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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 --- - Combine consecutive fences & atomic-related optimisations"
href="https://llvm.org/bugs/show_bug.cgi?id=31736">31736</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Combine consecutive fences & atomic-related optimisations
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>davide@freebsd.org
</td>
</tr>
<tr>
<th>CC</th>
<td>chandlerc@gmail.com, filcab@gmail.com, hboehm@google.com, llvm-bugs@lists.llvm.org, llvm-bugzilla@jfbastien.com, llvm-dev@redking.me.uk, simon.f.whittaker@gmail.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following function:
int tinkywinky(void) {
__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();__sync_synchronize();
__sync_synchronize();
return 15;
}
gets lowered into:
tinkywinky(): # @tinkywinky()
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mov eax, 15
ret
by clang-3.9
and
tinkywinky():
mfence
mov eax, 15
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
mfence
ret
by gcc-6.3 (interesting the different position of the `mov` in the final
relocatable.
This is because InstCombine/InstSimplify/* don't combine adjacent fences. The
real question here is if it's legal to do this (i.e. has no obvservable
behaviour). I had a quick chat about this with Chandler yesterday night and he
asked to open a bug & cc: some people so we can all discuss about this).
Probably it's worth starting a discussion in the standard (if there's not
already one) about the guarantees provided in this case (if any).</pre>
</div>
</p>
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