[PATCH] D45733: [DAGCombiner] Unfold scalar masked merge if profitable

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

> If the mask is constant, right now i always unfold it.

Let me make sure I understand. The fold in question is:

  %n0 = xor i4 %x, %y
  %n1 = and i4 %n0, C1
  %r  = xor i4 %n1, %y
  =>
  %mx = and i4 %x, C1
  %my = and i4 %y, ~C1
  %r = or i4 %mx, %my

If that's correct, we need to take a step back here. If the fold is universally good, then it can go in InstCombine, and there's no need to add code bloat to the DAG to handle the pattern unless something in the backend can create this pattern (seems unlikely).

But we need to take another step back before we add code bloat to InstCombine. Is there evidence that this pattern exists in source (bug report, test-suite, etc) and affects analysis/performance? If not, is it worth the cost of adding a matcher for the pattern? It's a simple matcher, so the expense bar is low...but if it never happens, do we care?


Repository:
  rL LLVM

https://reviews.llvm.org/D45733