[boost] [chrono] steady_clock efficiency

Subject: [boost] [chrono] steady_clock efficiency
From: Kenneth Porter (shiva.blacklist_at_[hidden])
Date: 2011-11-28 22:22:16

I expect most people will use boost::chrono::steady_clock to measure 
performance. I've been considering switching from my raw calls to the 
Windows peformance counter APIs to the steady_clock. I'm a little worried 
about the overhead of what should be a non-intrusive facility.

In examing the source, I see that steady_clock::now() reads the performance 
counter and then multiplies it by the reciprocal of the counter's frequency 
(acquired on first use of the clock) to return the time in nanoseconds. How 
expensive is this 64 bit multiply on a 32 bit system? How hard would it be 
to have now() return a time_point that stores the raw tick value and only 
converts it to nanoseconds when a calculation in real units is required? 
(Comparison to another time_point could still be done in performance 
counter ticks, saving the multiply.) Is it worth the extra complexity to 
demand-convert ticks to nanoseconds?

Source is in boost_1_48_0\boost\chrono\detail\inlined\win\chrono.cpp