[llvm] [MCP] Move dependencies if they block copy propagation (PR #105562)

[Quentin Colombet via llvm-commits]

================
@@ -92,6 +106,113 @@ static cl::opt<cl::boolOrDefault>
     EnableSpillageCopyElimination("enable-spill-copy-elim", cl::Hidden);
 
 namespace {
+// A ScheduleDAG subclass that is used as a dependency graph.
+class ScheduleDAGMCP : public ScheduleDAGInstrs {
+public:
+  void schedule() override {
+    llvm_unreachable("This schedule dag is only used as a dependency graph for "
+                     "Machine Copy Propagation\n");
+  }
+
+  ScheduleDAGMCP(MachineFunction &MF, const MachineLoopInfo *MLI,
+                 bool RemoveKillFlags = false)
+      : ScheduleDAGInstrs(MF, MLI, RemoveKillFlags) {
+    CanHandleTerminators = true;
+  }
+};
+
+static std::optional<llvm::SmallVector<MachineInstr *>>
+moveInstructionsOutOfTheWayIfWeCan(SUnit *Dst, SUnit *Src) {
+  MachineInstr *DstInstr = Dst->getInstr();
+  MachineInstr *SrcInstr = Src->getInstr();
+  MachineBasicBlock *MBB = SrcInstr->getParent();
+
+  if (DstInstr == nullptr || SrcInstr == nullptr)
+    return {};
+  assert("This function only operates on a basic block level." &&
+         MBB == SrcInstr->getParent());
+
+  int SectionSize =
+      std::distance(SrcInstr->getIterator(), DstInstr->getIterator());
+
+  // The bit vector representing the instructions in the section.
+  // This vector stores which instruction needs to be moved and which does not.
+  BitVector SectionInstr(SectionSize, false);
+
+  // The queue for the breadth first search.
+  std::queue<const SUnit *> Edges;
+
+  // Process the children of a node.
+  // Basically every node are checked before it is being put into the queue.
+  // A node is enqueued if it has no dependencies on the source of the copy
+  // (only if we are not talking about the destination node which is a special
+  // case indicated by a flag) and is located between the source of the copy and
+  // the destination of the copy.
+  auto ProcessSNodeChildren = [SrcInstr, &SectionSize, &SectionInstr](
+                                  std::queue<const SUnit *> &Queue,
+                                  const SUnit *Node, bool IsRoot) -> bool {
+    for (llvm::SDep I : Node->Preds) {
+      SUnit *SU = I.getSUnit();
+      MachineInstr &MI = *(SU->getInstr());
+      if (!IsRoot && &MI == SrcInstr)
+        return false;
----------------
qcolombet wrote:

What does the returned boolean mean?

https://github.com/llvm/llvm-project/pull/105562