linux-kernel - RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int

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Date:   Thu, 21 Dec 2017 14:48:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>
CC:     'Crt Mori' <cmo@...exis.com>, Jonathan Cameron <jic23@...nel.org>,
        "Ingo Molnar" <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kees Cook" <keescook@...omium.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        "Ian Abbott" <abbotti@....co.uk>,
        Larry Finger <Larry.Finger@...inger.net>,
        "Niklas Soderlund" <niklas.soderlund+renesas@...natech.se>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        Joe Perches <joe@...ches.com>
Subject: RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt

From: Peter Zijlstra
> Sent: 21 December 2017 14:12
...
> > > This part above looks like FLS
> > It also does the rest of the required shifts.
> 
> Still, fls() + shift is way faster on hardware that has an fls
> instruction.
> 
> Writing out that binary search doesn't make sense.

If the hardware doesn't have an appropriate fls instruction
the soft fls()will be worse.

If you used fls() you'd still need quite a bit of code
to generate the correct shift and loop count adjustment.
Given the cost of the loop iterations the 3 tests are noise.
The open coded version is obviously correct...

I didn't add the 4th one because the code always does 2 iterations.

If you were really worried about performance there are faster
algorithms (even doing 2 or 4 bits a time is faster).

	David