Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO

[Arnd Bergmann]

On Mon, Mar 30, 2026, at 14:03, Geert Uytterhoeven wrote:
> On Fri, 27 Mar 2026 at 10:02, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>
>> echo 'struct { short a : 3; short b :15; short c :14; } x; int y = sizeof(x);' | m68k-linux-gcc -xc - -S -o-
>>
>> this produces '4' on m68k-linux-gcc, but '6' everywhere else. I originally
>> thought this was related to this 2009 change in both compilers
>
> Oh, now I remember.  AFAIK (holding wood and a rabbit leg) we don't
> have any bitfield members spanning multiple base type instances in
> the kernel.

There are certainly very few of those, but two example I found in
UAPI are

struct dvd_layer {
        __u8 book_version       : 4;
        __u8 book_type          : 4;

        __u8 min_rate           : 4;
        __u8 disc_size          : 4;

        __u8 layer_type         : 4;
        __u8 track_path         : 1;
        __u8 nlayers            : 2;

        __u8 track_density      : 4; // crosses u8 boundary
        __u8 linear_density     : 4;
        __u8 bca                : 1;
        __u32 start_sector;
        __u32 end_sector;
        __u32 end_sector_l0;
};

struct usb_raw_ep_caps {
        __u32   type_control    : 1;
        __u32   type_iso        : 1;
        __u32   type_bulk       : 1;
        __u32   type_int        : 1;
        __u32   dir_in          : 1;
        __u32   dir_out         : 1;
       // 2 bit padding on m68k, 26 bits elsewhere
};

    Arnd