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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 - constexpr fibonacci is not computed at compile time"
href="https://bugs.llvm.org/show_bug.cgi?id=39109">39109</a>
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<th>Summary</th>
<td>constexpr fibonacci is not computed at compile time
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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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>
</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>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>constexpr uint32_t fib(const uint32_t n) {
if (n <= 1) return 1;
return fib(n - 1) + fib(n - 2);
}
int main() {
return fib(7);
}
int main2() {
const uint32_t res = fib(7);
return res;
}
Clang is unable to compute fib in main (but it can in main2) at compile time,
GCC can do it in both functions.
main: # @main
mov edi, 7
jmp fib(unsigned int)
While main2 is interesting.. Clang computes it as 21 but then it still calls
fib uselessly.
main2(): # @main2()
push rax
mov edi, 7
call fib(unsigned int)
mov eax, 21
pop rcx
ret
Latest GCC:
main:
mov eax, 21
ret
main2():
mov eax, 21
ret</pre>
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