From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:49655)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <aik@ozlabs.ru>) id 1VCSTf-0003Cs-1v
	for qemu-devel@nongnu.org; Thu, 22 Aug 2013 06:51:02 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <aik@ozlabs.ru>) id 1VCSTR-00059c-WD
	for qemu-devel@nongnu.org; Thu, 22 Aug 2013 06:50:54 -0400
Received: from mail-pb0-f45.google.com ([209.85.160.45]:42168)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <aik@ozlabs.ru>) id 1VCSTR-00059V-O7
	for qemu-devel@nongnu.org; Thu, 22 Aug 2013 06:50:41 -0400
Received: by mail-pb0-f45.google.com with SMTP id mc17so1605434pbc.4
	for <qemu-devel@nongnu.org>; Thu, 22 Aug 2013 03:50:40 -0700 (PDT)
Message-ID: <5215ECFB.6080908@ozlabs.ru>
Date: Thu, 22 Aug 2013 20:50:35 +1000
From: Alexey Kardashevskiy <aik@ozlabs.ru>
MIME-Version: 1.0
References: <1377159632-7446-1-git-send-email-aik@ozlabs.ru>
	<1377159632-7446-2-git-send-email-aik@ozlabs.ru>
	<5215D561.3050702@redhat.com>
	<CAFEAcA_WDhmGLpzo=+sodS+ZwTW-Kb-U9jw4SP3Sjz0P3Ds+1w@mail.gmail.com>
	<5215DE80.4030203@redhat.com>
In-Reply-To: <5215DE80.4030203@redhat.com>
Content-Type: text/plain; charset=KOI8-R
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH v2 1/3] int128: add int128_exts64()
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Alex Williamson <alex.williamson@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>

On 08/22/2013 07:48 PM, Paolo Bonzini wrote:
> Il 22/08/2013 11:47, Peter Maydell ha scritto:
>> On 22 August 2013 10:09, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>> Il 22/08/2013 10:20, Alexey Kardashevskiy ha scritto:
>>>> +static inline Int128 int128_exts64(int64_t a)
>>>> +{
>>>> +    return (Int128) { .lo = a, .hi = (a >> 63) ? -1 : 0 };
>>>> +}
>>>
>>> The "? -1 : 0" is not necessary, but the compiler will remove it at -O1
>>> or more (interestingly, or -O0 it will remove the shift and leave the
>>> conditional!).
>>
>> We can avoid relying on implementation defined
>> behaviour here by using
>>   .hi = (a < 0) ? -1 : 0;
>>
>> (I know we allow ourselves to assume right-shift of signed
>> ints is arithmetic shift, but I think it's nicer to avoid it unless
>> it really makes the code better.)
> 
> This is what Alexey proposed.  I suggested (a >> 63) without the ?: but
> he misunderstood my (probably not clear enough) suggestion.

Yes, I misunderstood. It was not obvious to me that (signed long
long)-1>>63 will be still -1. I really (really) envy people who can easily
read stuff like but I cannot :(

1) return (Int128) { .lo = a, .hi = (a < 0) ? -1 : 0 };
2) return (Int128) { .lo = a, .hi = (a < 0) };
3) return (Int128) { .lo = a, .hi = a >> 63 };

So with which one should I repost the patch?


-- 
Alexey