linux-kernel - Re: [PATCH 1/5] bitops: Introduce find_next_andnot

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Message-ID: <xhsmha683b2qs.mognet@vschneid.remote.csb>
Date:   Wed, 17 Aug 2022 11:09:31 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Tony Luck <tony.luck@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Gal Pressman <gal@...dia.com>, Tariq Toukan <tariqt@...dia.com>
Subject: Re: [PATCH 1/5] bitops: Introduce find_next_andnot_bit()

On 16/08/22 15:13, Yury Norov wrote:
> On Tue, Aug 16, 2022 at 07:07:23PM +0100, Valentin Schneider wrote:
>> @@ -59,7 +63,9 @@ unsigned long _find_next_bit(const unsigned long *addr1,
>>
>>              tmp = addr1[start / BITS_PER_LONG];
>>              if (addr2)
>> -			tmp &= addr2[start / BITS_PER_LONG];
>> +			tmp &= negate ?
>> +			       ~addr2[start / BITS_PER_LONG] :
>> +				addr2[start / BITS_PER_LONG];
>>              tmp ^= invert;
>>      }
>
> So it flips addr2 bits twice - first with new 'negate', and second
> with the existing 'invert'. There is no such combination in the
> existing code, but the pattern looks ugly, particularly because we use
> different inverting approaches. Because of that, and because XOR trick
> generates better code, I'd suggest something like this:
>
>         tmp = addr1[start / BITS_PER_LONG] ^ invert1;
>         if (addr2)
>                 tmp &= addr2[start / BITS_PER_LONG] ^ invert2;

That does look much better, and also gets rid of the ternary. Thanks!