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Date: Mon, 07 Apr 2014 18:22:38 +0300
From: Avi Kivity <avi@cloudius-systems.com>
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Subject: Re: [Qemu-devel] [Qemu-trivial] [PATCH 2/3] int128.h: Avoid
 undefined behaviours involving signed arithmetic
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: QEMU Trivial <qemu-trivial@nongnu.org>, Patch Tracking <patches@linaro.org>, Michael Tokarev <mjt@tls.msk.ru>, QEMU Developers <qemu-devel@nongnu.org>, Richard Henderson <rth@twiddle.net>

On 04/07/2014 06:17 PM, Peter Maydell wrote:
> On 7 April 2014 15:56, Avi Kivity <avi@cloudius-systems.com> wrote:
>> On 04/06/2014 01:18 PM, Peter Maydell wrote:
>>> The alternative would be to say that Int128 should have
>>> undefined behaviour on underflow/overflow and the test
>>> code is wrong, but that doesn't seem very useful to me.
>> Isn't the test broken here?  It is trying to add (or shift) -2^127 and
>> something else, and the result truly overflows.
> Well, the test code is assuming "semantics as per 2s
> complement arithmetic" and checking various corner cases.
> As I say, we could define that this is invalid and
> rewrite the test cases.

It is invalid.  The test thinks that -2^127 * 2 == 0, but if a guest 
could trigger it, it would probably be a security issue.