Burst Inference Failure 9 - 2020.2 English

Explanation

Burst inferencing will fail if it detects overlapping accesses of array elements. Consider rewriting the code to eliminate any overlapping accesses in the same region.

Examples

• Loop-carried dependency with RAW dependence:

  //////////// ORIGINAL ////////////
  void foo(int *a) {
    for (long i = 2; i < 256; ++i)
      a[i] = a[i-2];
  }
   
  //////////// UPDATED ////////////
  // Remove overlapping accesses in the same region
  void foo(int *a, int *b, int n) {
    int t0 = a[0];
    int t1 = a[1];
    for (int i = 2; i < n; ++i) {
      int t2 = t0;
      a[i] = t2;
      t2 = t1;
      t1 = t0;
      t0 = t2;
    }
  }

• Anti-dependency (WAR):

  //////////// ORIGINAL ////////////
  void foo(int *a, int n) {
    int buff[50];
    for (long i = 0; i < n; ++i) {
      buff[i] = a[i] + 100;
      ...
      a[i] = buff[i];
    }
  }
   
  //////////// UPDATED ////////////
  // Remove overlapping accesses in the same region
  void foo(int *a, int n) {
    int buff[50];
    for (long i = 0; i < n; ++i) {
      buff[i] = a[i] + 100;
      ...
    }
    for (long i = 0; i < n; ++i)
      a[i] = buff[i];
  }