Re: Can we drop upstream Linux x32 support?

Rich Felker
2018-12-14T16:13:35Z

On Fri, Dec 14, 2018 at 12:42:34PM +0100, Florian Weimer wrote:
> * Rich Felker:
>
> > On Thu, Dec 13, 2018 at 05:04:59PM +0100, Florian Weimer wrote:
> >> * Rich Felker:
> >>
> >> >> If the compiler can handle the zeroing, that would be great, though not
> >> >> sure how (some __attribute__((zero)) which generates a type constructor
> >> >> for such structure; it kind of departs from what the C language offers).
> >> >
> >> > The compiler fundamentally can't. At the very least it would require
> >> > effective type tracking, which requires shadow memory and is even more
> >> > controversial than -fstrict-aliasing (because in a sense it's a
> >> > stronger version thereof).
> >>
> >> It's possible to do it with the right types. See _Bool on 32-bit Darwin
> >> PowerPC for an example, which is four bytes instead of the usual one.
> >>
> >> Similarly, we could have integer types with trap representations.
> >> Whether it is a good idea is a different matter, but the amount of
> >> compiler magic required is actually limited.
> >
> > If you do this you just have LP64 with value range restricted to
> > 32-bit.
>
> You have to a type different from long int for the relevant struct
> fields. This type would have zero padding.

Just upthread (Message-ID: <20181212165237.GT23599@brightrain.aerifal.cx>)
I explained why this does not work:

>>> If on the other hand you tried to make just some pointers "wide
>>> pointers", you'd also be completely breaking the specified API
>>> contracts of standard interfaces. For example in struct iovec's
>>> iov_base, &foo->iov_base is no longer a valid pointer to an object of
>>> type void* that you can pass to interfaces expecting void**. Sloppy
>>> misunderstandings like what you're making now are exactly why x32 is
>>> already broken and buggy (&foo->tv_nsec already has wrong type for
>>> struct timespec foo).

Rich