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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 --- - Right-shift operator returns incorrect and inconsistent values"
href="http://llvm.org/bugs/show_bug.cgi?id=19500">19500</a>
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<th>Summary</th>
<td>Right-shift operator returns incorrect and inconsistent values
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.4
</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>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>chris.cooper@veranmedical.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, 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=12416" name="attach_12416" title="Short source code showing the problem">attachment 12416</a> <a href="attachment.cgi?id=12416&action=edit" title="Short source code showing the problem">[details]</a></span>
Short source code showing the problem
Compiling the attached test program with no flags (just "clang++ shift.cpp")
shows several problems with the right-shift operator.
Right-shifting by a large value should always return zero (according to my
reading of the C++ ISO spec, section 5.8.3).
1) Executing the program "as is" gives this output:
Main: 48271 >> 1291394886 = 754; rx=0=0x0=long:0; (lx>>ix)=long:754
Test: 48271 >> 1291394886; (lx>>ix)=754; rx=754=0x2f2=long:754;
(lx>>ix)=long:754
lx (8 bits) = 48271 = = bc8f
ix (4 bits) = 1291394886 = = 4cf91f46
1a) In the main() function, storing the result of the shift in a variable (rx)
results in the correct zero value, but passing the result of the shift either
to std::cout or to a simple print_value() function gives a non-zero result
(754).
1b) When the exact same values are passed to a function for evaluation, the
right-shift operator returns the same invalid value whether the result is
stored first or not.
Perhaps more disturbing:
Commenting out the last two lines of the main() function results in a
drastically wrong value being stored and reported:
Main: 48271 >> 1291394886 = 754;
rx=140734645460120=0x7fff568cdc98=long:140734645460120; (lx>>ix)=long:754</pre>
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