[PATCH] D144226: [Loop-Interchange] Allow inner-loop only reductions

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quic_aankit added a comment.

In D144226#4299835 <https://reviews.llvm.org/D144226#4299835>, @Meinersbur wrote:

> Summarize the algorithm in the description, in particular what a "removablePHIs" is and what distinguishes a inner-only reduction from an inner reduction?

@Meinersbur. Thank you for the review.

By inner-only reduction, I mean that the outer loop is not involved in the
reduction at all. For eg:

  for (j = 0; j < n; j++) {
      for (k = 0; k < l; k++) {
          z[j] += x[k]*y[n*k+j];
      }
  }

In the code above, reduction is performed on each z[j] only by the inner k loop. The
outer loop is not involved in the reduction at all. Loop interchange can handle such
cases if the load and store of z[j] remain within the inner loop. But due to LICM, the load
and store are moved into the outer loop, thereby creating a PHI in the inner-loop that
is not part of reduction across the outerloop. To handle this, we sink the loads
and hoist the stores to the inner loop. This eliminates the inner loop PHI and also
makes the two loops tightly nested, allowing us to loop interchange.

In the patch we try to keep track of such Load-LoadPHI-StorePHI-Store chains.
In places where we check if we can handle a PHI, we match if it's a "removablePHI" -a LoadPHI
or the StorePHI that can be removed by moving the loads and stores inwards

In the end, if we do decide that loop-interchange is profitable we move the loads
and store inwards and let LoopInterchange do the rest.


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https://reviews.llvm.org/D144226