PWR023: Add restrict for pointer function parameters to hint the compiler that vectorization is safe

Issue

Use restrict for pointer function parameters to hint the compiler that there is no pointer aliasing preventing vectorization.

Actions

Verify that there is no aliasing at the call sites and add restrict to the pointer function parameters.

Relevance

Compilers can automatically vectorize only those loops for which they can guarantee that the pointers do not alias one another, i.e. no memory address is accessible through two different pointers. The developer can use the restrict C keyword to inform the compiler that the specified block of memory is not aliased by any other block. Providing this information can help the compiler generate more efficient code or vectorize the loop. Therefore it is always recommended to use restrict whenever possible so that the compiler has as much information as possible to perform optimizations such as vectorization.

note

The compilers can emit runtime checks, however, in the presence of too many pointers, this is not possible.

Code example

The following loop exhibits potential pointer aliasing between pointers x and y:

int example(int *x, int *y) {
  int sum = 0;
  for (int i = 0; i < 10; ++i) {
    sum += x[i];
    y[i] = 2;
  }
  return sum;
}

If you are sure that no aliasing exists between the arguments, adding the restrict keyword will hint the compiler so that it can proceed with optimizations such as vectorization:

int example(int *restrict x, int *restrict y) {
  int sum = 0;
  for (int i = 0; i < 10; ++i) {
    sum += x[i];
    y[i] = 2;
  }
  return sum;
}