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

[Ryotaro Kasuga via llvm-commits]

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@@ -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());
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kasuga-fj wrote:

@dpenry 
Let me make sure. In terms of correctness, we can remove the for statement here, right? I added this for statement to preserve the behavior of the original implementation. This appends all predecessors of the SU, which may contain anti-dependencies from a PHI. There is a similar part in `normalizeNonPipelinedInstructions`, which I believe we can remove as well.

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