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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 - Inconsistent optimization of code using __builtin_ctz when changing the return type"
href="https://bugs.llvm.org/show_bug.cgi?id=46389">46389</a>
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<th>Summary</th>
<td>Inconsistent optimization of code using __builtin_ctz when changing the return type
</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>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>unsigned long f(uint64_t 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;
}
With -O3, LLVM outputs this :
f(unsigned long): # @f(unsigned long)
mov rax, rdi
shr rax, 32
tzcnt ecx, eax
or ecx, 32
tzcnt eax, edi
cmovb eax, ecx
ret
If the return type is changed from `unsigned long` to `unsigned int`, LLVM
outputs this :
f(unsigned long): # @f(unsigned long)
test edi, edi
je .LBB0_1
tzcnt eax, edi
ret
.LBB0_1:
shr rdi, 32
tzcnt eax, edi
or eax, 32
ret
This seems rather inconsistent considering that the two outputs are
functionally completely identical (at least, as far as I can see.</pre>
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