RFR (S): 8211175: Remove temporary clock initialization duplication

[Mikael Vidstedt]

Great cleanup, thanks for doing this! Looks good.

One question:

If I read this correctly _clock_gettime and _clock_getres are always set up together - they’re either both NULL, or both set to a valid function? (With this change the exception to that seems to be BSD (but not OSX/APPLE), but AFAICT the BSD code could/should also use the posix code.)

Asuming they’re both set up, should this (new code):

   if (pthread_getcpuclockid_func &&
       pthread_getcpuclockid_func(_main_thread, &clockid) == 0 &&
      os::Posix::clock_getres(clockid, &tp) == 0 && tp.tv_sec == 0) {
     _supports_fast_thread_cpu_time = true;
     _pthread_getcpuclockid = pthread_getcpuclockid_func;

Perhaps be this:

   if (pthread_getcpuclockid_func &&
       pthread_getcpuclockid_func(_main_thread, &clockid) == 0 &&
      os::Posix::supports_monotonic_clock() &&  // Added check here
      os::Posix::clock_getres(clockid, &tp) && // No need to check return value here
      tp.tv_sec == 0) {
     _supports_fast_thread_cpu_time = true;
     _pthread_getcpuclockid = pthread_getcpuclockid_func;

With that change, as far as I can tell there are no calls to os::Posix::clock_gettime or os::Posix::clock_getres which do *not* expect to get back status == 0, in which case the code in clock_gettime and clock_getres code could be rewritten to be an assert or guarantee instead, and never return -1 failure - they could even be void return functions. Thoughts?

Cheers,
Mikael

> On Oct 1, 2018, at 11:08 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> webrev: http://cr.openjdk.java.net/~dholmes/8211175/webrev/
> bug: https://bugs.openjdk.java.net/browse/JDK-8211175
> 
> This cleans up some code duplication between os::Linux and os::Posix. os::Posix now exports the necessary clock functions so the Linux code can use those.
> 
> Also cleaned up the clock_getres syscall as that is already provided via the direct clock_getres (no need for a syscall).
> 
> Finally, as code in os::Linux already assumed clock_gettime etc was always available at build-time (and run-time for that matter) this is now explicitly checked via:
> 
> + #ifndef SUPPORTS_CLOCK_MONOTONIC
> + #error "Build platform doesn't support clock_gettime and related functionality"
> + #endif
> 
> Testing: tiers 1-3 (mach5)
> 
> Thanks,
> David