<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 --- - Inline asm constraint alternatives ignored"
href="http://llvm.org/bugs/show_bug.cgi?id=20197">20197</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Inline asm constraint alternatives ignored
</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>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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>benny.kra@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>echristo@gmail.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>When multiple alternatives in an inline asm constraint are given we ignore all
of them but the most "general". This gives nasty artifacts in the code.
int bsr(unsigned v) {
int ret;
__asm__("bsr %1, %0" : "=&r"(ret) : "rm"(v) : "cc");
return ret;
}
$ clang -O3 -S -o - t.c
bsr:
movl %edi, -4(%rsp)
#APP
bsrl -4(%rsp), %eax
#NO_APP
retq
The spilling is totally unnecessary. GCC gets this one right. On 32 bit x86
it's even worse:
$ clang -O3 -S -o - t.c -m32
bsr:
pushl %eax
movl 8(%esp), %eax
movl %eax, (%esp)
#APP
bsrl (%esp), %eax
#NO_APP
popl %edx
retl
GCC knows a better way:
$ gcc-4.8 -O3 -S -o - t.c -m32
bsr:
#APP
bsr 4(%esp), %eax
#NO_APP
ret
The constraint "g" is just as bad, being translated into "imr" internally.</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>