[Patch, fortran] PR 56919 SYSTEM_CLOCK on Windows

[Tobias Burnus]

Janne Blomqvist wrote:
> Attached is an updated patch which uses GetTickCount for 
> system_clock_4; this should be fine as system_clock_4 wraps around in 
> ~25 days anyways. For system_clock_8 it uses 
> QueryPerformance{Counter,Frequency}. 

> The patch also adds an additional check for _POSIX_MONOTONIC_CLOCK.
> Ok for trunk?

Regarding the documentation, I wonder whether one should do the 
following additional changes:
- Explicitly suggest to use a kind=8 argument für system_clock (for 
higher resolution and to avoid overflows).
- To change the system_clock example to use an integer(8) argument. 
Possibly, via iso_fortran_env's int64 or via selected_int_kind(18)


+ #if defined(CLOCK_MONOTONIC) && defined(_POSIX_MONOTONIC_CLOCK)

I'd add
   && _POSIX_MONOTONIC_CLOCK >= 0
as POSIX states: "If a symbolic constant is defined with the value -1, 
the option is not supported."


+      uint32_t cnt = GetTickCount ();

I wonder whether a comment stating that GetTickCount instead of 
QueryPerformanceCounter is used as the extra precision and 49.7-days 
overflow do not matter with the 32bit system_clock - and as 
QueryPerformanceCounter has issues on some (very few) systems.


Otherwise, it looks fine to me.

Tobias