[FFmpeg-devel] [PATCH] avutil/crc: always use precalculated CRC tables for known polynomials

[James Almer]

On 10/23/2017 10:24 PM, Michael Niedermayer wrote:
> On Tue, Oct 24, 2017 at 01:39:03AM +0100, Mark Thompson wrote:
>> On 24/10/17 00:52, Michael Niedermayer wrote:
>>> Hi
>>>
>>> On Mon, Oct 23, 2017 at 04:43:19PM +0100, Mark Thompson wrote:
>>>> On 23/10/17 03:56, Michael Niedermayer wrote:
>>>>> the initialization should be thread safe as it never writes a different
>>>>> value in the same spot
>>>>
>>>> This is not true; please be very careful with assumptions like this.
>>>>
>>>> The C standard calls this a data race and it is undefined behaviour.
>>>
>>> Thats interresting, we definitly do not want undefined behavior
>>> can you quote the relevant parts of the C standard
>>> which make writing the same value, a data race ?
>>>
>>> I was not aware of this if its true.
>>> Also i dont immedeatly see this in the "latest" draft i am looking at
>>> atm
>>>
>>> What i see is this:
>>> 4 Two expression evaluations conflict if one of them modifies a memory location and the
>>>   other one reads or modifies the same memory location.
>>> ...
>>> 25 The execution of a program contains a data race if it contains two conflicting actions in
>>>    different threads, at least one of which is not atomic, and neither happens before the
>>>    other. Any such data race results in undefined behavior.
>>>
>>> This seems carefully worded to not include writing the same value.
>>> As "modification" implies change which does not occur when writing the
>>> same value.
>>
>> All writes are modifications.
>>
>> See section 3.1 note 2:
>> """
>> 3 NOTE 2   "Modify"€™ includes the case where the new value being stored is the same as the previous value.
>> """
> 
> That resolves the difference between our interpretation
> 
> thanks

Should i push this, or does someone want to give ff_thread_once() a try?