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Date:   Thu, 13 Dec 2018 11:28:28 -0500
From:   Rich Felker <dalias@libc.org>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Andy Lutomirski <luto@kernel.org>, tg@mirbsd.de,
        Linus Torvalds <torvalds@linux-foundation.org>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Mike Frysinger <vapier@gentoo.org>,
        "H. J. Lu" <hjl.tools@gmail.com>, x32@buildd.debian.org,
        Arnd Bergmann <arnd@arndb.de>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: Can we drop upstream Linux x32 support?
Message-ID: <20181213162828.GW23599@brightrain.aerifal.cx>
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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. This does not accomplish anything useful since it uses just as
much memory as normal LP64 but you don't get anything beneficial out
of it.

Rich