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

On Mon, 2024-10-28 at 04:07 +0100, Thorsten Glaser wrote:
> On Sun, 27 Oct 2024, Arnd Bergmann wrote:
> > On Sun, Oct 27, 2024, at 06:16, Finn Thain wrote:
> 
> > > Right. And I don't think the problem is going to go away. One solution
> > > that is sometimes raised is better tooling. I'm not sure how that would
> 
> No. You don’t understand the problem: applications are being
> written that require natural alignment for at least 32-bit and
> smaller quantities, some possibly for all quantities even. We
> need these applications to work, and we cannot redesign them
> (it’s common for them to store flags in the lowest pointer bits
> and do other awful things).

Yep, that's exactly the problem. Plus, the maintainers of 99% of the upstream
projects don't care about the alignment peculiarities of the Linux/m68k port.

> Could you please, if you cannot believe it yourself, accept the
> word of those actually doing the porting that it is direly needed
> in the current Linux ecosystem. Thank you.

Agreed 100%.

> > > > That struct is just 12 bytes for you then?
> > > 
> > > Right. i686 and m68k agree on that.
> > 
> > The other ones that have misaligned 'long long' variables are
> > 
> > arc
> > arm (OABI only)
> > csky
> > microblaze
> > nios2
> > openrisc
> > superh
> > i386
> 
> Okaaay. So, all “old” platforms or with low distro support.
> For Debian, we have superh and i386, out of these. It is
> entirely possible that Qt et al. can work with this, but
> these all have natural alignment for quantities ≤ 32 bits.

I'm not aware of any serious issues with alignment on i386.

i386 was the original target for 90% of the software in Debian, so if
there are issues, developers have already addressed them.

> Does Gentoo support more of them? Debian used to have arm
> OABI, but that’s ages ago. I know things like OpenADK support
> them, but that is very chiefly embedded.

Currently, Gentoo supports i386 out of that list.

> > The 32-bit architectures that use a natural 8-byte
> > alignment are
> > 
> > arm (EABI)
> > parisc
> > mips
> > powerpc
> > riscv
> > s390
> > sparc
> > xtensa
> 
> That’s majoritily represented in Debian. And ideally, we’d
> use that for all new architectures.
> 
> I have a feeling that GCC’s -malign-int will only push us
> into the former. I have no idea whether it’s feasible to
> also change GCC for this; that will probably depend on both
> the other “what parts of the toolchain need changing” and
> whether GCC can be switched with little effort.

Did you see how Chewi implemented 32-bit alignment? He actually didn't
use -malign-int but directly patched GCC itself, see:

> https://marc.info/?l=glibc-help&m=169303990426196&w=2

Adrian

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