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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Passing 0 to __atomic_fetch_add generates mfence instead of lock xadd"
href="https://llvm.org/bugs/show_bug.cgi?id=28498">28498</a>
</td>
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<th>Summary</th>
<td>Passing 0 to __atomic_fetch_add generates mfence instead of lock xadd
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>pierregousseau14@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Using clang 3.9 on trunk at r273567
Target: x86_64-unknown-linux-gnu
When passing a '0' constant to '__atomic_fetch_add', an mfence instruction is
generated instead of the usual 'lock xadd' which seems less efficient.
---
#include <stdint.h>
int64_t * g_ptr;
int foo() {
return __atomic_fetch_add(g_ptr, 0, __ATOMIC_RELAXED);
}
---
clang -O3 -S test.c
Generates:
movq %rdi, -8(%rsp)
movq g_ptr(%rip), %rax
mfence
movq (%rax), %rax
retq
Using gcc:
movq g_ptr(%rip), %rdx
movq %rdi, -8(%rsp)
xorl %eax, %eax
lock xaddq %rax, (%rdx)
ret</pre>
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