netdev - Re: [PATCH RFC] reciprocal_divide: correction/update of the algorithm

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Date:	Tue, 14 Jan 2014 10:07:05 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev@...r.kernel.org, dborkman@...hat.com,
	linux-kernel@...r.kernel.org, darkjames-ws@...kjames.pl
Subject: Re: [PATCH RFC] reciprocal_divide: correction/update of the
 algorithm

On Mon, 2014-01-13 at 22:42 +0100, Hannes Frederic Sowa wrote:
> This patch is a RFC and part of a series Daniel Borkmann and me want to
> do when introducing prandom_u32_range{,_ro} and prandom_u32_max{,_ro}
> helpers later this week.

> -static inline u32 reciprocal_divide(u32 A, u32 R)
> +struct reciprocal_value reciprocal_value(u32 d);
> +
> +static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
>  {
> -	return (u32)(((u64)A * R) >> 32);
> +	u32 t = (u32)(((u64)a * R.m) >> 32);
> +	return (t + ((a - t) >> R.sh1)) >> R.sh2;
>  }

I would rather introduce new helpers and convert users that really need
them.

For instance, just use a divide in BPF, because doing this on JIT might
be too complex for the gains. Strangely, libpcap doesn't seem to
optimize any divide, like divides by a power of two...

Reciprocal were added 7 years ago, for very specific uses, but current
cpus have reasonably fast dividers.



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