[PATCH] D41953: [LoopUnroll] Unroll and Jam

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

I was being a bit vague, what I meant was that if we have a pass structure like this, as we have now but with added unroll and jam:

  Loop Pass Manager
    Induction Variable Simplification
    Recognize loop idioms
    Delete dead loops
    Unroll and Jam
    Unroll loops

With a nest with inner and outer loops, the passes will be done in this order:

  IndVar simplify inner loop
  Recognise inner loop
  Delete Dead inner loop
  UnJ inner loop
  Unroll inner loop
  IndVar simplify outer loop
  Recognise outer loop
  Delete Dead outer loop
  UnJ outer loop
  Unroll outer loop

So when Unj gets to the outer loop, the inner loop may have been unrolled. So the unroll for the inner loop would have to know whether the loop nest was profitable to unroll and jam and -not- unroll in that case. To get the UnJ of the outer loop to happen before the unroll of the inner loop, two loop pass managers are needed (in this case one just for the Unroll Loops). This can affect code generation for existing cases, which was something I was hoping to avoid. I saw some cases of infinite loops being generated differently when I had it like that for testing.

What I am hoping is that we won't have to add UnJ here in the pass pipeline, we can add it later at the other place that we do unrolling. This should allow us to more easily have two loop pass managers, simplifying things a lot :)


Repository:
  rL LLVM

https://reviews.llvm.org/D41953