linux-kernel - Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}

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Message-ID: <ZLabOHmNQm2EOXWR@smile.fi.intel.com>
Date:   Tue, 18 Jul 2023 17:01:28 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Yury Norov <yury.norov@...il.com>, catalin.marinas@....com,
        will@...nel.org, pcc@...gle.com, andreyknvl@...il.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}_value()

On Tue, Jul 18, 2023 at 11:30:00AM +0200, Alexander Potapenko wrote:
> On Mon, Jul 17, 2023 at 5:51 PM Yury Norov <yury.norov@...il.com> wrote:
> > On Mon, Jul 17, 2023 at 01:37:04PM +0200, Alexander Potapenko wrote:

...

> > When nbits == 0, copy-like functions shouldn't touch any memory. See how
> > other bitmap and find_bit functions hold it.
> 
> I think this is different from what other bitmap functions do, but it
> should be enough to bail out on !nbits, i.e.:
> 
>     if (!nbits)
>         return 0;
> 
> You probably meant adding a __builtin_constant_p() (which is used all
> over the place in bitmap.h), but:
>  - the compiler won't have problem optimizing away the code for a
> constant nbits=0;
>  - we anyway need a dynamic check for the case nbits is not constant
> (for both bitmap_get_value() and bitmap_set_value(), I assume).
> 
> What do you think?

The idea behind is to eliminate the code completely for the cases nbits != 0.
In your case the dynamic check will be there. That's what we want to avoid.

-- 
With Best Regards,
Andy Shevchenko