netdev - RE: [PATCH net-next] filter: add bpf_optimize

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Date:	Fri, 21 Nov 2014 10:26:23 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Denis Kirjanov' <kda@...ux-powerpc.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	Alexei Starovoitov <ast@...mgrid.com>
Subject: RE: [PATCH net-next] filter: add bpf_optimize_div()

From: Denis Kirjanov
> optimize_div() found in  mips bpf jit is really usefull
> for other arches. So let's put it in filter.h
...
> +static inline int bpf_optimize_div(u32 *k)
> +{
> +	/* power of 2 divides can be implemented with right shift */
> +	if (!(*k & (*k-1))) {
> +		*k = ilog2(*k);
> +		return 1;
> +	}
> +
> +	return 0;
> +}

If 'k' is a constant then a 'multiply by reciprocal' is likely to
be as fast as the shift - and works for a wider range of values.

On some architectures the ilog2() may itself be expensive.

On x86 the divide might even be faster than the conditional mess
the above generates.

	David



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