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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - No strict aliasing for uint8_t"
href="https://llvm.org/bugs/show_bug.cgi?id=31410">31410</a>
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<th>Summary</th>
<td>No strict aliasing for uint8_t
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
</tr>
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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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>rsilvera@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Currently uint8_t is treated identically to an unsigned char for purposes
of strict aliasing. This is unnecessary and results in missed optimization,
since unsigned chars must alias all other types (but uint8_ts are not required
to).
There is a related discussion at
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66110">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66110</a>
Here is a trivial demonstration:
#include <stdint.h>
int strict(uint32_t *p, uint8_t *q) {
*p = 1;
*q = 0;
return *p;
}
With clang -O on x86-64, we get:
mov dword ptr [rdi], 1
mov byte ptr [rsi], 0
mov eax, dword ptr [rdi]
ret
Replacing uint8_t with uint16_t (note the load immediate into eax):
mov dword ptr [rdi], 1
mov word ptr [rsi], 0
mov eax, 1
ret</pre>
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