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Message-ID: <1513790375.1234.121.camel@perches.com> Date: Wed, 20 Dec 2017 09:19:35 -0800 From: Joe Perches <joe@...ches.com> To: David Laight <David.Laight@...LAB.COM>, 'Crt Mori' <cmo@...exis.com>, Peter Zijlstra <peterz@...radead.org> Cc: 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> Subject: Re: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt On Wed, 2017-12-20 at 16:46 +0000, David Laight wrote: > From: Crt Mori > > Sent: 20 December 2017 16:17 > > > > On 20 December 2017 at 17:00, Peter Zijlstra <peterz@...radead.org> wrote: > > > On Wed, Dec 20, 2017 at 02:39:26PM +0000, David Laight wrote: > > > > > > > With minor changes it ought to be possible to remove most of the > > > > 64bit arithmetic and shifts. > > > > > > > > If you care about performance then using 32 bit maths will be much faster. > > > > > > Some, u64 add/sub/shift isn't exactly expensive, but yes, I also > > > indicated that improvement is possible. At the very least y can be made > > > a u32 I suppose. > > > > OK, is there any more easy optimizations you see? > > I think this version works. > It doesn't have the optimisation for small values. > > unsigned int sqrt64(unsigned long long x) > { > unsigned int x_hi = x >> 32; > > unsigned int b = 0; > unsigned int y = 0; > unsigned int i; > Perhaps add: if (x <= UINT_MAX) return int_sqrt((unsigned long)x); > for (i = 0; i < 32; i++) { > b <<= 2; > b |= x_hi >> 30; > x_hi <<= 2; > if (i == 15) > x_hi = x; > y <<= 1; > if (b > y) > b -= ++y; > } > return y; > } > > Put it through cc -O3 -m32 -c -o sqrt64.o sqrt64.c and then objdump sqrt64.o > and compare to that of your version. > > David >
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