[Bug c/52991] attribute packed broken on mingw32?

[daniel.c.klauer at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52991

--- Comment #6 from daniel.c.klauer at web dot de 2012-10-06 23:02:22 UTC ---
Using the ms_struct attribute instead of compiling with -mms-bitfields
reproduces the packing issue, while using the gcc_struct attribute prevents the
issue from showing up even under -mms-bitfields.

These struct examples show the packing issue without nested structs:

struct __attribute__((ms_struct, packed)) A {
    int   a;
    short b;
};
struct __attribute__((ms_struct, packed)) B {
    short a;
    int   b;
};

sizeof(struct A) == 6         as expected
sizeof(struct B) == 8         unexpected, expected 6 instead
offsetof(struct B, b) == 4    unexpected, expected 2 instead

A message [1] from the mingw-w64-public mailing list indicates this behaviour:
Only the outer struct is packed, causing padding bytes at its end to be removed
as expected, but not the fields: padding bytes in front of a field according to
the field's natural alignment are not removed, despite the packed attribute,
which is unexpected.

Explicitly packing the fields does not help:

struct __attribute__((ms_struct)) C {
    int   a __attribute__((packed, aligned(1)));
    short b __attribute__((packed, aligned(1)));
};
struct __attribute__((ms_struct)) D {
    short a __attribute__((packed, aligned(1)));
    int   b __attribute__((packed, aligned(1)));
};

sizeof(struct C) == 6         as expected
sizeof(struct D) == 8         unexpected, expected 6 instead
offsetof(struct D, b) == 4    unexpected, expected 2 instead

[1] http://sourceforge.net/mailarchive/message.php?msg_id=29650589