[Patch, fortran] PR 56919 SYSTEM_CLOCK on Windows

[Tobias Burnus]

Janne Blomqvist wrote:
> the attached patch implements the SYSTEM_CLOCK intrinsics on the MinGW
> and Cygwin targets using the GetTickCount/GetTickCount64 functions.
> These should be quite robust monotonic clocks and AFAICS are the best
> we can do on Windows.

I think using QueryPerformanceCounter is the better approach. It is 
supported since Windows 2000 and recommended as high-performance 
counter: 
http://msdn.microsoft.com/en-us/library/windows/desktop/ms644900%28v=vs.85%29.aspx

I really dislike GetTickCount, which overflows after 50 days - that's 
not what you want to have. And GetTickCount64 only exists since 
Vista/2008. By contrast, QueryPerformanceCounter should allow for finer 
resolution and it is already available since Windows 2000.


- Also Cygwin uses QueryPerformanceCounter for clock_gettime, now as is, 
before it subtracted some offset which caused it to start with 0 from 
process startup time. (I belive QueryPerformanceCounter counts from 
system startup.)
- MinGW-w64 also uses it  for the monotonic clock_gettime - at least in 
experimental/winpthreads.
- I think MinGW does't support it (yet?).

* * *

Regarding clock_gettime: I really think we should check check 
_POSIX_MONOTONIC_CLOCK as well. Currently,  only MONOTONIC_CLOCK is 
checked, which is always available (on POSIX conform systems).

See GLIBCXX_ENABLE_LIBSTDCXX_TIME in libstdc++-v3/acinclude.m4 - and in 
particular ac_has_clock_monotonic.

Tobias