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

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Date:   Sun, 12 Feb 2023 20:38:06 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Bart Van Assche <bvanassche@....org>,
        Julia Lawall <julia.lawall@...ia.fr>
Cc:     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 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.

Thank you,
./drv

> 
> Thanks,
> 
> Bart.