[Bug c++/80334] New: Segfault when taking address of copy of unaligned struct

[jagerman at jagerman dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80334

            Bug ID: 80334
           Summary: Segfault when taking address of copy of unaligned
                    struct
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jagerman at jagerman dot com
  Target Milestone: ---

I am getting a segfault with g++ 7 when trying to copy an unaligned struct into
an aligned variable when the struct contains a member with alignment greater
than 8 (on my amd64 architecture).

I boiled my code down into the following simplified program which exhibits the
segfault under a recent g++ 7 snapshot (requires compiling with -O2 or above to
trigger):

test.cpp
=====
#include <iostream>

struct A { alignas(16) char c; };
struct B { A unpacked; char d; } __attribute__((packed));

int main() {
    std::cout << "sizeof(A) = " << sizeof(A) << ", sizeof(B) = " << sizeof(B)
<< "\n";
    alignas(16) B b[3];

    for (int i = 0; i < 3; i++) b[i].unpacked.c = 'a' + i;

    for (int i = 0; i < 3; i++) {
        std::cout << "i=" << i << "; copying..." << std::endl;
        auto a = new A(b[i].unpacked);
        std::cout << "copied value = " << a->c << std::endl;
    }
}
=====



If I change the `alignas(16)` on the member in `struct A` to `alignas(8)` or
`alignas(4)` there is no segfault; there also is no segfault under -O0 or -O1,
or under g++ 6.

(The `alignas(16) char` was a `long double` in the original code, which has
alignof == 16).

The alignas(16) on the array in main is just there to force alignment on the
first element of `b`: with that alignment, the *first* copy succeeds because
the `unpacked` member happens to be correctly aligned; the call in the second
iteration of the loop (when the member isn't aligned) triggers the segfault.