Re: Plan needed for switching m68k to 32-bit alignment

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Thorsten,

On Fri, Oct 25, 2024 at 11:38 PM Thorsten Glaser <tg@debian.org> wrote:
> On Fri, 25 Oct 2024, John Paul Adrian Glaubitz wrote:
> >as m68k has supported 32-bit alignment through the "-malign-int"
> >switch for a long time.
>
> That switch constitutes a fundamental ABI change, even if
> the effect is limited.
>
> Question is, do we need a kernel change for this at all?
> Is there anything in the kernel/userland interface that
> gets affected by this, or do these APIs use sufficient
> explicit padding? Has anyone looked at this?

As pointed out by Arnd, there are.

> (Another question: does -malign-int switch to natural
> alignment for all types, i.e. 64 bit for llong and double,
> etc. or are we going to have to change GCC first as well
> to avoid more surprises?)

It only changes alignment for short and larger from 2 to 4 bytes.
So long long and double are still aligned to 4 bytes instead of 8.
If we decide to change alignment, I'd rather change all types to
natural alignment, to avoid future nasty surprises.

What to do with long double, which is 12 bytes on m68k and i386?
I386 kept its alignment at 4, while amd64 changed both size and
alignment to 16.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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