-rw-r--r-- 1735 djbsort-20180729/cpucycles/dev4ns/cpucycles.c
#include <sys/types.h> #include <fcntl.h> #include <time.h> #include <stdio.h> #include <unistd.h> #include <sys/time.h> #include "cpucycles.h" static int fddev = -1; static int prev[3]; static unsigned long long prevcycles = 0; static int now[3]; static long long cyclespersec = 0; static void readdev(int *result) { if (read(fddev,result,12) == 12) return; result[0] = result[1] = result[2] = 0; } long long cpucycles(void) { unsigned long long delta4; int deltan; int deltas; unsigned long long guesscycles; if (fddev == -1) { fddev = open("/dev/cpucycles4ns",O_RDONLY); readdev(prev); } readdev(now); delta4 = (unsigned int) (now[0] - prev[0]); /* unsigned change in number of cycles mod 2^32 */ deltan = now[1] - prev[1]; /* signed change in number of nanoseconds mod 10^9 */ deltas = now[2] - prev[2]; /* signed change in number of seconds */ if ((deltas == 0 && deltan < 200000000) || (deltas == 1 && deltan < -800000000)) return prevcycles + delta4; prev[0] = now[0]; prev[1] = now[1]; prev[2] = now[2]; if ((deltas == 0 && deltan < 300000000) || (deltas == 1 && deltan < -700000000)) { // actual number of cycles cannot have increased by 2^32 in <0.3ms cyclespersec = 1000000000 * (unsigned long long) delta4; cyclespersec /= deltan + 1000000000 * (long long) deltas; } else { guesscycles = deltas * cyclespersec; guesscycles += (deltan * cyclespersec) / 1000000000; while (delta4 + 2147483648ULL < guesscycles) delta4 += 4294967296ULL; /* XXX: could do longer-term extrapolation here */ } prevcycles += delta4; return prevcycles; } long long cpucycles_persecond(void) { while (!cyclespersec) cpucycles(); return cyclespersec; }