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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang does not properly optimize xor expressions"
href="https://bugs.llvm.org/show_bug.cgi?id=36622">36622</a>
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<th>Summary</th>
<td>clang does not properly optimize xor expressions
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>5.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>FreeBSD
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<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>
<tr>
<th>Reporter</th>
<td>avg@FreeBSD.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>It appears that in some code sequences Clang fails to recognize that
x xor C xor C == x
where C is a compile-time constant.
For instance:
-----------------------------------------
uint32_t
combine(uint16_t x, uint16_t y)
{
uint32_t r = 0x5a5a7777;
r ^= (r ^ x) & 0xffff;
r ^= ((r >> 16) ^ y) << 16;
return (r);
}
-----------------------------------------
In this code the initial value of r is not important and the code should be
equivalent to:
-----------------------------------------
uint32_t
combine(uint16_t x, uint16_t y)
{
return (x | ((uint32_t)y << 16));
}
-----------------------------------------
But even with -O3 Clang produces the following assembler code:
-----------------------------------------
orl $1515847680, %edi # imm = 0x5A5A0000
xorl $23130, %esi # imm = 0x5A5A
shll $16, %esi
xorl %edi, %esi
movl %esi, %eax
-----------------------------------------
It's easy to see that 0x5A5A will always cancel out through double application
via xor, but Clang does not recognize that.</pre>
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