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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 - Codegen can result in multiple sequential mfence instructions"
href="https://bugs.llvm.org/show_bug.cgi?id=37690">37690</a>
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<th>Summary</th>
<td>Codegen can result in multiple sequential mfence instructions
</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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>redbeard0531@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This is from an example to prove that atomic_thread_fence does not prevent the
compiler from optimizing non-escaped memory.
<a href="https://godbolt.org/g/WXFdPL">https://godbolt.org/g/WXFdPL</a>
#include <utility>
#include <atomic>
struct Type {
Type(Type&&)=default;
int i;
};
Type func(Type t) {
auto out = Type(Type(std::move(t)));
std::atomic_thread_fence(std::memory_order_seq_cst);
return out;
}
auto func2(Type t) { return func(func(func(func(std::move(t))))); }
func(Type): # @func(Type)
mfence
mov eax, edi
ret
func2(Type): # @func2(Type)
mfence
mfence
mfence
mfence
mov eax, edi
ret</pre>
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</p>
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