[llvm-bugs] [Bug 38786] New: Erroneous loop optimization with -O2, accessing to wrong std::vector element

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38786

            Bug ID: 38786
           Summary: Erroneous loop optimization with -O2, accessing to
                    wrong std::vector element
           Product: libraries
           Version: 6.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: release blocker
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: sylvestre at debian.org
                CC: llvm-bugs at lists.llvm.org

Reported initially here http://bugs.debian.org/907649

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

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