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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __builtin_nan incorrectly has "const" attribute"
href="https://bugs.llvm.org/show_bug.cgi?id=37778">37778</a>
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<th>Summary</th>
<td>__builtin_nan incorrectly has "const" attribute
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>gfalcon@google.com
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>In Builtins.def, __builtin_nan and friends have the "c" (const) attribute, even
though these functions access the pointed-to memory. This leads the optimizer
into generating incorrect code.
The two functions below return different answers in -O1 and above:
double a() {
char buf[4];
buf[0] = buf[1] = buf[2] = '9';
buf[3] = '\0';
return __builtin_nan(buf);
}
double b() { return __builtin_nan("999"); }
In a(), the optimizer thinks that nothing reads the contents of `buf`, and
eliminates the assignments. See <a href="https://godbolt.org/g/sdn5Mh">https://godbolt.org/g/sdn5Mh</a>.</pre>
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