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

Muhammad Faiz mfcc64 at gmail.com
Tue Oct 24 12:32:29 EEST 2017

On Tue, Oct 24, 2017 at 8:36 AM, James Almer <jamrial at gmail.com> wrote:
> 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?

I've posted a patch.


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