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From:   Florian Weimer <fweimer@redhat.com>
To:     Rich Felker <dalias@libc.org>
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?
References: <CALCETrWgpAX7FV23zHmid83SsgnwFMKD4a_-xSEgB6v0kJR5sA@mail.gmail.com>
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        <CALCETrUYn=S=hmJ0tKdm2LoSgkWchY2_65sH7hJZp7wfS30giw@mail.gmail.com>
        <20181212165237.GT23599@brightrain.aerifal.cx>
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        <20181213155744.GU23599@brightrain.aerifal.cx>
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        <20181213162828.GW23599@brightrain.aerifal.cx>
Date:   Fri, 14 Dec 2018 12:42:34 +0100
In-Reply-To: <20181213162828.GW23599@brightrain.aerifal.cx> (Rich Felker's
        message of "Thu, 13 Dec 2018 11:28:28 -0500")
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* 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.

Florian