netdev - RE: [PATCH v2 net] net: solve a NAPI race

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Date:   Thu, 2 Mar 2017 10:24:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>
CC:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "Tariq Toukan" <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: RE: [PATCH v2 net] net: solve a NAPI race

From: Eric Dumazet
> Sent: 01 March 2017 17:33
> On Wed, 2017-03-01 at 08:14 -0800, Alexander Duyck wrote:
> 
> > What build flags are you using?  With -Os or -O2 I have seen it
> > convert the /b * c into a single shift.
> >
> 
> 
> Because b & c are unsigned in our case.
> 
> I presume David tried signed integers, this is why gcc does that.

I was using integer constants but an unsigned variable.
Changing the constants to unsigned makes no difference (they would get
promoted anyway).

After some experiments I can get gcc to generate a single shift if the
variable being tested is 32bits, the constants are 64bits and a 64bit
result is required.
In every other case it does either and-shr-shl or shr-and-shl (all the
shr are logical).

	David