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Re: [PATCH] rs6000: Add a builtin to read the time base register on PowerPC


On Wed, 29 Aug 2012, Michael Meissner wrote:
> On Wed, Aug 29, 2012 at 01:56:05PM -0400, Hans-Peter Nilsson wrote:
> > On Wed, 29 Aug 2012, Segher Boessenkool wrote:
> > > On some systems the timebase runs at a rather low frequency, say 20MHz.
> > > This test will spuriously fail there.  Waste a million CPU cycles before
> > > reading TB the second time?
> >
> > Waste said million cycles portably by calling sched_yield()?
> > (Available only on POSIX systems. :)
>
> Well only for a test environment.  You don't want to call sched_yield in the
> normal case, since the apps that do this many millions of times need this to be
> as a fast as possible.

Surely, but IMHO what goes for the normal case is not a valid
reading of "waste"..."millions of cycles". ;)

Point being, for simulator environments, you may not want the
loop that was suggested later.  On the other hand, that might
not be an observable period, either.

brgds, H-P


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