linux-kernel - Re: [PATCH] scsi: qla1280: Replace arithmetic addition by bitwise OR

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Date:   Sun, 12 Feb 2023 20:55:06 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     Bart Van Assche <bvanassche@....org>, Michael Reed <mdr@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] scsi: qla1280: Replace arithmetic addition by bitwise OR

On Sun, Feb 12, 2023 at 04:11:58PM +0100, Julia Lawall wrote:
> 
> 
> 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.

I think my interpretation of the patch log for [1] was not accurate. The line
"Running time is divided by 3 ..." made me believe OR'ing would replace "F+A+R"
instructions by a single operation. My bad.

[1] https://lore.kernel.org/all/alpine.DEB.2.20.1711130649370.2483@hadrien/

> 
> julia