From: David Laight <David.Laight@ACULAB.COM>
To: "'Peter Zijlstra'" <peterz@infradead.org>
Cc: "'Crt Mori'" <cmo@melexis.com>,
Jonathan Cameron <jic23@kernel.org>,
"Ingo Molnar" <mingo@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
"Kees Cook" <keescook@chromium.org>,
Rusty Russell <rusty@rustcorp.com.au>,
"Ian Abbott" <abbotti@mev.co.uk>,
Larry Finger <Larry.Finger@lwfinger.net>,
"Niklas Soderlund" <niklas.soderlund+renesas@ragnatech.se>,
Thomas Gleixner <tglx@linutronix.de>,
Krzysztof Kozlowski <krzk@kernel.org>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
Joe Perches <joe@perches.com>
Subject: RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Date: Thu, 21 Dec 2017 14:48:14 +0000 [thread overview]
Message-ID: <79658484b99a4f65bb3a1937b3d156ad@AcuMS.aculab.com> (raw)
In-Reply-To: <20171221141130.cdng2mysnjj6j4i6@hirez.programming.kicks-ass.net>
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
next prev parent reply other threads:[~2017-12-21 14:48 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-20 14:20 [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt Crt Mori
2017-12-20 14:39 ` David Laight
2017-12-20 15:41 ` Crt Mori
2017-12-20 16:00 ` Peter Zijlstra
2017-12-20 16:17 ` Crt Mori
2017-12-20 16:46 ` David Laight
2017-12-20 17:19 ` Joe Perches
2017-12-21 10:02 ` David Laight
2017-12-20 17:30 ` Crt Mori
2017-12-21 10:08 ` David Laight
2017-12-21 10:59 ` David Laight
2017-12-21 11:43 ` David Laight
2017-12-21 13:17 ` Crt Mori
2017-12-21 13:56 ` David Laight
2017-12-21 14:11 ` Peter Zijlstra
2017-12-21 14:48 ` David Laight [this message]
2017-12-22 13:44 ` Crt Mori
2018-01-09 15:18 ` Crt Mori
2018-01-12 9:41 ` David Laight
2018-01-15 8:17 ` Crt Mori
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