[all-commits] [llvm/llvm-project] 5c7bbe: [MachinePipeliner] Refine the RecMII calculation

[lsaba via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 5c7bbe3659a04c1d17deb3b50ab5b88204327842
      https://github.com/llvm/llvm-project/commit/5c7bbe3659a04c1d17deb3b50ab5b88204327842
  Author: Lama <lama.saba at intel.com>
  Date:   2020-04-13 (Mon, 13 Apr 2020)

  Changed paths:
    M llvm/include/llvm/CodeGen/MachinePipeliner.h

  Log Message:
  -----------
  [MachinePipeliner] Refine the RecMII calculation

In the case of more than one SDep  between two successor SUnits in the Nodeset, the current implementation sums the latencies of the dependencies, which could create a larger RecMII than necessary.

for example, in case there is both a data dependency and an output dependency (with latency > 0) between successor nodes:
SU(1) inst1:
  successors:
    SU(2): out  latency = 1
    SU(2): data latency = 1
SU(2) inst2:
  successors:
    SU(3): out  latency = 1
    SU(3): data latency = 1
SU(3) inst3:
  successors:
    SU(1): out  latency = 1
    SU(1): data latency = 1

the NodeSet latency returned would be 6, whereas it could be 3 if we take the max for each successor SUnit.
In general this can be extended to finding the shortest path in the recurrence..
thoughts?

Unfortunately I had a hard time creating a test for this in Hexagon/PowerPC, so help would be appreciated.

Reviewed By: bcahoon

Differential Revision: https://reviews.llvm.org/D75918