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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Suboptimal atomic_fetch_add"
href="https://bugs.llvm.org/show_bug.cgi?id=37557">37557</a>
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<th>Summary</th>
<td>Suboptimal atomic_fetch_add
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: C
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nruslan_devel@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following example:
#include <stdatomic.h>
int func(_Atomic(unsigned long) * obj, unsigned long a)
{
return atomic_fetch_add(obj, a) == -a;
}
Basically, it suffices to use 'lock add' and check whether ZF flag is set.
However, the following code (that uses xadd) is generated (-O2):
func: # @func
.cfi_startproc
# %bb.0:
movq %rsi, %rcx
lock xaddq %rcx, (%rdi)
xorl %eax, %eax
addq %rsi, %rcx
sete %al
retq</pre>
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