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

[Steve Lhomme]

On 2018-06-22 9:19 PM, Rémi Denis-Courmont wrote:
> As I already pointed out, CLOCK_FREQ must remain a multiple of one 
> million and a divisor of one billion. Too much problems with overflow 
> and rounding otherwise.

100,000,000 is one of these.



>
> Besides POSIX, ISO and Microsoft all agree there.
>
> Le 22 juin 2018 14:28:08 GMT+01:00, Steve Lhomme <robux4 at ycbcr.xyz> a 
> écrit :
>
>     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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