[vlc-devel] [PATCH 09/10] vlc_tick: add msftime_t to express 100ns time

[Steve Lhomme]

On 2018-06-22 3:17 PM, Romain Vimont wrote:
> On Fri, Jun 22, 2018 at 02:53:26PM +0200, Steve Lhomme wrote:
>> Such values currently have a better resolution than vlc_tick_t
>> Also provide conversion macros/functions to/from vlc_tick_t
>> ---
>>   include/vlc_tick.h | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/include/vlc_tick.h b/include/vlc_tick.h
>> index 9e7f909e149..6953ba346d4 100644
>> --- a/include/vlc_tick.h
>> +++ b/include/vlc_tick.h
>> @@ -138,6 +138,24 @@ static inline int64_t ns_from_vlc_tick(vlc_tick_t vtk)
>>       return vtk * INT64_C(1000000000) / CLOCK_FREQ;
>>   }
>>   
>> +/**
>> + * Typically Microsoft time with 100ns resolutions
>> + */
>> +typedef int64_t msftime_t;
>> +#define MSFTIME_FROM_VLC_TICK(vtk)  ((msftime_t)((vtk) * (INT64_C(10000000) / CLOCK_FREQ)))
>> +#define VLC_TICK_FROM_MSFTIME(msft) ((vlc_tick_t)((msft) / (INT64_C(10000000) / CLOCK_FREQ)))
>> +#define MSFTIME_FROM_SEC(sec)       ((msftime_t)((sec) * INT64_C(10000000)))  /* seconds in msftime_t */
>> +
>> +static inline vlc_tick_t vlc_tick_from_msftime(msftime_t t)
>> +{
>> +    return t * CLOCK_FREQ / INT64_C(10000000);
> The order of operations is not the same as in the macro, so in theory
> the results (in extreme cases) might be different.
>
> If 10000000 is a multiple of CLOCK_FREQ, the macro implementation is
> always better (less overflows). In practice, this is probably not
> important.

It doesn't overflow because of the current value of CLOCK_FREQ. If 
CLOCK_FREQ changes (to 100,000,000 for example) then the macro is doing 
division by 0. The function is correct except it might overflow.

To avoid this I would like the add static asserts in the various 
functions added by these patches. But that means moving the code. Or 
putting the assert somewhere else (with relevant explanation).

We could also do some #if depending on CLOCK_FREQ > divider or not and 
do the operations the other way around in that case.

>
>> +}
>> +
>> +static inline msftime_t msftime_from_vlc_tick(vlc_tick_t vtk)
>> +{
>> +    return vtk * INT64_C(10000000) / CLOCK_FREQ;
>> +}
>> +
>>   /*****************************************************************************
>>    * MSTRTIME_MAX_SIZE: maximum possible size of mstrtime
>>    *****************************************************************************
>> -- 
>> 2.17.0
>>
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