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

[Quentin Colombet via llvm-commits]

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@@ -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());
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qcolombet wrote:

Can the size be negative?
I.e., are we sure `DstInstr` appears after `SrcInstr`?

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