Re: [dpdk-dev] [PATCH 1/1] net: fix aliasing issue in checksum computation

["Morten Brørup" <mb@smartsharesystems.com>]

Georg, I apologize for calling you Geoff below! Just realized my mistake.

Med venlig hilsen / Kind regards,
-Morten Brørup


> -----Original Message-----
> From: Morten Brørup
> Sent: Saturday, 16 October 2021 10.21
> To: 'Georg Sauthoff'
> Cc: 'dev@dpdk.org'; 'Ferruh Yigit'; 'Olivier Matz'; 'Thomas Monjalon';
> 'David Marchand'
> Subject: RE: [dpdk-dev] [PATCH 1/1] net: fix aliasing issue in checksum
> computation
> 
> Geoff,
> 
> I have given this some more thoughts.
> 
> Most bytes transferred in real life are transferred in large packets,
> so faster processing of large packets is a great improvement!
> 
> Furthermore, a quick analysis of a recent packet sample from an ISP
> customer of ours shows that less than 8 % of the packets are odd size.
> Would you consider adding an unlikely() to the branch handling the odd
> byte at the end?
> 
> -Morten
> 
> > -----Original Message-----
> > From: Morten Brørup
> > Sent: Thursday, 14 October 2021 22.22
> >
> > > -----Original Message-----
> > > From: Ferruh Yigit [mailto:ferruh.yigit@intel.com]
> > > Sent: Thursday, 14 October 2021 19.20
> > >
> > > On 9/18/2021 12:49 PM, Georg Sauthoff wrote:
> > > > That means a superfluous cast is removed and aliasing through a
> > > uint8_t
> > > > pointer is eliminated. Note that uint8_t doesn't have the same
> > > > strict-aliasing properties as unsigned char.
> > > >
> > > > Also simplified the loop since a modern C compiler can speed up
> > (i.e.
> > > > auto-vectorize) it in a similar way. For example, GCC auto-
> > vectorizes
> > > it
> > > > for Haswell using AVX registers while halving the number of
> > > instructions
> > > > in the generated code.
> > > >
> > > > Signed-off-by: Georg Sauthoff <mail@gms.tf>
> > >
> > > + Morten. (Because of past reviews on cksum code)
> >
> > Thanks, Ferruh.
> >
> > I have not verified the claimed benefits of the patch, but I have
> > reviewed the code thoroughly, and it looks perfectly good to me.
> >
> > Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
> >
> > BTW: It makes me wonder if other parts of DPDK could benefit from the
> > same treatment. Especially some of the older DPDK code, where we were
> > trying to optimize by hand what a modern compiler can optimize for us
> > today.
> >
> > >
> > > > ---
> > > >   lib/net/rte_ip.h | 27 ++++++++-------------------
> > > >   1 file changed, 8 insertions(+), 19 deletions(-)
> > > >
> > > > diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
> > > > index 05948b69b7..386db94c85 100644
> > > > --- a/lib/net/rte_ip.h
> > > > +++ b/lib/net/rte_ip.h
> > > > @@ -141,29 +141,18 @@ rte_ipv4_hdr_len(const struct rte_ipv4_hdr
> > > *ipv4_hdr)
> > > >   static inline uint32_t
> > > >   __rte_raw_cksum(const void *buf, size_t len, uint32_t sum)
> > > >   {
> > > > -	/* workaround gcc strict-aliasing warning */
> > > > -	uintptr_t ptr = (uintptr_t)buf;
> > > > +	/* extend strict-aliasing rules */
> > > >   	typedef uint16_t __attribute__((__may_alias__)) u16_p;
> > > > -	const u16_p *u16_buf = (const u16_p *)ptr;
> > > > -
> > > > -	while (len >= (sizeof(*u16_buf) * 4)) {
> > > > -		sum += u16_buf[0];
> > > > -		sum += u16_buf[1];
> > > > -		sum += u16_buf[2];
> > > > -		sum += u16_buf[3];
> > > > -		len -= sizeof(*u16_buf) * 4;
> > > > -		u16_buf += 4;
> > > > -	}
> > > > -	while (len >= sizeof(*u16_buf)) {
> > > > +	const u16_p *u16_buf = (const u16_p *)buf;
> > > > +	const u16_p *end = u16_buf + len / sizeof(*u16_buf);
> > > > +
> > > > +	for (; u16_buf != end; ++u16_buf)
> >
> > Personally I would prefer post-incrementing here. It makes no
> > difference, so I don't see any need to revise the patch.
> >
> > > >   		sum += *u16_buf;
> > > > -		len -= sizeof(*u16_buf);
> > > > -		u16_buf += 1;
> > > > -	}
> > > >
> > > > -	/* if length is in odd bytes */
> > > > -	if (len == 1) {
> > > > +	/* if length is odd, keeping it byte order independent */
> > > > +	if (len % 2) {
> 
> I assume that the compiler already optimizes "% 2" to "& 1".
> 
> > > >   		uint16_t left = 0;
> > > > -		*(uint8_t *)&left = *(const uint8_t *)u16_buf;
> > > > +		*(unsigned char*)&left = *(const unsigned char *)end;
> > > >   		sum += left;
> > > >   	}
> > > >