From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: torvalds@linux-foundation.org, akpm@linux-foundation.org,
dave@stgolabs.net, aksgarg1989@gmail.com, tglx@linutronix.de,
mingo@kernel.org, joe@perches.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] lib/int_sqrt: Optimize initial value compute
Date: Tue, 25 Jul 2017 12:50:40 +0100 [thread overview]
Message-ID: <20170725115040.GB27768@arm.com> (raw)
In-Reply-To: <20170724153455.710696863@infradead.org>
Hi Peter,
On Mon, Jul 24, 2017 at 05:16:32PM +0200, Peter Zijlstra wrote:
> The initial value (@m) compute is:
>
> m = 1UL << (BITS_PER_LONG - 2);
> while (m > x)
> m >>= 2;
>
> Which is a linear search for the highest even bit smaller or equal to @x
> We can implement this using a binary search using __fls() (or better
> when its hardware implemented).
>
> m = 1UL << (__fls(x) & ~1UL);
>
> Especially for small values of @x; which are the more common
> arguments; the linear search is near to worst case, while the binary
> search of __fls() is a constant 6 branches.
>
> cycles: branches: branch-misses:
>
> PRE:
>
> hot: 43.633557 +- 0.034373 45.333132 +- 0.002277 0.023529 +- 0.000681
> cold: 207.438411 +- 0.125840 45.333132 +- 0.002277 6.976486 +- 0.004219
>
> SOFTWARE FLS:
>
> hot: 29.576176 +- 0.028850 26.666730 +- 0.004511 0.019463 +- 0.000663
> cold: 165.947136 +- 0.188406 26.666746 +- 0.004511 6.133897 +- 0.004386
>
> HARDWARE FLS:
>
> hot: 24.720922 +- 0.025161 20.666784 +- 0.004509 0.020836 +- 0.000677
> cold: 132.777197 +- 0.127471 20.666776 +- 0.004509 5.080285 +- 0.003874
>
> Averages computed over all values <128k using a LFSR to generate
> order. Cold numbers have a LFSR based branch trace buffer 'confuser'
> ran between each int_sqrt() invocation.
The hardware fls version works nicely for arm64, where it can be implemented
using the clz instruction (via the __builtin_clzl intrinsic).
Acked-by: Will Deacon <will.deacon@arm.com>
Cheers,
Will
next prev parent reply other threads:[~2017-07-25 11:50 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-24 15:16 [PATCH 0/3] lib/int_sqrt: Fix, optimize and document Peter Zijlstra
2017-07-24 15:16 ` [PATCH 1/3] lib/int_sqrt: Optimize small argument Peter Zijlstra
2017-07-24 15:16 ` [PATCH 2/3] lib/int_sqrt: Optimize initial value compute Peter Zijlstra
2017-07-24 17:35 ` Linus Torvalds
2017-07-25 8:17 ` Peter Zijlstra
2017-07-25 15:43 ` Linus Torvalds
2017-07-25 16:08 ` Peter Zijlstra
2017-07-25 11:50 ` Will Deacon [this message]
2017-07-24 15:16 ` [PATCH 3/3] lib/int_sqrt: Adjust comments Peter Zijlstra
2017-07-24 15:42 ` [PATCH 0/3] lib/int_sqrt: Fix, optimize and document Joe Perches
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