[all-commits] [llvm/llvm-project] aadf55: [NFC] EliminateDuplicatePHINodes(): small-size opt...

[Roman Lebedev via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: aadf55d1cea24a4e5384ab8546c3d794cb1ec724
      https://github.com/llvm/llvm-project/commit/aadf55d1cea24a4e5384ab8546c3d794cb1ec724
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2020-09-17 (Thu, 17 Sep 2020)

  Changed paths:
    M llvm/lib/Transforms/Utils/Local.cpp

  Log Message:
  -----------
  [NFC] EliminateDuplicatePHINodes(): small-size optimization: if there are <= 32 PHI's, O(n^2) algo is faster (geomean -0.08%)

This is functionally equivalent to the old implementation.

As per https://llvm-compile-time-tracker.com/compare.php?from=5f4e9bf6416e45eba483a4e5e263749989fdb3b3&to=4739e6e4eb54d3736e6457249c0919b30f6c855a&stat=instructions
this is a clear geomean compile-time regression-free win with overall geomean of `-0.08%`

32 PHI's appears to be the sweet spot; both the 16 and 64 performed worse:
https://llvm-compile-time-tracker.com/compare.php?from=5f4e9bf6416e45eba483a4e5e263749989fdb3b3&to=c4efe1fbbfdf0305ac26cd19eacb0c7774cdf60e&stat=instructions
https://llvm-compile-time-tracker.com/compare.php?from=5f4e9bf6416e45eba483a4e5e263749989fdb3b3&to=e4989d1c67010d3339d1a40ff5286a31f10cfe82&stat=instructions

If we have more PHI's than that, we fall-back to the original DenseSet-based implementation,
so the not-so-fast cases will still be handled.

However compile-time isn't the main motivation here.
I can name at least 3 limitations of this CSE:
1. Assumes that all PHI nodes have incoming basic blocks in the same order (can be fixed while keeping the DenseMap)
2. Does not special-handle `undef` incoming values (i don't see how we can do this with hashing)
3. Does not special-handle backedge incoming values (maybe can be fixed by hashing backedge as some magical value)

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87408