrl $8, %eax
movl %eax, %edx
movl %edi, %eax
andl $16711935, %eax
sall $8, %eax
orl %edx, %eax
roll $16, %eax
ret
byteswap_gcc:
movl %edi, %eax
bswap %eax
ret
byteswap:
movl %edi, %eax
bswap %eax
ret
Tested both -m32 and -m64, with options: -Ofast -S
Tested versions:
- Debian clang version 3.5.0-+rc1-2 (tags/RELEASE_35/rc1) (based on LLVM 3.5.0)
Target: x86_64-pc-linux-gnu
- gcc (Debian 4.9.1-11) 4.9.1 Target: x86_64-linux-gnu</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>