[Bug c++/77388] New: Reference to a packed structure member

["andre.simoesdiasvieira at arm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77388

            Bug ID: 77388
           Summary: Reference to a packed structure member
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andre.simoesdiasvieira at arm dot com
  Target Milestone: ---

As initially reported by Michal on
https://answers.launchpad.net/gcc-arm-embedded/+question/345145 gcc seems to be
showing some weird behavior when it comes to passing a reference to a member in
a packed structure.

I was able to reduce the testcase presented in that launchpad ticket to the
following program:

$cat t.cpp:
#define PACKED __attribute__ ((packed))

#define TYPE_C short

typedef struct {
    TYPE_C c;
} PACKED test_struct;

class A
{
  const TYPE_C &c;
public:
  A (const TYPE_C & _c) :
    c(_c) {};
};

class B
{
public:
  B();
  A foo ();
private:
  test_struct * s;
};

A B::foo ()
{
  return A (s->c);
}


Compiling this with
$arm-none-eabi-g++ -mcpu=cortex-m7 -mthumb -S -O1 t.cpp -fdump-tree-optimized

Will yield the following dump:
;; Function A B::foo() (_ZN1B3fooEv, funcdef_no=3, decl_uid=4607, cgraph_uid=3,
symbol_order=3)

A B::foo() (struct B * const this)
{
  const short int D.4636;
  struct A D.4650;

  <bb 2>:
  MEM[(struct A *)&D.4650] = &D.4636;
  D.4636 ={v} {CLOBBER};
  return D.4650;

}

As you can see, it will not load the struct's field.

Changing the 'TYPE_C' define to 'char' will yield the following  dump:
;; Function A B::foo() (_ZN1B3fooEv, funcdef_no=3, decl_uid=4607, cgraph_uid=3,
symbol_order=3)

A B::foo() (struct B * const this)
{
  struct A D.4649;
  struct test_struct * _1;
  char * _2;

  <bb 2>:
  _1 = this_4(D)->s;
  _2 = &_1->c;
  MEM[(struct A *)&D.4649] = _2;
  return D.4649;

}

Now when the type is 'char' it seems to be able to get the fields address.

Can anyone shine some light on this for me? Is referencing a packed structure's
member that is not guaranteed to be aligned (so not char) undefined behavior?