netdev - Re: [PATCH] netfilter: unfold two critical loops in ip_packet

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Message-ID: <87ocxox0bu.fsf@basil.nowhere.org>
Date:	Fri, 30 Jan 2009 16:47:33 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	Linux Network Development list <netdev@...r.kernel.org>
Subject: Re: [PATCH] netfilter: unfold two critical loops in ip_packet_match()

Eric Dumazet <dada1@...mosbay.com> writes:

> While doing oprofile tests I noticed two loops are not properly unrolled by gcc

That's because nobody passed -funroll-loops. Did you try that for
that file? Likely will need -O2 too

> +static unsigned long ifname_compare(const void *_a, const void *_b, const void *_mask)
> +{
> +	const unsigned long *a = (const unsigned long *)_a;
> +	const unsigned long *b = (const unsigned long *)_b;
> +	const unsigned long *mask = (const unsigned long *)_mask;
> +	unsigned long ret;
> +
> +	ret = (a[0] ^ b[0]) & mask[0];
> +	ret |= (a[1] ^ b[1]) & mask[1];
> +	if (IFNAMSIZ > 2 * sizeof(unsigned long))
> +		ret |= (a[2] ^ b[2]) & mask[2];
> +	if (IFNAMSIZ > 3 * sizeof(unsigned long))
> +		ret |= (a[3] ^ b[3]) & mask[3];

That will silently break for IFNAMSIZ >= 4*sizeof(unsigned long)
You should add a dummy loop for that or at least a BUILD_BUG_ON

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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