Date: Mon, 7 Mar 2016 21:38:08 +0000
From: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Stas Sergeev <stsp@list.ru>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        X86 ML <x86@kernel.org>, Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: sigaltstack breaks swapcontext()
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> Anyway, maybe Linus or the x86 maintainers have some idea of how AC is
> used.  If there are people who use it for a whole program and if libc
> can survive the experience, then they might expect even signal
> handlers to run with AC set.  But if they're sane and protect just
> critical pieces of code being tested with AC, we could be polite and
> clear AC on signal handler entry.

No idea about AC and signals but the main use I have seen is type
checking, particularly in old 16bit code.

Basically code does add type to address, deference. If the type is wrong
for the address you get an alignment trap.

Alan