[Bug c++/70603] New: gcc alignas issue with pointers to template class

[mhadji at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70603

            Bug ID: 70603
           Summary: gcc alignas issue with pointers to template class
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mhadji at gmail dot com
  Target Milestone: ---

I am using gcc 4.9.2 and I am trying to properly align statically initialized
arrays for use with AVX. Here is the gist of the code that segfaults due to
alignment issues:

#include <iostream>
#include <cstddef>

struct B {
    alignas(32) double x[1] = {0};
};

struct A
{
    A() { b1 = new B(); b2 = new B(); }

    B* b1;
    B* b2;
};

int main(int argc, char** argv) {
    A a;

    int ret = (ptrdiff_t) a.b1->x % 32 + (ptrdiff_t) a.b2->x % 32;

    std::cout << (ptrdiff_t) a.b1->x % 32 << "," << (ptrdiff_t) a.b2->x % 32 <<
"\n";

    return ret;
}
On my system array a.b2->x is not aligned on a 32 byte boundary. The size of x
does not matter, as long as it is an array (so "double x = 0" works fine). If I
make the pointers to B statically allocated members instead, it works properly.
If I create local variables *b1 and *b2 inside main, it works properly. If I
use dynamically allocated arrays inside class A and posix_malloc, it works
properly.