[llvm] [MachinePipeliner] Add an abstract layer to manipulate Data Dependenc… (PR #109918)

[Ryotaro Kasuga via llvm-commits]

================
@@ -3184,18 +3171,21 @@ SmallSet<SUnit *, 8> SMSchedule::computeUnpipelineableNodes(
     if (SU.isInstr() && PLI->shouldIgnoreForPipelining(SU.getInstr()))
       Worklist.push_back(&SU);
 
+  const SwingSchedulerDDG *DDG = SSD->getDDG();
   while (!Worklist.empty()) {
     auto SU = Worklist.pop_back_val();
     if (DoNotPipeline.count(SU))
       continue;
     LLVM_DEBUG(dbgs() << "Do not pipeline SU(" << SU->NodeNum << ")\n");
     DoNotPipeline.insert(SU);
-    for (auto &Dep : SU->Preds)
-      Worklist.push_back(Dep.getSUnit());
-    if (SU->getInstr()->isPHI())
-      for (auto &Dep : SU->Succs)
-        if (Dep.getKind() == SDep::Anti)
-          Worklist.push_back(Dep.getSUnit());
+    for (const auto &IE : DDG->getInEdges(SU))
+      Worklist.push_back(IE.getSrc());
+
+    // To preserve previous behavior and prevent regression
+    // FIXME: Remove if this doesn't have significant impact on
+    for (const auto &OE : DDG->getOutEdges(SU))
+      if (OE.getDistance() == 1)
+        Worklist.push_back(OE.getDst());
----------------
kasuga-fj wrote:

>  I think those edges are actually in the new DDG, the dependent nodes should already be in the worklist

This is true.

Then, it looks like that it's safe to remove both of them. I'd like to remove them in the future to simplify the code, if there is no performance degradation. Thank you for your review!

https://github.com/llvm/llvm-project/pull/109918