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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang++ wrong result on -O3 and above"
href="https://bugs.llvm.org/show_bug.cgi?id=50719">50719</a>
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<th>Summary</th>
<td>clang++ wrong result on -O3 and above
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>12.0
</td>
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<tr>
<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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<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>
<tr>
<th>Reporter</th>
<td>shaohua.li@inf.ethz.ch
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<div>
<pre>Hi there,
I found that for a piece of code, clang++ would give out different results when
compiled with different opt flags.
$ clang++ -v
Ubuntu clang version
12.0.1-++20210605063018+0826268d59c6-1~exp1~20210605043737.99
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64
$
$ clang++ -O0 a.cpp ; ./a.out
492559612348245.812500
$
$ clang++ -O3 a.cpp ; ./a.out
492559612348245.875000
$
$ cat a.cpp
#include <iostream>
#include <cstdint>
#include <iomanip>
#include <math.h>
#include <vector>
int main() {
std::vector<unsigned short> value = {19126};
double out = pow(2.0, value[0]/384.0-1.0);
std::cout << std::fixed << out << std::endl;
}
$</pre>
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