Timing code on Intel CPUs using "core clock cycles"?

Question

What is this method of timing code on Intel processors, referred to by Agnor fod as "core clock cycles":

http://gcc.gnu.org/ml/gcc/2008-07/msg00424.html

My test results, referred to above, uses the "core clock cycles" performance counter on Intel and RDTSC on AMD. It's the highest resolution you can get.

I always thought RDTSC was the best way to measure on Intel CPUs too? What is this other technique he speaks about and how do you measure using it?

Answer 1

He is referring to the Intel PCM, the Performance Counter Monitor.

http://software.intel.com/en-us/articles/intel-performance-counter-monitor-a-better-way-to-measure-cpu-utilization

By the way in the original post linked article, http://www.agner.org/optimize/optimizing_cpp.pdf

the poster explicity refers to it:

The time stamp counter is a little inaccurate on microprocessors that can change the clock frequency (Intel SpeedStep® technology). A more accurate measurement can be obtained with a performance monitor counter for "core clock cycles", using the test program mentioned above