21.4 restrict Pointer Example

Here are examples where restrict enables real optimization.

In this example, restrict assures GCC that the array out points to does not overlap with the array in points to.

void
process_data (const char *in,
              char * restrict out,
              size_t size)
{
  for (i = 0; i < size; i++)
    out[i] = in[i] + in[i + 1];
}

Here’s a simple tree structure, where each tree node holds data of type PAYLOAD plus two subtrees.

struct foo
  {
    PAYLOAD payload;
    struct foo *left;
    struct foo *right;
  };

Now here’s a function to null out both pointers in the left subtree.

void
null_left (struct foo *a)
{
  a->left->left = NULL;
  a->left->right = NULL;
}

Since *a and *a->left have the same data type, they could legitimately alias (see Aliasing). Therefore, the compiled code for null_left must read a->left again from memory when executing the second assignment statement.

We can enable optimization, so that it does not need to read a->left again, by writing null_left in a less obvious way.

void
null_left (struct foo *a)
{
  struct foo *b = a->left;
  b->left = NULL;
  b->right = NULL;
}

A more elegant way to fix this is with restrict.

void
null_left (struct foo *restrict a)
{
  a->left->left = NULL;
  a->left->right = NULL;
}

Declaring a as restrict asserts that other pointers such as a->left will not point to the same memory space as a. Therefore, the memory location a->left->left cannot be the same memory as a->left. Knowing this, the compiled code may avoid reloading a->left for the second statement.