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

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Message-ID: <CAG_fn=X+NshSQXD9H7ajuMsDM7oA60-HF-bPrt3bQuPL63W=6A@mail.gmail.com>
Date:   Mon, 17 Jul 2023 18:29:52 +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()

On Mon, Jul 17, 2023 at 5:03 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> 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.

I think that the compiler might consider this transformation invalid
because "GENMASK(nbits - 1, 0) << offset" and "GENMASK(nbits + offset
- 1, offset)" have different values in the case "nbits + offset"
exceed 64.
Restricting the domain of bitmap_set_value() might result in better
code, but it is easier to stick to your version, which prevents the
overflow.

> > > > 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.

This "nbits vs. nbits - 1" was not detected by test_set_get_value(),
so I'll add an extra test case for it.