[Bug c++/41788] New: -Wpacked option changes the layout of packed non-POD structs

[nenad at tensilica dot com]

When a packed non-POD struct is included in another packed struct,
the outer struct layout changes depending on whether -Wpacked is used or not.
It seems wrong that a warning option affects data layout and code generation.

The same behavior is observed with 4.4.2, 4.3.4, 4.2.1, 4.1.2.
It could be related to bug 26670.

% cat packed_nonpod.cpp

extern "C" int printf (__const char *__restrict __format, ...);

struct INNER {
  virtual int foo() const { return 1; }
} __attribute__ ((packed));

struct OUTER {
  char c;
  INNER inner;
} __attribute__ ((packed));

int main()
{
  OUTER outer;
  printf("sizeof(outer) = %u\n", sizeof(outer));
  printf("offset(inner) = %u\n", (char *)&outer.inner - (char *)&outer);
  return 0;
}

% g++ packed_nonpod.cpp
% a.out
sizeof(outer) = 5
offset(inner) = 1

% g++ packed_nonpod.cpp -Wpacked
packed_nonpod.cpp:3: warning: packed attribute is unnecessary for 'INNER'
packed_nonpod.cpp:9: warning: ignoring packed attribute because of unpacked
non-POD field 'INNER OUTER::inner'
% a.out
sizeof(outer) = 8
offset(inner) = 4


-- 
           Summary: -Wpacked option changes the layout of packed non-POD
                    structs
           Product: gcc
           Version: 4.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: nenad at tensilica dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41788