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

On Sun, 2024-10-27 at 14:47 +1100, Finn Thain wrote:
> On Sun, 27 Oct 2024, Thorsten Glaser wrote:
> 
> > > https://gcc.gnu.org/wiki/RustFrontEnd
> > 
> > That’s assuming it can build the same stuff the same way so it can be 
> > used in packaging.
> > 
> 
> Not an assumption. I simply pointed out an opportunity for collaboration, 
> because I see the primary problem facing the m68k port is a lack of 
> resources.

There is hardly anyone to collaborate with. Plus, the problem is that 99%
the upstream projects will not carry any patches that unbreak their software
on m68k.

> > > I expect alignment assumptions like that will end up putting more 
> > > platforms in the same predicament in future.
> > 
> > No, all the other platforms use natural alignment.
> > 
> > > "Natural" alignment is meaningless in the context of portable data
> > > structures, as they exist without reference to any particular integer
> > 
> > Yeah, but in practice, all we have is ILP32 and LP64; 
> 
> ... for now.

Sorry, but this a moot argument. Arguing with what could be in 50 years isn't
helpful here if it just intended to quench the discussion.

> > the Windows® world has LLP64 in addition. And natural alignment just 
> > means that all the data types’ alignment is their size. (Which kind of 
> > makes sense, so you can read them without getting an unalignment trap on 
> > SPARC or so.)
> > 
> 
> That would mean __alignof__(foo.b) == sizeof(foo.b) but that's not the 
> case on my Linux/i686 system. 4 != 8:
> 
> struct baa {
>         int a;
>         long long b;
> } foo;
> 
> > > Q. What is the size of this struct, assuming baa.b is naturally aligned?
> > > 
> > > struct baa {
> > >        int a;
> > >        long long b;
> > > };
> > 
> > For ILP32, LP64 and LLP64, it’s 4 (a) + 4 (padding) + 8 (b) = 16 chars.
> 
> So natural alignment is portable if you first assume a data model. But the 
> struct definition itself is not portable, since it doesn't specify a data 
> model.
> 
> > More importantly, and relevant for Qt, struct baa is also 8-byte
> > aligned...
> > 
> 
> If the struct itself is not naturally aligned, it too will eventually 
> break someone's assumption of natural alignment.

All this discussion doesn't help solve the real world problems with the m68k port.

Adrian

-- 
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