Fixes from n64 branch

[clktmr]

Here are some changes from #6 that are unrelated to mips64 support. I created a separate PR to get your opinion on this. Please have a look at the commits.

[clktmr]

Should be an atomic to do this properly

[michalderkacz]

Nice and simple solution.

Yes, use internal/atomic.Xadd64. It should work on a 64-bit CPU without any problem but I don't know what about 32-bit ARM.

Try test it on your Teensy if you can. Or apply to the the wip and try to use GOTARGET=noostest GOARCH=thumb emgo test. You must set the system timer back to this implementation because the ARMv7-M SYSTICK is used by default. Don't forgot to set the start value of dummyNanoseconds to something bigger that the last rtos.Nanotime().

For now I plan to collect such new noos tests here: https://github.com/embeddedgo/noostest/tree/master/tests

If you are unsure how to do all this please ask me. I can also try to invent a proper test myself if yow want.

[michalderkacz]

I cherry-picked 4b8a81f into master-embedded.

According dummyNanotime I checked all usleep and usleep_no_g calls:

runtime/proc.go:1014.3-1014.8:    usleep(1000)
runtime/proc.go:1029.3-1029.8:    usleep(1000)
runtime/proc.go:1032.2-1032.7:    usleep(1000)
runtime/proc.go:1034.2-1034.7:    usleep(1000)
runtime/proc.go:5973.3-5973.8:    usleep(delay)
runtime/proc.go:6841.8-6841.13:  usleep(3)

runtime/proc.go:576.2-576.12:      usleep_no_g(100)
runtime/proc.go:2547.4-2547.14:  usleep_no_g(1)

I think we can try to implement both usleep and usleep_no_g as no-op function (like in os_js.go).

As I see, the usleep is used mainly to give the another threads a chance to run before something is done, but it's always a "best effort" action. So maybe we can implement usleep like this:

func usleep(us uint32) {
  for i := range (1 + us/16) {
    osyield()
    // or maybe yield_m()
  }
}

[michalderkacz]

It seems that sysmon uses usleep as some kind of real timer. So I propose to implement the usleep and usleep_no_g functions this way:

func usleep(us uint32) {
  if thetasker.nanotime != dummyNanotime && tasker.newnanotime == nil {
    nanosleep(uint64(us) * 1000)
    return
  }
  for i := range (1 + us/32) {
    osyield()
  }
}

The syssetsystim1 should set tasker.newnanotime to nil after setting tasker.nanotime.