Re: [PATCH] Introduce compat_u64 and compat_s64 types

[Matthew Wilcox <matthew@wil.cx>]

On Fri, Jun 15, 2007 at 02:42:23PM +0200, Arnd Bergmann wrote:
> That's what I thought as well at first, since this is how the gcc 
> documentation seems to describe it. However, recent version of gcc
> complain about this:
> 
> gcc-4.1 -Wall -O2 test.c  -c
> test.c:1: warning: 'packed' attribute ignored
> 
> I have tested versions 2.95, 3.3 and 4.1, an they all ignore do the
> right thing when you do not specify the packed attribute.

...

> We might ask the gcc developers to clarify the documentation, which as of 4.1
> states:
> 
>      The `aligned' attribute can only increase the alignment; but you
>      can decrease it by specifying `packed' as well.  See below.
> 
> My understanding is that this only applies to statically allocated variables,
> but not to automatic stack variables and to usage of the type inside of
> a data structure.

Here's a program which illustrates the source of confusion:

#include <stdio.h>
#include <stddef.h>

typedef unsigned long long __attribute__((aligned(4))) compat_u64;

struct foo {
        int y;
        unsigned long long __attribute__((aligned(4))) x;
};

struct bar {
        int y;
        compat_u64 x;
};

int main(void)
{
        printf("offset of foo->x is %lu\n", offsetof(struct foo, x));
        printf("offset of bar->x is %lu\n", offsetof(struct bar, x));
        return 0;
}

output (on ia64, and I'm told other 64-bit platforms) is:

$ ./test 
offset of foo->x is 8
offset of bar->x is 4

I'll try and come up with some wording that works for the GCC manual.
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