Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}_value()

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Alexander Potapenko <glider@google.com>
Cc: Yury Norov <yury.norov@gmail.com>,
	catalin.marinas@arm.com, will@kernel.org, pcc@google.com,
	andreyknvl@gmail.com, linux@rasmusvillemoes.dk,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, eugenis@google.com,
	syednwaris@gmail.com, william.gray@linaro.org,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}_value()
Date: Tue, 18 Jul 2023 17:01:28 +0300	[thread overview]
Message-ID: <ZLabOHmNQm2EOXWR@smile.fi.intel.com> (raw)
In-Reply-To: <CAG_fn=UPqyFhEMLDz3nhc7fgpCvprLsYrNjUnw3z4KfWCJGTHA@mail.gmail.com>

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@gmail.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

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