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From:   Yun Levi <ppbuk5246@gmail.com>
Date:   Thu, 3 Dec 2020 08:16:16 +0900
Message-ID: <CAM7-yPQfzyOAyjyeFp2cMDXMrk292_AN6+6ZvahugS_h387SyA@mail.gmail.com>
Subject: 
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Yury Norov <yury.norov@gmail.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>, dushistov@mail.ru,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        William Breathitt Gray <vilhelm.gray@gmail.com>,
        richard.weiyang@linux.alibaba.com, joseph.qi@linux.alibaba.com,
        skalluru@marvell.com, Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch@vger.kernel.org
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On Thu, Dec 3, 2020 at 3:55 AM Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
>
> On Wed, Dec 02, 2020 at 08:51:27PM +0200, Andy Shevchenko wrote:
> > On Thu, Dec 03, 2020 at 03:27:33AM +0900, Yun Levi wrote:
> > > On Thu, Dec 3, 2020 at 2:36 AM Andy Shevchenko
> > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > On Wed, Dec 02, 2020 at 09:26:05AM -0800, Yury Norov wrote:
>
> ...
>
> > > > Side note: speaking of performance, any plans to fix for_each_*_bit*() for
> > > > cases when the nbits is known to be <= BITS_PER_LONG?
> > > >
> > > > Now it makes an awful code generation (something like few hundred bytes of
> > > > code).
> >
> > > Frankly Speaking, I don't have an idea in now.....
> > > Could you share your idea or wisdom?
> >
> > Something like (I may be mistaken by names, etc, I'm not a compiler expert,
> > and this is in pseudo language, I don't remember all API names by hart,
> > just to express the idea) as a rough first step
> >
> > __builtin_constant(nbits, find_next_set_bit_long, find_next_set_bit)
> >
> > find_next_set_bit_long()
> > {
> >       unsigned long v = BIT_LAST_WORD(i);
> >       return ffs_long(v);
> > }
> >

I think this idea is hard to apply to find_next_set_bit.
because __builtin_constant should be not only to size but also to offset.
though we find size && offset is const under BITS_PER_LONG,
I'm not sure it could be implemented as const expression..

> > Same for find_first_set_bit() -> map it to ffs_long().
> >
> > And I believe it can be optimized more.

In case of the find_first_set_bit, I think it would be possible,
But I think it much better to separate as another patch set.
So I want to focus on adding find_prev_*_bit, find_last_zero_bit to
this patchset and mail-thread. Frankly speaking I need time to see
that suggestion and think so, in next patch v2,
it wouldn't be included.

>
> Btw it will also require to reconsider test cases where such constant small
> nbits values are passed (forcing compiler to avoid optimization somehow, one
> way is to try random nbits for some test cases).
>
> --
> With Best Regards,
> Andy Shevchenko
>
>


if my understanding and attitude are wrong, I really apologize for my
rudeness and stubbornness but please let me know what thing is wrong.

Sincerely
Levi.