On Mon, Apr 09, 2018 at 04:38:11PM +0000, Van Haaren, Harry wrote: > > From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Dumitrescu, Cristian > > Sent: Monday, April 9, 2018 4:59 PM > > To: Stephen Hemminger <step...@networkplumber.org>; Singh, Jasvinder > > <jasvinder.si...@intel.com>; Richardson, Bruce <bruce.richard...@intel.com> > > Cc: dev@dpdk.org > > Subject: Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8 > > > > > > > > > -----Original Message----- > > > From: Stephen Hemminger [mailto:step...@networkplumber.org] > > > Sent: Monday, April 9, 2018 4:10 PM > > > To: Singh, Jasvinder <jasvinder.si...@intel.com> > > > Cc: dev@dpdk.org; Dumitrescu, Cristian <cristian.dumitre...@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.si...@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.si...@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
