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

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Message-ID: <ZLVQ0NCC8SI5F8xR@smile.fi.intel.com>
Date:   Mon, 17 Jul 2023 17:31:44 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     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 05:29:12PM +0300, Andy Shevchenko wrote:
> On Mon, Jul 17, 2023 at 04:14:57PM +0200, Alexander Potapenko wrote:

...

> > > > +             map[index] &= ~(GENMASK(nbits + offset - 1, offset));
> > >
> > > 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
> 
> Wow, indeed! Perhaps time to report to clang and GCC people. I believe the root
> cause is that in the original version compiler can't prove that l is constant
> for GENMASK().
> 
> > > To fix that I found an easy refactoring:
> > >
> > >                 map[index] &= ~(GENMASK(nbits, 0) << offset));

nbits - 1 it should be, btw. In any case it seems the code is still better.

> > I'll take this one.
> > 
> > > (*) don't remember the actual versions, though, but anyway...

-- 
With Best Regards,
Andy Shevchenko