Identical loops that execute in different but consistent times

same-function.c

// gcc -std=gnu99 -O3 -Wall -Wextra same-function.c -o same-function
// Identical loops that execute in different but consistent times

#if COPY_AND_RUN_TO_TEST
for n in 0 1 2 3 4 5 6 7 8 9;
do echo same-function ${n}:;
/usr/bin/time -f "%e seconds" same-function ${n};
/usr/bin/time -f "%e seconds" same-function ${n};
/usr/bin/time -f "%e seconds" same-function ${n};
done
#endif

#include <stdio.h>
#include <stdint.h>

#define COMPILER_NO_INLINE __attribute__((noinline))

#define COUNT (1000 * 1000 * 1000)

volatile uint64_t counter = 0;

COMPILER_NO_INLINE void loop0(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop1(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop2(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop3(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop4(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop5(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop6(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop7(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop8(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

COMPILER_NO_INLINE void loop9(void) {
  for (int j = COUNT; j != 0; j--) {
    uint64_t val = counter;
    val = val + 1;
    counter = val;
  }
  return;
}

int main(int argc, char** argv) {
  if (argc <= 1) goto die_usage;

  char firstLetter = argv[1][0];
  switch (firstLetter) {
  case '0': loop0(); break;
  case '1': loop1(); break;
  case '2': loop2(); break;
  case '3': loop3(); break;
  case '4': loop4(); break;
  case '5': loop5(); break;
  case '6': loop6(); break;
  case '7': loop7(); break;
  case '8': loop8(); break;
  case '9': loop9(); break;
  default: goto die_usage;
  }

  return 0;

  die_usage:
  printf("Usage: [time] %s [0-9]\n", argv[0]);
  return 1;
}