Re: [dpdk-dev] [PATCH v2] mbuf: fix compile by removing struct from function

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 11 Jan 2019 03:01:43 +0000
"Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com> wrote:

> > -----Original Message-----
> > From: dev <dev-bounces@dpdk.org> On Behalf Of Dumitrescu, Cristian
> > Sent: Friday, January 11, 2019 2:40 AM
> > To: Van Haaren, Harry <harry.van.haaren@intel.com>; dev@dpdk.org
> > Cc: Pattan, Reshma <reshma.pattan@intel.com>; thomas@monjalon.net;
> > olivier.matz@6wind.com
> > Subject: Re: [dpdk-dev] [PATCH v2] mbuf: fix compile by removing struct
> > from function
> >
> >
> >  
> > > -----Original Message-----
> > > From: Van Haaren, Harry
> > > Sent: Thursday, January 10, 2019 6:07 PM
> > > To: dev@dpdk.org
> > > Cc: Van Haaren, Harry <harry.van.haaren@intel.com>; Pattan, Reshma
> > > <reshma.pattan@intel.com>; Dumitrescu, Cristian
> > > <cristian.dumitrescu@intel.com>; thomas@monjalon.net;
> > > olivier.matz@6wind.com
> > > Subject: [PATCH v2] mbuf: fix compile by removing struct from function
> > >
> > > Although C compilation works with the struct rte_mbuf_sched
> > > declared inside the struct rte_mbuf namespace, C++ fails to
> > > compile.
> > >
> > > This fix removes the temporary struct rte_mbuf_sched, instead
> > > reading from the mbuf directly for each struct member. As the
> > > struct is now not used directly, the C++ compiler doesn't need
> > > to know about the struct, resolving the issue.
> > >
> > > Fixes: 5d3f72100904 ("mbuf: implement generic format for sched field")
> > >
> > > Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
> > >
> > > ---
> > >
> > > Cc: reshma.pattan@intel.com
> > > Cc: cristian.dumitrescu@intel.com
> > > Cc: thomas@monjalon.net
> > > Cc: olivier.matz@6wind.com
> > >
> > > See mailing list for v1 discussion, perhaps this solution is more
> > > readable due to leaving sched struct in-line in the mbuf struct.
> > > ---
> > >  lib/librte_mbuf/rte_mbuf.h | 16 ++++++----------
> > >  1 file changed, 6 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > > index bc562dc8a..1b260efd5 100644
> > > --- a/lib/librte_mbuf/rte_mbuf.h
> > > +++ b/lib/librte_mbuf/rte_mbuf.h
> > > @@ -2344,11 +2344,9 @@ rte_mbuf_sched_get(const struct rte_mbuf  
> > *m,  
> > > uint32_t *queue_id,
> > >  uint8_t *traffic_class,
> > >  uint8_t *color)
> > >  {
> > > -struct rte_mbuf_sched sched = m->hash.sched;  
> 
> Did C++ complain *sched? *sched is better with less stack mem footprint.
> If the pointer works, the code looks better.

Using *sched will cause compiler to generate multiple references which
is the real performance issue here.