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

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Date:   Mon, 17 Jul 2023 16:53:51 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.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()

> >
> > 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.

>
> > 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?