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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed opportunity for BTR instruction"
href="https://bugs.llvm.org/show_bug.cgi?id=37938">37938</a>
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<th>Summary</th>
<td>Missed opportunity for BTR instruction
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</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>
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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>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
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<pre>Hello,
Code:
uint64_t btr_maybe(uint64_t x, unsigned bit) {
return x & ~(1ULL << bit);
}
Clang -03 gives:
btr_maybe: # @btr_maybe
mov rax, -2
mov ecx, esi
rol rax, cl
and rax, rdi
ret
But BTR instruction can be used (and GCC 8+ emits it):
btr_maybe(unsigned long, unsigned int):
mov rax, rdi
btr rax, rsi
ret</pre>
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