Re: [PATCH rdma-next 08/12] overflow.h: Add arithmetic shift helper

From: Leon Romanovsky <leon@kernel.org>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Jason Gunthorpe <jgg@mellanox.com>,
	Doug Ledford <dledford@redhat.com>,
	Kees Cook <keescook@chromium.org>,
	RDMA mailing list <linux-rdma@vger.kernel.org>,
	Hadar Hen Zion <hadarh@mellanox.com>,
	Matan Barak <matanb@mellanox.com>,
	Michael J Ruhl <michael.j.ruhl@intel.com>,
	Noa Osherovich <noaos@mellanox.com>,
	Raed Salem <raeds@mellanox.com>,
	Yishai Hadas <yishaih@mellanox.com>,
	Saeed Mahameed <saeedm@mellanox.com>,
	linux-netdev <netdev@vger.kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH rdma-next 08/12] overflow.h: Add arithmetic shift helper
Date: Wed, 27 Jun 2018 20:39:11 +0300	[thread overview]
Message-ID: <20180627173911.GS30877@mtr-leonro.mtl.com> (raw)
In-Reply-To: <CAKwiHFgchr+6HYOZ4e4e1vzL9cFabe6eonNNM8NTWZypazcuKA@mail.gmail.com>

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On Wed, Jun 27, 2018 at 11:36:03AM +0200, Rasmus Villemoes wrote:
> On 26 June 2018 at 19:54, Jason Gunthorpe <jgg@mellanox.com> wrote:
>
> > On Tue, Jun 26, 2018 at 10:07:07AM +0200, Rasmus Villemoes wrote:
> > >    On 25 June 2018 at 19:11, Jason Gunthorpe <[1]jgg@mellanox.com>
> > wrote:
> > >
> >
> > When thinking about signed cases.. The explicit u64 cast, and
> > implict promotion to typeof(d), produce something counter intuitive,
> > eg:
> >
> >   (u64)(s32)-1 == 0xffffffffffffffff
> >
> > Which would result in a shift oucome that is not what anyone would
> > expect, IMHO... So the first version isn't what I'd expect either..
> >
>
> Wouldn't check_shift_overflow(-1, 4, &someint) just put -16 in someint and
> report no overflow? That's what I'd expect, if negative values are to be
> supported at all.

Most if not all the times we don't do shifts on negative values, so I
don't think that we should support them.

Thanks

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