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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 - In O0 optimization, -nan cannot be calculated properly."
href="https://bugs.llvm.org/show_bug.cgi?id=49304">49304</a>
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<th>Summary</th>
<td>In O0 optimization, -nan cannot be calculated properly.
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>8.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++11
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>2077213809@qq.com
</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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<p>
<div>
<pre>Simple test cases
#include <iostream>
#include <stdio.h>
#include <limits>
int main()
{
double a = std::numeric_limits<double>::quiet_NaN();
double b = -a;
std::cout << b << std::endl;
return 0;
}
clang++ -O0 nan_bug.cpp
./a.out
nan
clang++ -O2 nan_bug.cpp
./a.out
-nan
when we use -O0 optimize, The negative operation does not seem to take effect.
The operation becomes sub 0.
the IR is
47 define dso_local i32 @main() #4 {
48 entry:
...
53 %call = call double @_ZNSt14numeric_limitsIdE9quiet_NaNEv() #3
54 store double %call, double* %a, align 8
55 %0 = load double, double* %a, align 8
56 %sub = fsub double -0.000000e+00, %0 // Desired Negative Operation
57 store double %sub, double* %b, align 8
58 %1 = load double, double* %b, align 8
...
when we use -O2, the -nan is directly calculated, so there's no problem.
%call.i6 = tail call dereferenceable(272) %"class.std::basic_ostream"*
@_ZNSo9_M_insertIdEERSoT_(%"class.std::basic_ostream"* nonnull @_ZSt4cout,
double 0xFFF8000000000000)</pre>
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