[PATCH] D78030: [TimeProfiler] Emit clock synchronization point

[Sergej Jaskiewicz via Phabricator via cfe-commits]

broadwaylamb marked an inline comment as done.
broadwaylamb added inline comments.


================
Comment at: llvm/lib/Support/TimeProfiler.cpp:266
+      const auto BeginningOfTimeUs = SystemTime - ProcessLocalTime;
+      J.attribute("beginningOfTime",
+                  BeginningOfTimeUs.time_since_epoch().count());
----------------
MaskRay wrote:
> broadwaylamb wrote:
> > broadwaylamb wrote:
> > > MaskRay wrote:
> > > > What is the logic here?
> > > > 
> > > > Two events in the same process may have different `beginningOfTime`. Is that intended?
> > > > 
> > > > Noe that `system_clock::now()` and `steady_clock::now()` are not measured at the same time.
> > > >Two events in the same process may have different beginningOfTime. Is that intended?
> > > 
> > > How would that be possible? We only compute `beginningOfTime` when writing the data of the time profiler to a JSON file. AFAICT it happens only once. So, this code is only called once.
> > > 
> > > >Noe that system_clock::now() and steady_clock::now() are not measured at the same time.
> > > I understand. But the difference should be very small (certainly less than a microsecond), since there are only a few arithmetic operations in between.
> > Although we could set the beginning of time in `TimeProfiler`'s constructor and assign it to a field. That would be a lot simpler, I guess. What do you think?
> OK, but I am still confused by the logic and the comment. Does the code just want to compute the start time in system_clock?
Exactly, yes.


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https://reviews.llvm.org/D78030