linux-kernel - Re: your mail

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Message-ID: <20201202185127.GO4077@smile.fi.intel.com>
Date:   Wed, 2 Dec 2020 20:51:27 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yun Levi <ppbuk5246@...il.com>
Cc:     Yury Norov <yury.norov@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>, dushistov@...l.ru,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        richard.weiyang@...ux.alibaba.com, joseph.qi@...ux.alibaba.com,
        skalluru@...vell.com, Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org
Subject: Re: your mail

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@...ux.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);
}

Same for find_first_set_bit() -> map it to ffs_long().

And I believe it can be optimized more.

-- 
With Best Regards,
Andy Shevchenko