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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Suboptimal codegen with __builtin_ctz (regressed in 12)"
href="https://bugs.llvm.org/show_bug.cgi?id=51589">51589</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Suboptimal codegen with __builtin_ctz (regressed in 12)
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>unsigned long f(unsigned long long value)
{
unsigned int result;
if ((value & 0xFFFFFFFF) == 0)
{
result = __builtin_ctz(value >> 32) + 32;
}
else
{
if ((unsigned int)value != 0)
result = __builtin_ctz((unsigned int)value);
}
return result;
}
LLVM 11 -O2
f(unsigned long long): # @f(unsigned long
long)
mov rax, rdi
shr rax, 32
bsf ecx, eax
or ecx, 32
bsf eax, edi
cmove eax, ecx
ret
LLVM12+ -O2:
f(unsigned long long): # @f(unsigned long
f(unsigned long long): # @f(unsigned long
long)
test edi, edi
je .LBB0_1
test edi, edi
je .LBB0_3
bsf eax, edi
mov eax, eax
ret
.LBB0_1:
shr rdi, 32
bsf eax, edi
or eax, 32
mov eax, eax
ret
.LBB0_3:
mov eax, eax
ret
<a href="https://godbolt.org/z/M9q4d13nq">https://godbolt.org/z/M9q4d13nq</a></pre>
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