Re: [dpdk-dev] [PATCH] ixgbe: fix compile error with gcc4.4 (used RHEL 6)

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Sep 18, 2014 at 11:03:12AM -0400, Neil Horman wrote:
> On Thu, Sep 18, 2014 at 12:35:21PM +0000, Richardson, Bruce wrote:
> > > -----Original Message-----
> > > From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > > Sent: Thursday, September 18, 2014 1:25 PM
> > > To: Thomas Monjalon
> > > Cc: Richardson, Bruce; dev@dpdk.org
> > > Subject: Re: [dpdk-dev] [PATCH] ixgbe: fix compile error with gcc4.4 (used RHEL
> > > 6)
> > > 
> > > On Thu, Sep 18, 2014 at 01:09:16PM +0200, Thomas Monjalon wrote:
> > > > > The refcnt field is contained within an anonymous union within the mbuf
> > > > > data structure, and gcc 4.4 gives an error about an unknown field unless
> > > > > the initialiser for the field is contained within extra braces.
> > > > >
> > > > > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > > >
> > > > Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>
> > > >
> > > > Thanks Bruce, it is now applied.
> > > 
> > > Hang on here, we use anonymous unions all the time in RHEL6, and make
> > > assignments to them frequently, and the compiler doesn't complain (see the
> > > dropcount variable in sk_buff for an example).  Not saying that this is a big
> > > deal, but can you explain a little more about what you're seeing when this error
> > > occurs, before we just paper over it?
> > > 
> > 
> > Originally reported on RHEL6 as a build failure. When I use gcc4.4 on Fedora 20, I get the following without this change:
> > 
> >   CC ixgbe_rxtx_vec.o
> > == Build lib/librte_table
> > /home/bruce/dpdk.org/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c: In function 'ixgbe_rxq_vec_setup':
> > /home/bruce/dpdk.org/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c:726: error: unknown field 'refcnt' specified in initializer
> > compilation terminated due to -Wfatal-errors.
> > make[5]: *** [ixgbe_rxtx_vec.o] Error 1
> > make[4]: *** [librte_pmd_ixgbe] Error 2
> > make[4]: *** Waiting for unfinished jobs....
> > make[3]: *** [lib] Error 2
> > make[2]: *** [all] Error 2
> > make[1]: *** [x86_64-native-linuxapp-gcc_install] Error 2
> > make: *** [install] Error 2
> > 
> > Regards,
> > /Bruce
> > 
> 
> 
> Thank you, I've reproduced the problem here, and you're right, it is a compiler
> bug, but I really hate the idea of just throwing braces around something to shut
> the compiler up, especially when the compiler has since been fixed.  Looking at
> it a bit more closely it seems like the the thing to do is actually just
> consistently use rte_mbuf_refcnt_set, since thats what the rte_mbuf header
> documentation says to do, and protects you if the internal structure changes, as
> well as prevents you from having to spread ifdef RTE_MBUF_REFCNT all over the
> place.
> 
> This patch does a bit more than that too.  With a little creative typedef-ing we
> don't need the anonymous union at all, and lets us just use a single refcnt
> variable, and I think eliminate that odd refcnt_reserved member of the union,
> that, as far as I can see, does nothing.
> 
> Thoughts?
> 

I like the idea of using a typedef, though are we not adjusting the mbuf 
layout if the refcount is disabled? A follow-on question would also be - do 
we really need an option to disable the reference count, do we not always 
just leave it on?

As for using refcnt_set - that would be a solution that would work too, but 
that approach needs to be checked for performance impacts as we are going 
from a constant initializer value to having an extra assignment instruction 
in the fast-path.

The simplest fix is probably just to add the braces, which isn't really much 
of a big deal.

/Bruce