From: Thomas Gleixner <tglx@linutronix.de>
To: zhong jiang <zhongjiang@huawei.com>
Cc: Ingo Molnar <mingo@kernel.org>,
akpm@linux-foundation.org, mingo@redhat.com, minchan@kernel.org,
mhocko@suse.com, hpa@zytor.com, x86@kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative
Date: Thu, 29 Jun 2017 08:33:20 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.20.1706290832140.1861@nanos> (raw)
In-Reply-To: <59545DD6.3030508@huawei.com>
On Thu, 29 Jun 2017, zhong jiang wrote:
> On 2017/6/29 6:13, Thomas Gleixner wrote:
> > That's simply wrong. If oparg is negative and the SHIFT bit is set then the
> > result is undefined today and there is no way that this can be used at
> > all.
> >
> > On x86:
> >
> > 1 << -1 = 0x80000000
> > 1 << -2048 = 0x00000001
> > 1 << -2047 = 0x00000002
> but I test the cases in x86_64 all is zero. I wonder whether it is related to gcc or not
>
> zj.c:15:8: warning: left shift count is negative [-Wshift-count-negative]
> j = 1 << -2048;
> ^
> [root@localhost zhongjiang]# ./zj
> j = 0
Which is not a surprise because the compiler can detect it as the shift is
a constant. oparg is not so constant ...
Thanks,
tglx
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next prev parent reply other threads:[~2017-06-29 6:34 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-21 11:43 zhong jiang
2017-06-21 16:40 ` Ingo Molnar
2017-06-28 4:35 ` zhong jiang
2017-06-28 21:43 ` hpa
2017-06-29 2:12 ` zhong jiang
2017-06-29 4:29 ` hpa
2017-06-29 5:57 ` zhong jiang
2017-06-28 22:13 ` Thomas Gleixner
2017-06-29 1:54 ` zhong jiang
2017-06-29 6:33 ` Thomas Gleixner [this message]
2017-06-29 7:04 ` zhong jiang
2017-08-25 5:21 ` zhong jiang
2017-08-25 21:13 ` Thomas Gleixner
2017-08-26 2:51 ` zhong jiang
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