by zombie.fml:

Summary: __attribute__((__packed__)) is broken on Windows with gcc 4.7
using the default compiler options.

I noticed this problem when compiling go on Windows. The build failed when testing
misc/cgo/test at `testParallelSleep`. It reported a very long duration between
two parallel 1s sleeps.

This is my system information:

gcc.exe (GCC) 4.7.2/mingw ** NOTE gcc version is important
go version go1.1 windows/386
8g
Windows 7 32-bit

With gcc 4.7, Windows mingw targets are using the -mms-bitfields option by default.
See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52991 and
http://gcc.gnu.org/gcc-4.7/changes.html

This flag causes __attribute__((__packed__)) not working as intended on structs.
cgo generates code that relies on __attribute__((__packed__)) which is then broken.

I wrote a simple test code at http://play.golang.org/p/VnXvU4yQPw
The expected output is four 0x12345678. But the output is:

$ go run 64.go
x0 = 0x12345678, x1 = 0x1234567800000000, x2 = 0x12345678, x3 = 0x1234567800000000, x4 =
0x12345678

I modified cgo generated intermediate c code during compiling to output the
struct size. Like this one:

void
_cgo_f861c44e4fb7_Cfunc_foo1(void *v)
{
    struct foo {
        void* p0;
        long long int r;
    } __attribute__((__packed__)) *a = v;
    int i = 0;
    unsigned char *p = (unsigned char*)a;
    a->r = foo1((void*)a->p0);
    for (i = 0; i < sizeof(*a); i++, p++) {
        printf("0x%x ", *p);
    }
    printf("--foo1, sizeof = %d, offset(r) = %d\n", sizeof(*a), offsetof(struct foo, r));
}

And I got this output:

$ go run 64.go
x0 = 0x12345678, x1 = 0x1234567800000000, x2 = 0x12345678, x3 = 0x1234567800000000, x4 =
0x12345678
0x78 0x56 0x34 0x12 0x0 0x0 0x0 0x0 --foo0, sizeof = 8, offset(r) = 0
0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x78 0x56 0x34 0x12 0x0 0x0 0x0 0x0 --foo1, sizeof = 16,
offset(r) = 8
0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x78 0x56 0x34 0x12 0x0 0x0 0x0 0x0 --foo2, sizeof = 16,
offset(r) = 8
0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x78 0x56 0x34 0x12 0x0
0x0 0x0 0x0 --foo3, sizeof = 24, offset(r) = 16
0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x78 0x56 0x34 0x12 0x0
0x0 0x0 0x0 --foo4, sizeof = 24, offset(r) = 16

There's a 4-byte padding in foo1 and foo3 which is not expected. The ·_Cfunc_foo1
and ·_Cfunc_foo3 functions in _cgo_defun.c passes a 12 and 20 bytes buffer to 
_cgo_f861c44e4fb7_Cfunc_foo1 and _cgo_f861c44e4fb7_Cfunc_foo3 respectively. But
the actual struct sizes are 16 and 24 respectively.