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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - APFloat::fusedMultiplyAdd is broken when addend is 0"
href="http://llvm.org/bugs/show_bug.cgi?id=20832">20832</a>
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<th>Summary</th>
<td>APFloat::fusedMultiplyAdd is broken when addend is 0
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Support Libraries
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>greg_bedwell@sn.scee.net
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>I believe the following testcase demonstrates a bug in
APFloat::fusedMultiplyAdd. I would expect this to produce a result of 56
((7*8)+0) but instead it produces 7. The addend being 0 seems to be the key.
----------------------------------------------------------------------
declare double @llvm.fma.f64(double, double, double) nounwind
define double @_Z4testv() nounwind {
%1 = call double @llvm.fma.f64(double 7.000000e+00, double 8.000000e+00,
double 0.000000e+00)
ret double %1
}
----------------------------------------------------------------------
Using r216885:
llc test.ll -mtriple=x86_64-unknown-unknown -o -
we get the following constant generated:
.quad 4619567317775286272 # double 7
Within the function APFloat::fusedMultiplyAdd it calls multiplySpecials which
returns opOK allowing the call to addOrSubtract to occur, however at no point
is the actual multiplication performed, meaning that we essentially do 7+0,
ignoring the multiplicand entirely.</pre>
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