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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - xor %a, C -> xor %a, -1 canonicalization"
href="https://bugs.llvm.org/show_bug.cgi?id=37610">37610</a>
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<th>Summary</th>
<td>xor %a, C -> xor %a, -1 canonicalization
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Clang sometimes generates from
#include <cstdint>
uint8_t and_0(uint32_t cache, uint8_t count) {
return uint8_t(cache) & uint8_t((uint8_t(1) << uint8_t(count)) - uint8_t(1));
}
this
define dso_local zeroext i8 @and_0(unsigned int, unsigned char)(i32, i8
zeroext) local_unnamed_addr #0 {
%3 = zext i8 %1 to i32
%4 = shl i32 1, %3
%5 = add i32 %4, 255
%6 = and i32 %5, %0
%7 = trunc i32 %6 to i8
ret i8 %7
}
Because the constant is not -1, the final code is suboptimal
<a href="https://godbolt.org/g/DeY8sr">https://godbolt.org/g/DeY8sr</a>
Should we canonicalize it somehow?
<a href="https://rise4fun.com/Alive/sEB">https://rise4fun.com/Alive/sEB</a></pre>
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