Re: Can we drop upstream Linux x32 support?

On Fri, Dec 14, 2018 at 12:42:34PM +0100, Florian Weimer wrote:

* Rich Felker:

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On Thu, Dec 13, 2018 at 05:04:59PM +0100, Florian Weimer wrote:

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* Rich Felker:

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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: [ref])
I explained why this does not work:

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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