Pointer aliasing

Pointer aliasing happens when a single memory location is accessible through different symbolic names in the programs. The name pointer aliasing applies specifically to accessing the memory location through pointers, although the same effect can be achieved when accessing the memory location through references, or any other way.

Pointer aliasing is notorious for breaking compiler optimizations and introducing hidden loop dependencies. Consider the following example:

void foo(double *a, double *b, int n) {
  for (int i = 0; i < n; i++) {
    b[i] += a[i];
  }
}

In case pointers a and b point to distinct memory blocks, this code is perfectly parallelizable. But, consider that someone called the function with the following arguments:

foo(a, a + 1, n - 1);

If this is the case, pointers a and b alias and the loop cannot be parallelized because of the loop-carried dependency.