Re: Question on BIGGEST_ALIGNMENT in GCC on NetBSD/m68k

On Mon, 16 Jun 2025 at 10:33, Laurent Vivier <[email protected]> wrote:
> Le 16/06/2025 à 10:14, John Paul Adrian Glaubitz a écrit :
> > On Mon, 2025-06-16 at 10:00 +0200, Laurent Vivier wrote:
> >>> Could you point me to these packages which assume 2 bytes alignment? I'm genuinely
> >>> interested as I would like to put these up on the wiki. So far I haven't found any.
> >>> I have used Debian's rebootstrap to create an initial set of m68k packages with
> >>> 4 bytes alignment and didn't run into any problems yet.
> >>>
> >>> The Gentoo developers have also successfully created a chroot with 4 bytes alignment:
> >>>
> >>> https://dev.gentoo.org/~dilfridge/m68k/
> >>>
> >>> I have tried it and it worked fine for me with qemu-user.
> >>>
> >>> So, if there is any software that is incompatible with 4 bytes alignment on m68k, I
> >>> would like to hear about it.
> >>
> >> qemu-user is one of these softwares that know m68k has alignment of
> >> 2byte. All the structures translated from the m68k chroot to the host
> >> kernel are with a 2byte alignment.

The Linux kernel also knows m68k has an alignment of 2 bytes.

> > And yet it works without any problems so far. Please try the Gentoo chroot yourself.
>
> Because most of the structures are correctly aligned by default, but
> some of them not. You must run glibc test suite and LTP to be sure there
> is no regression.

Indeed.

> In QEMU, alignment is defined here:
>
> include/user/abitypes.h
>
> for m68k the values to change are:
>
> #ifdef TARGET_M68K
> #define ABI_INT_ALIGNMENT 2
> #define ABI_LONG_ALIGNMENT 2
> #define ABI_LLONG_ALIGNMENT 2
> #endif

Note that it also used to have:

    #ifdef TARGET_CRIS
    #define ABI_SHORT_ALIGNMENT 1
    #define ABI_INT_ALIGNMENT 1
    #define ABI_LONG_ALIGNMENT 1
    #define ABI_LLONG_ALIGNMENT 1
    #endif

and still has:

    #if (defined(TARGET_I386) && !defined(TARGET_X86_64)) \
        || defined(TARGET_SH4) \
        || defined(TARGET_OPENRISC) \
        || defined(TARGET_MICROBLAZE) \
        || defined(TARGET_NIOS2)
    #define ABI_LLONG_ALIGNMENT 4
    #endif

which will probably become the next victim of the 64-bit little-endian
natural-alignment monoculture...

> And there will be a problem with binfmt_misc because we can't rely on
> the ELF signature to know which qemu-user to run, the one with 2byte
> alignment or the one with 4byte alignment?

Exactly.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds