PWR017: Using countable while loops instead of for loops may inhibit vectorization

Issue

Using countable while loops instead of for loops may inhibit vectorization.

Actions

Rewrite the loop so that its number of iterations can be counted.

Relevance

Many loops can benefit from automatic compiler vectorization. However, while loops are more difficult to vectorize than for loops. For a compiler to be able to vectorize a loop, certain requirements need to be met. One of them being that the loop is countable, i.e. the number of iterations is known before entering the loop so that the compiler can chunk the data into vectors. Normally, compilers can successfully count the number of iterations of most for loops; however, they tend to struggle for other types such as while loops. This is especially true for older compilers. Thus, the use of for loops is encouraged.

Code examples

Consider the following loop:

while (i < n) {
    b[i] = a[i];
    i++;
}
int example(int *A) {
  int sum = 0;
  int i = 0;
  while (i < 1000) {
     sum += A[i++];
  }
  return sum;
}

The number of iterations for this loop is known before the loop starts executing, and this loop is therefore countable. It can be converted to a for loop, like this:

int sum = 0;
for (int i = 0; i < 1000; i++) {
    sum += A[i];
}

Related resources

• PWR017 examples at GitHub

References

• Vectorization
• OpenMP canonical form