Effectiveness of GCC optmization on bit operations

Question

Here are two ways to set an individual bit in C on x86-64:

inline void SetBitC(long *array, int bit) {
   //Pure C version
   *array |= 1<

        

Answer 1

For such code:

#include 
#include 

int main() {
  volatile long long i = 0;
  time_t start = time (NULL);
  for (long long n = 0; n < (1LL << 32); n++) {
    i |= 1 << 10;
  }
  time_t end = time (NULL);
  printf("C took %ds\n", (int)(end - start));
  start = time (NULL);
  for (long long n = 0; n < (1LL << 32); n++) {
    __asm__ ("bts %[bit], %[i]"
                  : [i] "=r"(i)
                  : "[i]"(i), [bit] "i" (10));
  }
  end = time (NULL);
  printf("ASM took %ds\n", (int)(end - start));
}

the result was:

C took 12s
ASM took 10s

My flag was (-std=gnu99 -O2 -march=core2). Without the volatile the loop was optimized out. gcc 4.4.2.

No difference was with:

__asm__ ("bts %[bit], %[i]"
              : [i] "+m"(i)
              : [bit] "r" (10));

So probably the answer was - noone cares. In microbenchmark the only difference is the one between those two methods but in real life I belive such code does not take much CPU.

Additionally for such code:

#include 
#include 

int main() {
  volatile long long i = 0;
  time_t start = time (NULL);
  for (long long n = 0; n < (1L << 32); n++) {
    i |= 1 << (n % 32);
  }
  time_t end = time (NULL);
  printf("C took %ds\n", (int)(end - start));
  start = time (NULL);
  for (long long n = 0; n < (1L << 32); n++) {
    __asm__ ("bts %[bit], %[i]"
                  : [i] "+m"(i)
                  : [bit] "r" (n % 32));
  }
  end = time (NULL);
  printf("ASM took %ds\n", (int)(end - start));
}

The result was:

C took 9s
ASM took 10s

Both results were 'stable'. Testing CPU 'Intel(R) Core(TM)2 Duo CPU T9600 @ 2.80GHz'.