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

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Date:   Mon, 17 Jul 2023 18:03:32 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, linux@...musvillemoes.dk,
        yury.norov@...il.com, 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 Mon, Jul 17, 2023 at 04:53:51PM +0200, Alexander Potapenko wrote:

...

> > > I remember that this construction may bring horrible code on some architectures
> > > with some version(s) of the compiler (*).
> >
> > Wow, even the trunk Clang and GCC seem to generate better code for
> > your version of this line: https://godbolt.org/z/36Kqxhe6j
> 
> Oh wait.
> First, my Godbolt reproducer is incorrect, it is using sizeof(unsigned
> long) instead of 8 * sizeof(unsigned long) for BITS_PER_LONG.

Still slightly better. And note, that the same GENMASK() is used at the
beginning of the function. Compiler actually might cache it.

> > > To fix that I found an easy refactoring:
> > >
> > >                 map[index] &= ~(GENMASK(nbits, 0) << offset));
> > >
> 
> Second, the line above should probably be:
>   map[index] &= ~(GENMASK(nbits - 1, 0) << offset));
> 
> , right?

Yes.

-- 
With Best Regards,
Andy Shevchenko