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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - -freciprocal-math flag doesn't generate 'arcp' in LLVM IR"
href="http://llvm.org/bugs/show_bug.cgi?id=20912">20912</a>
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<th>Summary</th>
<td>-freciprocal-math flag doesn't generate 'arcp' in LLVM IR
</td>
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<th>Product</th>
<td>clang
</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>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>I'm filing this purely as a clang bug to generate the 'arcp' fast math flag
when -freciprocal-math is specified as a parameter. More changes are needed to
produce the correct machine code.
Reference: The addition of fast-math flags to LLVM IR happened in 2012:
<a href="http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/054999.html">http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/054999.html</a>
...but there appears to have been very little follow-up in making those flags
actually do anything (see also <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - no-signed-zero optimization flag isn't honored for an fneg operation"
href="show_bug.cgi?id=20870">bug 20870</a>).
$ ./clang -v
clang version 3.6.0 (217587)
Target: x86_64-apple-darwin13.3.0
Thread model: posix
$ cat reciprocal.c
float reciprocal(float x) {
return 1.0f / x;
}
$ ./clang -O1 -freciprocal-math reciprocal.c -emit-llvm -S -o -
...
%div = fdiv float 1.000000e+00, %x ; "arcp" ???</pre>
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