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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - double-double constant expression evaluation can produce spurious NaN"
href="https://bugs.llvm.org/show_bug.cgi?id=49132">49132</a>
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<th>Summary</th>
<td>double-double constant expression evaluation can produce spurious NaN
</td>
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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>Other
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>bbaren@google.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=24512" name="attach_24512" title="minimal example">attachment 24512</a> <a href="attachment.cgi?id=24512&action=edit" title="minimal example">[details]</a></span>
minimal example
When compiling for POWER (-target powerpc64le-linux-gnu), Clang can incorrectly
simplify long double (double-double) expressions to NaN:
$ uname -m
ppc64le
$ cat ldbl.c
extern int printf(const char* restrict, ...);
int main() {
long double a = -0x1.1e282e024debdb875f47f6c85p+1021L;
printf("%Le\n", a + 0x1.fffffffffffff7ffffffffffff8p+1023L);
}
$ clang -Wall -Wextra -O1 -o ldbl ldbl.c
$ ./ldbl
nan
Inspecting the generated bitcode shows that Clang has statically evaluated the
double-double addition to a NaN, but by IEEE 754, adding two non-NaN inputs
should never produce a NaN output.
GCC produces the correct answer for this computation, though only when
optimizations are enabled (see
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048</a>).</pre>
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