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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - __GCC_ATOMIC_LLONG_LOCK_FREE only set to 1 on X86"
href="http://llvm.org/bugs/show_bug.cgi?id=19355">19355</a>
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<th>Summary</th>
<td>__GCC_ATOMIC_LLONG_LOCK_FREE only set to 1 on X86
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.4
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>gjasny@googlemail.com
</td>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>Hello,
when compiling the Boost Atomic library I noticed a test case failure which
expects that boost::atomic<long long> is always lock-free on X86 [1].
This is caused by the fact that clang sets the __GCC_ATOMIC_LLONG_LOCK_FREE to
1.
In comparison gcc (4.8) sets this preprocessor symbol to 2:
$ gcc --version
gcc (Debian 4.8.2-19) 4.8.2
$ gcc -dM -E - -m32 < /dev/null|grep __GCC_ATOMIC_LLONG_LOCK_FREE
#define __GCC_ATOMIC_LLONG_LOCK_FREE 2
In contrast:
$ clang --version
Debian clang version 3.4-2 (tags/RELEASE_34/final) (based on LLVM 3.4)
Target: x86_64-pc-linux-gnu
Thread model: posix
$ clang -dM -E - -m32 < /dev/null|grep __GCC_ATOMIC_LLONG_LOCK_FREE
#define __GCC_ATOMIC_LLONG_LOCK_FREE 1
Is this difference a bug? Andrey Semashev stated in the Boost Atomic bug report
that Core2 CPUs all should support cmpxchg8b and thus be always completely lock
free.
Thanks,
Gregor
[1] See <a href="https://svn.boost.org/trac/boost/ticket/9842">https://svn.boost.org/trac/boost/ticket/9842</a></pre>
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