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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Performance bug: Clang or LLVM failure to eliminate tail recursion in Linux"
href="https://bugs.llvm.org/show_bug.cgi?id=34662">34662</a>
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<th>Summary</th>
<td>Performance bug: Clang or LLVM failure to eliminate tail recursion in Linux
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>5.0
</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>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>thecppzoo@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Hello. For this code:
__int128 factorial(unsigned arg, __int128 soFar) {
if(arg <= 1) { return soFar; }
return factorial(arg - 1, arg * soFar);
}
Clang 5.0.0 and others fails to eliminate the tail recursion, as can be seen at
the compiler explorer:
<a href="https://godbolt.org/g/iqLwgJ">https://godbolt.org/g/iqLwgJ</a>
(optimization level -O3)
factorial(unsigned int, __int128): #
@factorial(unsigned int, __int128)
mov rcx, rdx
mov rax, rsi
cmp edi, 2
jb .LBB0_2
push rax
mov edx, edi
dec edi
imul rcx, rdx
mul rdx
add rdx, rcx
mov rsi, rax
call factorial(unsigned int, __int128)
mov rcx, rdx
add rsp, 8
.LBB0_2:
mov rdx, rcx
ret
Curiously, -target aarch64-linux-gnu also with optimization -O3 produces
non-tail-recursive code.
GCC eliminates the tail recursion</pre>
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