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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - AArch64 atomic operations disrupted by fast-regalloc"
href="https://llvm.org/bugs/show_bug.cgi?id=25526">25526</a>
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<th>Summary</th>
<td>AArch64 atomic operations disrupted by fast-regalloc
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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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
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<th>Component</th>
<td>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>t.p.northover@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>When compiling this at -O0:
define {i64, i1} @foo(i64* %addr, i64 %desired, i64 %new) {
%old = cmpxchg i64* %addr, i64 %desired, i64 %new seq_cst seq_cst
ret {i64, i1} %old
}
Fast-regalloc seems to insert many spills between the ldaxr and the stlxr. If
these are on the same cache-line as %addr (in practice) then they can reset the
monitor causing the operation to fail.
ARM is almost certainly affected too. But both only at -O0.
I'm still not sure what to do about this one. Some obvious options are:
1. Go back to lots of pseudo-instructions for various different uses. Extremely
nasty, we worked very hard to get rid of those for better codegen.
2. Disable fast-regalloc in all cases. Probably bad for debug quality, so I
doubt this is viable.
3. Emit libcalls at -O0. I'm not sure all platforms support these though, we'd
have to make sure they're available.</pre>
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</p>
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