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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Optimized assembly raises div-by-zero exception though C source code takes pains to avoid it"
href="https://llvm.org/bugs/show_bug.cgi?id=23522">23522</a>
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<th>Summary</th>
<td>Optimized assembly raises div-by-zero exception though C source code takes pains to avoid it
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>Macintosh
</td>
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<th>OS</th>
<td>MacOS X
</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>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>mick@nag.co.uk
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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>Created <span class=""><a href="attachment.cgi?id=14326" name="attach_14326" title="Program showing clang optimization bug">attachment 14326</a> <a href="attachment.cgi?id=14326&action=edit" title="Program showing clang optimization bug">[details]</a></span>
Program showing clang optimization bug
The attached C program takes care to avoid division by zero when presented with
a = b = 0.0. It works correctly when compiled without optimization (using -O0).
When compiler optimization level -O1 or above is used, the assembly generated:
movsd -16(%rbp), %xmm0
mulsd %xmm0, %xmm0
movsd -24(%rbp), %xmm1
mulsd %xmm1, %xmm1
addsd %xmm0, %xmm1
movsd LCPI0_0(%rip), %xmm0
divsd %xmm1, %xmm0
cmpltsd LCPI0_1(%rip), %xmm1
movapd %xmm1, %xmm2
andnpd %xmm0, %xmm2
movsd LCPI0_2(%rip), %xmm0
andpd %xmm1, %xmm0
orpd %xmm2, %xmm0
sqrtsd %xmm0, %xmm0
performs a floating-point division before checking for zero denominator and
hence raises a div-by-zero exception:
clang -O1 optbug.c
./a.out
Input a, b: 0 0
Floating exception</pre>
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