Re: [dpdk-dev] [PATCH v5 4/8] net/ether: use bitops to speedup comparison

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 2 Jul 2019 11:26:15 +0200
Olivier Matz <olivier.matz@6wind.com> wrote:

> On Tue, Jul 02, 2019 at 09:53:14AM +0200, Olivier Matz wrote:
> > Hi,
> > 
> > On Mon, Jun 24, 2019 at 01:44:31PM -0700, Stephen Hemminger wrote:  
> > > Using bit operations like or and xor is faster than a loop
> > > on all architectures. Really just explicit unrolling.
> > > 
> > > Similar cast to uint16 unaligned is already done in
> > > other functions here.
> > > 
> > > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > > Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
> > > ---
> > >  lib/librte_net/rte_ether.h | 17 +++++++----------
> > >  1 file changed, 7 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/lib/librte_net/rte_ether.h b/lib/librte_net/rte_ether.h
> > > index 8edc7e217b25..feb35a33c94b 100644
> > > --- a/lib/librte_net/rte_ether.h
> > > +++ b/lib/librte_net/rte_ether.h
> > > @@ -81,11 +81,10 @@ struct rte_ether_addr {
> > >  static inline int rte_is_same_ether_addr(const struct rte_ether_addr *ea1,
> > >  				     const struct rte_ether_addr *ea2)
> > >  {
> > > -	int i;
> > > -	for (i = 0; i < RTE_ETHER_ADDR_LEN; i++)
> > > -		if (ea1->addr_bytes[i] != ea2->addr_bytes[i])
> > > -			return 0;
> > > -	return 1;
> > > +	const unaligned_uint16_t *w1 = (const uint16_t *)ea1;
> > > +	const unaligned_uint16_t *w2 = (const uint16_t *)ea2;
> > > +
> > > +	return ((w1[0] ^ w2[0]) | (w1[1] ^ w2[1]) | (w1[2] ^ w2[2])) == 0;
> > >  }
> > >  
> > >  /**
> > > @@ -100,11 +99,9 @@ static inline int rte_is_same_ether_addr(const struct rte_ether_addr *ea1,
> > >   */
> > >  static inline int rte_is_zero_ether_addr(const struct rte_ether_addr *ea)
> > >  {
> > > -	int i;
> > > -	for (i = 0; i < RTE_ETHER_ADDR_LEN; i++)
> > > -		if (ea->addr_bytes[i] != 0x00)
> > > -			return 0;
> > > -	return 1;
> > > +	const unaligned_uint16_t *w = (const uint16_t *)ea;
> > > +
> > > +	return (w[0] | w[1] | w[2]) == 0;
> > >  }
> > >  
> > >  /**  
> > 
> > I wonder if using memcmp() isn't faster with recent compilers (gcc >= 7).
> > I tried it quickly, and it seems the generated code is good (no call):
> > https://godbolt.org/z/9MOL7g
> > 
> > It would avoid the use of unaligned_uint16_t, and the next patch that
> > adds the alignment constraint.  
> 
> As pointed out by Konstantin privately (I guess he wanted to do a
> reply-all), the size of addr_bytes is wrong in my previous link (8
> instead of 6). Thanks for catching it.
> 
> With 6, the gcc code is not as good: there is still no call to memcmp(),
> but there are some jumps. With the latest clang, the generated code is
> nice: https://godbolt.org/z/nfptnY

gcc matters most for current DPDK users.