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

[Neil Horman <nhorman@tuxdriver.com>]

On Mon, Apr 09, 2018 at 04:38:11PM +0000, Van Haaren, Harry wrote:
> > 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 :)
> 
Why not make the hash_func pointer in the rte_hash_parameters structure an
anonymous union, and reserve a bit in the extra_flag field to denote if the
function pointer has 3 arguments or 4?  Then rte_hash_hash can use the
appropriate calling convention on hash_func.

Neil