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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 4/9/2018 5:38 PM, 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?

+1 to use RTE_NEXT_ABI and deliver fixed code, and agree this is kind of pushing
decision to distros.

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