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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Regression: NaN handling differs from LLVM 3.1-3.3"
href="http://llvm.org/bugs/show_bug.cgi?id=18048">18048</a>
</td>
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<th>Summary</th>
<td>Regression: NaN handling differs from LLVM 3.1-3.3
</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>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>release blocker
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kai@redstar.de
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=11601" name="attach_11601" title="IR file demonstrating the problem.">attachment 11601</a> <a href="attachment.cgi?id=11601&action=edit" title="IR file demonstrating the problem.">[details]</a></span>
IR file demonstrating the problem.
The attached IR code is reduced from a D program. It can be compiled and run on
Linux/x86_64.
Compiled with llc -filetype=obj bug.ll ; gcc -o bug bug.o ; ./bug it produces
the output:
0000ffff - 0000ffff
but compiled with opt -O3 <bug.ll | llc -filetype=obj >bug.o ; gcc -o bug bug.o
; ./bug it produces the output:
0000ffff - 00007fff
The root cause seems to be that -real.nan is negativ but 2* -real.nan is
positiv (with -O3). That is really surprising.
The original D program is:
import core.stdc.stdio : printf;
ushort sign(real x)
{
ushort* pxe = cast(ushort *)&x;
return pxe[4];
}
void main()
{
real x = -real.nan;
real y = 2*x;
printf("%08x - %08x\n", sign(x), sign(y));
}
This happens only with LLVM3.4+, not with LLVM 3.1/3.2/3.3.</pre>
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