[Bug libstdc++/69553] New: Optimizations O1/O2 makes std::array value incorrect when passed to function

[gee at ptilouk dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69553

            Bug ID: 69553
           Summary: Optimizations O1/O2 makes std::array value incorrect
                    when passed to function
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gee at ptilouk dot net
  Target Milestone: ---

Created attachment 37521
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37521&action=edit
Source code generating the bug

The following example displays incorrect values when compiling with
optimizations -O1 or -O2 (but not without optimization and not with -O3):

----------------------
#include <iostream>
#include <array>

typedef std::array<std::array<double, 2>, 2> Matrix;

void foo(const double &px, const double &py)
{
  std::cerr << px << " " << py << std::endl;
}

std::ostream& operator<<(std::ostream& os, const std::array<double, 2>& p)
{
  return os << p[0] << " " << p[1];
}

void test (const Matrix& t)
{
  std::cerr << "In test: " << std::endl
            << t[0] << std::endl
            << t[1] << std::endl;

  std::cerr << "In foo: " << std::endl;
  foo(t[0][0], t[0][1]);
  foo(t[1][0], t[1][1]);
}

int main (int, char**)
{
  Matrix t = {{ {{1,2}}, {{3,4}} }};
  test (t);
  return 0;
}
----------------------

(Compiled with 'g++-6 -std=c++11 -O1 test_bug_array.cpp')

Without optimization (or with O3), i have the following output:
In test: 
1 2
3 4
In foo: 
1 2
3 4

Which makes sense. Now if I turn on O1 or O2, I get:
In test: 
1 2
3 4
In foo: 
1 3
3 4

The bug does not appear when using boost::array or when replacing the structure
by a simple C-array (double[2][2]) or by pairs (std::pair<std::pair<double> >).