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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Erroneous loop optimization with -O2, accessing to wrong std::vector element"
href="https://bugs.llvm.org/show_bug.cgi?id=38786">38786</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Erroneous loop optimization with -O2, accessing to wrong std::vector element
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>release blocker
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sylvestre@debian.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Reported initially here <a href="http://bugs.debian.org/907649">http://bugs.debian.org/907649</a>
Clang 5.0 doesn't have the issue.
I can reproduce the issue from Clang 6 => 8
----
#include <iostream>
#include <vector>
class Foo {
private:
std::vector<double> _a;
std::vector<double> _d;
public:
Foo(const std::vector<double> &x, const std::vector<double> &y)
: _a(x.size()), _d(x.size()) {
for (unsigned int i = 0; i < x.size() - 1; i++) {
_a[i] = x[i + 1] - x[i];
_d[i] = (y[i + 1] - y[i]) / _a[i];
}
}
const std::vector<double> &a() const noexcept { return _a; }
};
int main() {
// Read input file
std::vector<double> x, y;
while (std::cin) {
double xi, yi;
if (std::cin >> xi >> yi) {
x.push_back(xi);
y.push_back(yi);
}
}
// Create Foo instance
Foo foo(x, y);
// Print computed data
for (auto a : foo.a()) std::cout << a << '\n';
return 0;
}
----
$> clang++-6.0 -std=c++11 -O2 -o bug bug.cpp
$> paste <(seq 1 5) <(seq 1 5) | ./bug
the expected result is:
1
1
1
1
0
But one gets:
1
2
2
0
0
Works with -O1</pre>
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