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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - No warnings for invalid memory orderings for atomic accesses"
href="http://llvm.org/bugs/show_bug.cgi?id=21179">21179</a>
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<th>Summary</th>
<td>No warnings for invalid memory orderings for atomic accesses
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<th>Product</th>
<td>clang
</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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++11
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>morisset@google.com
</td>
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<th>CC</th>
<td>dgregor@apple.com, jfb@google.com, llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>I believe that it is undefined behaviour for an atomic load to have the
memory_order_release or memory_order_acquire_release, or for an atomic store to
have the memory_order_acquire or memory_order_acquire_release. Similarily, the
failure memory ordering for a compare-exchange cannot have a release component,
and must be weaker than the success case. In all of these cases, clang
currently emits either a relaxed operation (or the strongest valid ordering
that is weaker than the success ordering for cmpxchg) without emitting any
warning.
Here is a simple reproduction:
```
#include <atomic>
using namespace std;
atomic<int> x;
int f () {
x.store(42, memory_order_acquire);
return x.load(memory_order_release);
}
```
I would expect warnings on both the store and the load.
The relevant code appears to be the lines 256, 786, 792, 798 of
lib/CodeGen/CGAtomic.cpp, where the undefined behaviour is noted in the
comment, then a default choice is made.. all without any warning.</pre>
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</p>
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