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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - APFloat::fusedMultiplyAdd is broken, leading to incorrect constant folding for libc fmal calls."
href="http://llvm.org/bugs/show_bug.cgi?id=20728">20728</a>
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<th>Summary</th>
<td>APFloat::fusedMultiplyAdd is broken, leading to incorrect constant folding for libc fmal calls.
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Support Libraries
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lhames@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>APFloat::multiplySignificand generates incorrect results when called with
Addend != 0 (i.e. from APFloat::fusedMultiplyAdd). This results in incorrect
results from libc fmal calls with constant arguments. I have tested this X86,
but I believe it will hit any architecture that uses APFloat to handle long
double constants.
The output is only incorrect for some constants.
E.g. fmal(3.0L, 4.0L, 5.0L) == 1.0 instead of 17.0L.
However fmal(-3.0L, 4.0L, 5.0L) == -7.0L as expected.
Compiling the following program with 'clang -O3 -o foo foo.c' on X86
demonstrates the bug:
#include <math.h>
#include <stdio.h>
int main(void) {
printf("%Lf\n", fmal(3.0L, 4.0L, 5.0L));
return 0;
}
The misbehavior for the specific constants above was introduced by r181715. The
commit message for that revision was: "Fix a bug that
APFloat::fusedMultiplyAdd() mistakenly evaluate "14.5f * -14.5f + 225.0f" to
225.0f.". I have confirmed that rolling back to r181714 causes "fmal(3.0L,
4.0L, 5.0L)" to generate correct results, but breaks "14.5f * -14.5f + 225.0f".</pre>
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