Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8

["Van Haaren, Harry" <harry.van.haaren@intel.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Dumitrescu, Cristian
> Sent: Monday, April 9, 2018 4:59 PM
> To: Stephen Hemminger <stephen@networkplumber.org>; Singh, Jasvinder
> <jasvinder.singh@intel.com>; Richardson, Bruce <bruce.richardson@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8
> 
> 
> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > Sent: Monday, April 9, 2018 4:10 PM
> > To: Singh, Jasvinder <jasvinder.singh@intel.com>
> > Cc: dev@dpdk.org; Dumitrescu, Cristian <cristian.dumitrescu@intel.com>
> > Subject: Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8
> >
> > On Mon,  9 Apr 2018 13:49:48 +0100
> > Jasvinder Singh <jasvinder.singh@intel.com> wrote:
> >
> > > Fix build error with gcc 8.0 due to cast between function types.
> > > Fixes: 5a80bf0ae613 ("table: add cuckoo hash")
> > >
> > > Signed-off-by: Jasvinder Singh <jasvinder.singh@intel.com>
> > > ---
> > >  lib/librte_table/rte_table_hash_cuckoo.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/lib/librte_table/rte_table_hash_cuckoo.c
> > b/lib/librte_table/rte_table_hash_cuckoo.c
> > > index dcb4fe9..f7eae27 100644
> > > --- a/lib/librte_table/rte_table_hash_cuckoo.c
> > > +++ b/lib/librte_table/rte_table_hash_cuckoo.c
> > > @@ -103,11 +103,13 @@ rte_table_hash_cuckoo_create(void *params,
> > >  		return NULL;
> > >  	}
> > >
> > > +	void *hash_func = p->f_hash;
> > > +
> > >  	/* Create cuckoo hash table */
> > >  	struct rte_hash_parameters hash_cuckoo_params = {
> > >  		.entries = p->n_keys,
> > >  		.key_len = p->key_size,
> > > -		.hash_func = (rte_hash_function)(p->f_hash),
> > > +		.hash_func = (rte_hash_function) hash_func,
> > >  		.hash_func_init_val = p->seed,
> > >  		.socket_id = socket_id,
> > >  		.name = p->name
> >
> > This is just tricking the compiler into not complaining.
> > I would really rather see the two hash functions made the same.
> 
> (Adding Bruce as well to consolidate all conversations in a single thread.)
> 
> What we want to do here is be able to use the librte_hash under the same API
> as the several hash table flavors implemented in librte_table.
> 
> Both of these libraries allow configuring the hash function per each hash
> table instance. Problem is: hash function in librte_hash has only 3 parameters
> (no key mask), while hash function in librte_table has 4 parameters (includes
> key mask). The key mask helps a lot for practical protocol implementations by
> avoiding key copy & pre-process on lookup.
> 
> So then: how to plug in librte_hash under the same API as the suite of hash
> tables in librte_table? We don't want to re-implement cuckoo hash from
> librte_hash, we simply want to invoke it as a low-level primitive, similarly
> to how the LPM and ACL tables are plugged into librte_table.
> 
> Solution is: as an exception, pass a 3-parameter hash function to cuckoo hash
> flavor under the librte_table. Maybe this should be documented better. This
> currently triggers a build warning with gcc 8, which is easy to fix, hence
> this trivial patch.
> 
> Ideally, for every 3-parameter hash function, I would like to generate the
> corresponding 4-parameter hash function on-the-fly, but unfortunately this is
> not what C language can do.
> 
> Of course, IMO the best solution is to add key mask support to librte_hash.


Looking at the previous discussion I see the following as a possible solution;

Given the current code looks broken it should be fixed in this release.
Given the actual code fix is an API / ABI break (depending on solution) it cannot be merged official in this release.
We have a NEXT_ABI macro - it allows us to break API/ABI conditionally at compile time.

With the above 3 points, I think the best solution is to correctly fix the problem that GCC 8 is identifying, and putting that new API inside the NEXT_ macros.

In this case, we can preserve backwards (buggy) behavior if required, and provide correct (but API/ABI breaking) code as well. This is a tough decision - particularly for distros - what do they package?

Given the current code, I don't see a better solution - but I hope I'm wrong :)