Date:   Sun, 12 Feb 2023 16:11:58 +0100 (CET)
From:   Julia Lawall <julia.lawall@inria.fr>
To:     Deepak R Varma <drv@mailo.com>
cc:     Bart Van Assche <bvanassche@acm.org>, Michael Reed <mdr@sgi.com>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Saurabh Singh Sengar <ssengar@microsoft.com>,
        Praveen Kumar <kumarpraveen@linux.microsoft.com>
Subject: Re: [PATCH] scsi: qla1280: Replace arithmetic addition by bitwise
 OR
In-Reply-To: <Y+kA1j10JCsLG9mN@ubun2204.myguest.virtualbox.org>
Message-ID: <alpine.DEB.2.22.394.2302121611010.9268@hadrien>
References: <Y+I7/QpQYjBXutLf@ubun2204.myguest.virtualbox.org> <aed35866-507c-870c-7e8a-c1868bcaa084@acm.org> <Y+kA1j10JCsLG9mN@ubun2204.myguest.virtualbox.org>
User-Agent: Alpine 2.22 (DEB 394 2020-01-19)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk


On Sun, 12 Feb 2023, Deepak R Varma wrote:

> On Sat, Feb 11, 2023 at 03:25:03PM -0800, Bart Van Assche wrote:
> > On 2/7/23 03:54, Deepak R Varma wrote:
> > > When adding two bit-field mask values, an OR operation offers higher
> > > performance over an arithmetic operation. So, convert such addition to
> > > an OR based expression.
> >
> > Where is the evidence that supports this claim? On the following page I read
> > that there is no performance difference when using a modern CPU: https://cs.stackexchange.com/questions/75811/why-is-addition-as-fast-as-bit-wise-operations-in-modern-processors
> >
>
> Hello Bart,
> You are correct. Modern CPU designs have improved addition and the performance
> is at par with the bitwise operation. The document I had read earlier mentioned
> a performance improvement for old CPUs and microprocessors, which today is not
> the case. Thank you for sharing the link.
>
> > > Issue identified using orplus.cocci semantic patch script.
> >
> > Where is that script located? Can it be deleted such that submission of
> > patches similar to this patch stops?
>
> I have added Julia to this email to understand how to best use this semantic
> patch. I already discussed with her on improving the Semantic patch such that it
> doesn't suggest making change when constants are involved.

FWIW, the semantic patch was never motivated by efficiency, but rather
with the goal of making the code more understandable.

julia