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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 - LLVM prefer lzcnt too strongly - extra computation required"
href="https://bugs.llvm.org/show_bug.cgi?id=50187">50187</a>
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<th>Summary</th>
<td>LLVM prefer lzcnt too strongly - extra computation required
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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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
</td>
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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>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
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<p>
<div>
<pre>long long clz64(long long num) {
return 63LL - __builtin_clzll(num | 1);
}
int clz32(unsigned num) {
return 31 - __builtin_clz(num | 1);
}
int clz32_a(unsigned num) {
return 31 - __builtin_clz(num);
}
LLVM -march=haswell -O3
clz64(long long): # @clz64(long long)
or rdi, 1
lzcnt rax, rdi
xor rax, 63
ret
clz32(unsigned int): # @clz32(unsigned int)
or edi, 1
lzcnt eax, edi
xor eax, 31
ret
clz32_a(unsigned int): # @clz32_a(unsigned int)
lzcnt eax, edi
xor eax, 31
ret
GCC -march=haswell -O3
clz64(long long):
or rdi, 1
bsr rax, rdi
ret
clz32(unsigned int):
or edi, 1
bsr eax, edi
ret
clz32_a(unsigned int):
bsr eax, edi
ret
<a href="https://gcc.godbolt.org/z/r4bYfocKG">https://gcc.godbolt.org/z/r4bYfocKG</a></pre>
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