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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine propagates values across omp barrier, after D75010"
href="https://bugs.llvm.org/show_bug.cgi?id=45635">45635</a>
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<th>Summary</th>
<td>InstCombine propagates values across omp barrier, after D75010
</td>
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<th>Product</th>
<td>OpenMP
</td>
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<th>Version</th>
<td>unspecified
</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>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Clang Compiler Support
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>chang-sun.lin.jr@intel.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=23394" name="attach_23394" title="test case">attachment 23394</a> <a href="attachment.cgi?id=23394&action=edit" title="test case">[details]</a></span>
test case
The attached test started giving different results @ O2, after commit
D75010/rG72b51d6f93b5. It has a code sequence like this:
#pragma omp critical
shared_var++;
#pragma omp barrier
printf("expecting 2: %d\n", shared_var);
D75010 marks the kmpc_barrier function with inaccessiblemem_or_argmemonly,
which allows InstCombine to propagate the local thread's shared_var value
across the barrier, to the printf. The different threads print different values
of shared_var. The standard seems to say that barrier implies a flush of the
data visible to the thread, ensuring that all threads see the same view of
memory after the barrier.
Compiled with:
clang -fopenmp -O2 barrier.c
Expecting this output:
expecting 2: 2
expecting 2: 2
Got this output:
expecting 2: 2
expecting 2: 1</pre>
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