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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 --- - clang miscompiles atomics by using optimized libcalls for underaligned storage"
href="https://llvm.org/bugs/show_bug.cgi?id=23263">23263</a>
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<th>Summary</th>
<td>clang miscompiles atomics by using optimized libcalls for underaligned storage
</td>
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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>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
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<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>Per <a href="https://gcc.gnu.org/wiki/Atomic/GCCMM/LIbrary">https://gcc.gnu.org/wiki/Atomic/GCCMM/LIbrary</a>, "All the optimized routines
expect that the object will be properly aligned for a data type of the
specified size".
However, Clang and GCC get this wrong:
struct X { int a, b; };
X f(X *p) { X r; __atomic_load(p, &r, 5); return r; }
... calls __atomic_load_8. And GCC's libatomic implements __atomic_load_8 on
x86_64 with a single 8-byte mov, which is only atomic for a correctly-aligned
operand.
We should instead emit a call to __atomic_load(8, p, &r, 5) because we have no
reason to think that p is 8 byte aligned.
The same happens for __atomic_store and probably all the other atomic libcalls.</pre>
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</p>
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