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HP_TIMING uses native timestamping instructions if available, thus greatly reducing the overhead of recording start and end times for function calls. For architectures that don't have HP_TIMING available, we fall back to the clock_gettime bits. One may also override this by invoking the benchmark as follows: make USE_CLOCK_GETTIME=1 bench and get the benchmark results using clock_gettime. One has to do `make bench-clean` to ensure that the benchmark programs are rebuilt.