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From: David Laight <David.Laight@ACULAB.COM>
To: "'Peter Zijlstra'" <peterz@infradead.org>
CC: "'Crt Mori'" <cmo@melexis.com>, Jonathan Cameron <jic23@kernel.org>,
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Subject: RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Thread-Topic: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
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Date: Thu, 21 Dec 2017 14:48:14 +0000
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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