[PATCH] Break dependencies in large loops containing reductions (LoopVectorize)

[Olivier Sallenave]

Hi Michael,

> Probably I miss somethings, but what dependencies would unrolling of the outer loop break?


It would break the dependencies between the reduction operations. With the example below, we can dispatch twice more instructions (if target permits), which is profitable to exploit more ILP.

  // Original loop.
  for (int i = 0; i < n; i++) 
      for (int j = 0; j < 3; j++)
          r += arr[i][j];
  
  // After unrolling innermost loop.
  for (int i = 0; i < n; i++)  {
      r += arr[i][0];
      r += arr[i][1];
      r += arr[i][2];
  }
  
  // After unrolling outermost loop (in vectorizer, which breaks dependencies).
  for (int i = 0; i < n; i += 2)  {
      r += arr[i][0];
      r_0 += arr[i+1][0];
      r += arr[i][1];
      r_0 += arr[i+1][1];
      r += arr[i][2];
      r_0 += arr[i+1][2];
  }
  r += r_0;


http://reviews.llvm.org/D7514

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