netdev - Re: [PATCH] xfrm: optimize ipv4 selector matching

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Message-ID: <20111122145940.GA24909@p183.telecom.by>
Date:	Tue, 22 Nov 2011 17:59:40 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] xfrm: optimize ipv4 selector matching

On Tue, Nov 22, 2011 at 02:50:59PM -0000, David Laight wrote:
>  
> > +static inline int addr4_match(const __be32 a1, const __be32 
> > a2, const u8 prefixlen)
> > +{
> > +	/* C99 6.5.7 (3): u32 << 32 is undefined behaviour */
> > +	if (prefixlen == 0) {
> > +		/* Matching constants result in smaller assembly. */
> > +		return 0xFFFFFFFFu;
> > +	}
> > +	return !((a1 ^ a2) & htonl(0xFFFFFFFFu << (32 - prefixlen)));
> > +}
> > +
> 
> It would probably be clearer to 'return 1' when prefixlen is zero.

"return 1" results in bigger code.
This function used only in boolean context, so exact return value doesn't matter.

> If this is a common path, might be worth caching
>     htonl(0xFFFFFFFFu << (32 - prefixlen))
> in the enclosing structure.

This means one more branch, which wouldn't be a win compared
to current code.
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