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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __atomic_load_8 assumes 8 byte alignment"
href="https://bugs.llvm.org/show_bug.cgi?id=47188">47188</a>
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<th>Summary</th>
<td>__atomic_load_8 assumes 8 byte alignment
</td>
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<th>Product</th>
<td>compiler-rt
</td>
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<th>Version</th>
<td>11.0
</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>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>builtins
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>tijl@coosemans.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>For the following code (found in Glasgow Haskell Compiler), clang generates a
function call to __atomic_load_8 on x86 (32-bit) because long long is 8 bytes
in size but only 4 byte aligned:
long long test( long long *x ) {
return( __atomic_load_n( x, __ATOMIC_SEQ_CST ));
}
For 8 byte aligned types clang uses the FILDLL instruction, which (according to
the Intel SDM Vol 3) loads 8 bytes atomically as long as they don't cross a
cache line boundary, which is guaranteed when they are 8 byte aligned but not
with 4 byte alignment.
The problem now is that libcompiler_rt casts the argument of __atomic_load_8 to
an _Atomic( long long ) pointer which has 8 byte alignment so clang uses the
FILDLL instruction when compiling libcompiler_rt.
I don't know if the bug is in libcompiler_rt or in clang. Maybe
__atomic_load_n should only accept atomic types like __c11_atomic_load does.</pre>
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</p>
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