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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 --- - clang misscompiles “|=” if the right side is a int{char{T}}"
href="http://llvm.org/bugs/show_bug.cgi?id=16903">16903</a>
</td>
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<tr>
<th>Summary</th>
<td>clang misscompiles “|=” if the right side is a int{char{T}}
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.3
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>florian.weber@sfz-bw.de
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, 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=11041" name="attach_11041" title="the preprocessed source">attachment 11041</a> <a href="attachment.cgi?id=11041&action=edit" title="the preprocessed source">[details]</a></span>
the preprocessed source
The following program causes clang to create wrong code (plattform:
x86_64-unknown-linux-gnu):
#include <iostream>
template<typename T>
void fun (T it) {
int m = 0;
for(int i = 0; i < 4; ++i, ++it){
m |= int{char{*it}};
std::cout << int(*it) << ", " << m << ", " << i << std::endl;
}
}
int main () {
char in[4] = {0,0,0,0};
fun(in);
}
Expected output:
0, 0, 0
0, 0, 1
0, 0, 2
0, 0, 3
The real output depends on the optimization-level, eg with -O0:
0, 256, 0
0, 110578432, 1
0, 110578432, 2
0, 110578432, 3
(110578432 is a number unique to every run if adress-space-randomisation is
enabled, otherwise it is always -7936)
with -O3 the output is always:
0, 224, 0
0, 224, 1
0, 224, 2
0, 224, 3
Additional information:
My commandline is:
clang++ -O0 -std=c++11 main.cpp / clang++ -O3 -std=c++11 main.cpp
The preprocessed source is attached.
As the Bug has some (though small) similarities, it may be related to #16894.
A look into the assembler showed that the only “or”-instruction in the TU
involves the register %ecx which is not used in any other place of the
object-file (I don't known details of the x64-assembler, so this might be
unrelated but it took long enough to find and looks suspicious enough that I
note it here).
clang++ --version:
clang version 3.3 (tags/RELEASE_33/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
plattform: Arch Linux</pre>
</div>
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