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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Analyzer says undefined behavior in uniform_int_distribution<size_t>"
href="https://llvm.org/bugs/show_bug.cgi?id=26395">26395</a>
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<th>Summary</th>
<td>Analyzer says undefined behavior in uniform_int_distribution<size_t>
</td>
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<th>Product</th>
<td>libc++
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<th>Version</th>
<td>3.7
</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>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>jared.grubb@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The clang static analyzer flags this code as having undefined behavior. Note
that if I change "size_t" to "unsigned" then things look fine. It looks like
the engine is 32-bit but it's going to try a 64-bit shift maybe? I'm totally
guessing here.
.../c++/v1/algorithm:2920
warning: The result of the '>>' expression is undefined
__mask0_ = __w0_ > 0 ? _Engine_result_type(~0) >> (_EDt - __w0_) :
~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
To reproduce:
$ cat x.cpp
#include <random>
#include <cstddef>
size_t genRandom(size_t maxValue) {
std::minstd_rand engine;
std::uniform_int_distribution<size_t> dist{0, maxValue};
return dist(engine);
}
$ clang++ x.cpp -std=c++14 --analyze</pre>
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