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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - -Watomic-alignment not using available alignment info"
href="https://bugs.llvm.org/show_bug.cgi?id=51076">51076</a>
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<th>Summary</th>
<td>-Watomic-alignment not using available alignment info
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>12.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>rth@twiddle.net
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>For i686:
----------------------------------
#include <stdint.h>
uint64_t foo(uint64_t *p)
{
p = __builtin_assume_aligned(p, 8);
return __atomic_load_n(p, __ATOMIC_RELAXED);
}
uint64_t bar(uintptr_t p)
{
if (p & 7) return 0;
return __atomic_load_n((uint64_t *)p, __ATOMIC_RELAXED);
}
----------------------------------
$ clang-12 -Wall -O -c z.c
z.c:6:10: warning: misaligned atomic operation may incur significant
performance penalty; the expected alignment (8 bytes) exceeds the actual
alignment (4 bytes) [-Watomic-alignment]
return __atomic_load_n(p, __ATOMIC_RELAXED);
^
z.c:12:10: warning: misaligned atomic operation may incur significant
performance penalty; the expected alignment (8 bytes) exceeds the actual
alignment (4 bytes) [-Watomic-alignment]
return __atomic_load_n((uint64_t *)p, __ATOMIC_RELAXED);
^
2 warnings generated.
In both cases, additional alignment information is available
beyond that of the uint64_t type, and it is not being used.</pre>
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