Bug#907649: clang-6.0: Erroneous loop optimisation with -O2, accessing to wrong std::vector element

[Juan Carlos Garcia Hernandez]

Package: clang-6.0
Version: 1:6.0.1-5
Severity: normal

Dear Maintainer,

When compiling with -O2 the code below, resulting loop is wrongly optimized leading
to plainly wrong results (not an approximation), it seems that elements in 
std::vector are not accessed in the right order.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
#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;
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

When executing the following commands (assuming that file bug.cpp contains code above)

$> 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

Nevertheless, the expected result is obtained with -O1.

Best regards,

Juan


-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'stable-updates'), (500, 'unstable'), (500, 'stable'), (500, 'oldstable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.17.0-3-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages clang-6.0 depends on:
ii  binutils                 2.31.1-4
ii  libc6                    2.27-5
ii  libc6-dev                2.27-5
ii  libclang-common-6.0-dev  1:6.0.1-5
ii  libclang1-6.0            1:6.0.1-5
ii  libgcc-8-dev             8.2.0-4
ii  libgcc1                  1:8.2.0-4
ii  libjsoncpp1              1.7.4-3
ii  libllvm6.0               1:6.0.1-5
ii  libobjc-8-dev            8.2.0-4
ii  libstdc++-8-dev          8.2.0-4
ii  libstdc++6               8.2.0-4

Versions of packages clang-6.0 recommends:
ii  libomp-dev    6.0.1-1
ii  llvm-6.0-dev  1:6.0.1-5
ii  python        2.7.15-3

Versions of packages clang-6.0 suggests:
pn  clang-6.0-doc  <none>
pn  gnustep        <none>
pn  gnustep-devel  <none>

-- no debconf information