Re: Question on BIGGEST_ALIGNMENT in GCC on NetBSD/m68k
Hi Greg,
On Wed, 2025-06-18 at 22:21 +1000, Greg Ungerer wrote:
> > Could you please elaborate this a bit more, please?
> >
> > Coldfire is handled as a separate target via TARGET_COLDFIRE in GCC, so we
> > would certainly be able to toggle the alignment settings independent of
> > what's done on classic m68k.
>
> The net out is that it is the same gcc compiler, m68k-linux-gcc.
> ColdFire just needs specific code generation via command line switches,
> like -m5200 (or -m5206e or -m5307 or -mcfv4e, etc). This is the same way
> you would specify 680x0 level - m68020, -m68030, etc.
Yes, but there is a TARGET_COLDFIRE macro as I mentioned above which could
be used to trigger which alignment to use by default. I don't see how that
would complicate things.
> The bulk of the instruction set is the same. Asm code will look totally
> familiar to anyone who knows m68k :-) One notable difference is that
> there is a more limited set addressing modes for some instructions.
True, but you won't be able to run any classic m68k binaries on ColdFire
and the other way around, are you?
> FWIW ColdFire currently uses the same ABI as all other m68k, so it uses
> 2-byte alignment today.
I know. And one user on the LKML has already demonstrated that his Coldfire
board booted fine with buildroot set to 4 bytes alignment.
> > In the Linux kernel, Coldfire is also a separate
> > arch, so the alignment settings can also be handled there separately if necessary.
>
> ColdFire is not handled as a separate architecture in linux, it is just a
> variant of m68k - so uses arch/m68k in the source.
Well, it's a separate sub-architecture, similar to what's done for sh3/sh4 on sh
or the various ARM flavors. My point was just that there is a way to separate
classic m68k and Coldfire code if necessary.
> > It's not really necessary to enforce this on Coldfire. However, since buildroot
> > builds completely from source, it wouldn't even be a problem to change the alignment
> > there as well.
>
> Yes, that is totally right in my experience. Certainly in my ColdFire work
> it is pretty much always a build-everything approach via buildroot or similar.
> I wouldn't think an ABI change would actually worry too many ColdFire uses,
> they don't use distributions like debian on them. (I would love to hear from
> anyone who does!).
Thanks a lot for confirming this!
Adrian
--
.''`. John Paul Adrian Glaubitz
: :' : Debian Developer
`. `' Physicist
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