Memcmp is the most efficient way to compare large chunks of data (mymicrocontroller.com)

The author's proposed for-loop version is:

  int *p = (int *)0x80000000;
  int *q = (int *)0x90000000;
  while(p < q){
	if(*p++ != *q++){
		flag = 1;
	}	
  }

There must have been a transcription error as that while loop doesn't look like it will exit until q reaches the end of memory and wraps back to 0.

On my machine, the following takes about 161 ms (that's the best of 5 timings on a non-quiet machine):

  result = memcmp(p, q, SIZE);

The author's for-loop is optimistic, and doesn't short-circuit exit on mismatch, so I'll replace it with a non-branching equivalent:

  result = 0;
  for (int i=0; i<SIZE/sizeof(long); i++) {
      result = result || (p[i] != q[i]);
  }

That takes 310 ms (compiled with Apple LLVM version 8.0.0, using -O3). About 1/2 the performance of built-in.

Replace the int with a long and it's 200ms. Manually unroll by 4 and it's 175ms. My final code is:

  result = 0;
  for (int i=0; i<SIZE/sizeof(long); i+=4) {
     result = result
       || (p[i] != q[i])
       || (p[i+1] != q[i+1])
       || (p[i+2] != q[i+2])
       || (p[i+3] != q[i+3]);
  }

That's only somewhat slower than the memcmp, not the 10x that the author reports.

Of course, memcmp does short-circuit evaluation, and I know the size is a multiple of sizeof(long), so this test case is biased in favor of my code.

I would, of course, always favor the tested, often optimized library code over rolling my own unless there are good reasons otherwise.