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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 - __atomic_{load,store} on i686 uses libcalls"
href="https://bugs.llvm.org/show_bug.cgi?id=36860">36860</a>
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<th>Summary</th>
<td>__atomic_{load,store} on i686 uses libcalls
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</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>LLVM Codegen
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>pirama@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This was originall reported at the Android NDK issue tracker
(<a href="https://github.com/android-ndk/ndk/issues/521">https://github.com/android-ndk/ndk/issues/521</a>).
$ cat atomic.cpp
unsigned long long
f(unsigned long long* x)
{
unsigned long long v;
__atomic_load(x, &v, __ATOMIC_SEQ_CST);
return v;
}
void
g(unsigned long long* x, unsigned long long v)
{
__atomic_store(x, &v, __ATOMIC_SEQ_CST);
}
$ clang++ -std=gnu++11 -O2 --target=i386-linux-android -S -o - atomic.cpp
-fno-exceptions -fno-rtti
Clang generates library calls to __atomic_{load,store}_8 while gcc implements
the operations using movq. Marking the pointers as 8-byte aligned causes Clang
to generate cmpxchg8b.
The frontend [1] and the backend[2] delegate atomic loads and stores to
libcalls when the alighnment of the type is smaller than the size. For i686,
where long long is aligned to 4 bytes, this causes Clang to always generate the
library call.
A possible fix is to specialize 'hasBuiltinAtomic' in TargetInfo for x86_32 and
allowing atomic operations on 4-byte-aligned pointers to 8-byte types.
[1]
<a href="https://github.com/llvm-mirror/clang/blob/f353d86deb3ea8299f37c5477dcbb9ea30558fb1/lib/CodeGen/CGAtomic.cpp#L758">https://github.com/llvm-mirror/clang/blob/f353d86deb3ea8299f37c5477dcbb9ea30558fb1/lib/CodeGen/CGAtomic.cpp#L758</a>
[2]
<a href="https://github.com/llvm-mirror/llvm/blob/74173207d108cc27f8a02ae06ad90cad42b69b91/lib/CodeGen/AtomicExpandPass.cpp#L188">https://github.com/llvm-mirror/llvm/blob/74173207d108cc27f8a02ae06ad90cad42b69b91/lib/CodeGen/AtomicExpandPass.cpp#L188</a></pre>
</div>
</p>
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