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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 - [InstCombine] miscompile of __builtin_fmod"
href="https://bugs.llvm.org/show_bug.cgi?id=34870">34870</a>
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</tr>
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<th>Summary</th>
<td>[InstCombine] miscompile of __builtin_fmod
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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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>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#include <stdio.h>
double foo(int cond, double x, double y) {
double z = cond ? 0.0 : y;
return __builtin_fmod(x, z);
}
int main() {
printf("fmod(%f, %f) = %f\n", 1.0, 2.0, foo(0, 1.0, 2.0));
// If the 1st param of foo() is true (non-zero), the 3rd param is replaced
printf("fmod(%f, %f) = %f\n", 1.0, 0.0, foo(1, 1.0, 2.0));
return 0;
}
$ clang -O0 fmod.c ; ./a.out
fmod(1.000000, 2.000000) = 1.000000
fmod(1.000000, 0.000000) = nan
$ clang -O1 fmod.c ; ./a.out
fmod(1.000000, 2.000000) = 1.000000
fmod(1.000000, 0.000000) = 1.000000
-----------------------------------------------------------------------------
The optimized code is wrong for the 2nd case -- assuming that __builtin_fmod()
should behave like the libm function fmod().
<a href="http://en.cppreference.com/w/c/numeric/math/fmod">http://en.cppreference.com/w/c/numeric/math/fmod</a>
"If y is ±0 and x is not NaN, NaN is returned"</pre>
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