Re: your mail

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Yun Levi <ppbuk5246@gmail.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
Subject: Re: your mail
Date: Wed, 2 Dec 2020 20:51:27 +0200	[thread overview]
Message-ID: <20201202185127.GO4077@smile.fi.intel.com> (raw)
In-Reply-To: <CAM7-yPSWvsySweXSmbvW2hucce8T7BOSkz-eF5t7PJE6zv5tjg@mail.gmail.com>

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);
}

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

And I believe it can be optimized more.

-- 
With Best Regards,
Andy Shevchenko