Explaining the Data Dependency Issue Exposed in "A cache miss is not a cache miss"

iterate-linked-list.c

struct pair { int first, int second };

/* LOAD() is just a pointer dereference. Written this way to make it stand out. */
#define LOAD(expr) (*(expr))

void iterate_linked_list(vector)
{
	register void *vector_start = vector->start;
	register long index = 0;
	register long sum = 0;
	register void *ppair;

LOOP:   ppair = vector_start + index*sizeof(struct pair);
	value = LOAD(ppair + sizeof(int));   /* pair.second */
	sum += value; 
	index = LOAD(ppair);                 /* pair.first  */
	if (index != 0)
		goto LOOP;

	/* Data dependency between loop iterations
	 * "value = LOAD(ppair)" depends on the previous iteration of itself:
	 *
	 * "value = LOAD(ppair)" depends on 'ppair'
	 * 'ppair' depends on 'index'
	 * 'index' depends on the previous iteration of "LOAD(ppair)".
	 *
	 * This data dependency doesn't exist in iterate_pointers(). */

OUTPUT: output = sum;
	return;
}

iterate-pointers.c

void iterate_pointers(vector)
{
	register void *vector_start = vector->start;
	register void *vector_finish = vector->finish;
	register void *ppair = vecotr_start;
	register long sum = 0;

LOOP:   if (ppair == vector_finish)
		goto OUTPUT;
	index = LOAD(ppair);
	value = LOAD(vector_start + index*sizeof(struct pair) + sizeof(int));
	sum += value;
	ppair += sizeof(struct pair);
	goto LOOP;

OUTPUT: output = sum;
	return;
}