Re: [PATCH net-next] etherdevice: Optimize compare_ether_addr/ether_addr_equal

[Joe Perches <joe@perches.com>]

On Sat, 2013-08-31 at 09:43 -0700, Stephen Hemminger wrote:
> On Sat, 31 Aug 2013 01:54:16 -0700
> Joe Perches <joe@perches.com> wrote:
> 
> > When CONFIG_HAS_EFFICIENT_UNALIGNED_ACCESS is set,
> > optimize compare_ether_addr a little by removing an
> > xor and or by using a u32 and u16 comparison
> > instead of 3 separate u16 comparisons.
> > 
> > Make the ether_addr_equal_64bits code a bit simpler
> > by adding a test for CONFIG_64BIT and calling
> > ether_addr_equal otherwise.
> > 
> > This also slightly improves ether_addr_equal_64bits
> > by removing the zap_last_2bytes shifts in the !64bit
> > case.
> > 
> > Signed-off-by: Joe Perches <joe@perches.com>
> > ---
> >  include/linux/etherdevice.h | 17 ++++++++++-------
> >  1 file changed, 10 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/etherdevice.h b/include/linux/etherdevice.h
> > index c623861..2514d17 100644
> > --- a/include/linux/etherdevice.h
> > +++ b/include/linux/etherdevice.h
> > @@ -208,11 +208,19 @@ static inline void eth_hw_addr_random(struct net_device *dev)
> >   */
> >  static inline unsigned compare_ether_addr(const u8 *addr1, const u8 *addr2)
> >  {
> > +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> > +	u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2));
> > +	fold |= ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4)));
> > +
> > +	BUILD_BUG_ON(ETH_ALEN != 6);
> > +	return fold != 0;
> > +#else
> >  	const u16 *a = (const u16 *) addr1;
> >  	const u16 *b = (const u16 *) addr2;
> >  
> >  	BUILD_BUG_ON(ETH_ALEN != 6);
> >  	return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;
> > +#endif
> >  }
> >  
> 
> If you really want to be efficient do it as one 64 bit mask and compare.

Nope.

That's what ether_addr_equal_64bits does
when it's known that a 64 bit test can be done.

Otherwise, there's no guarantee that 64 bits
are available to be read from 48 bits of data.