[llvm] [MachinePipeliner] Introduce a new class for loop-carried deps (PR #137663)

[via llvm-commits]

================
@@ -266,6 +266,82 @@ struct SUnitWithMemInfo {
   bool getUnderlyingObjects();
 };
 
+/// Add loop-carried chain dependencies. This class handles the same type of
+/// dependencies added by `ScheduleDAGInstrs::buildSchedGraph`, but takes into
+/// account dependencies across iterations.
+class LoopCarriedOrderDepsTracker {
+  // Type of instruction that is relevant to order-dependencies
+  enum class InstrTag {
+    Barrier = 0,      ///< A barrier event instruction.
+    LoadOrStore = 1,  ///< An instruction that may load or store memory, but is
+                      ///< not a barrier event.
+    FPExceptions = 2, ///< An instruction that does not match above, but may
+                      ///< raise floatin-point exceptions.
+  };
+
+  struct TaggedSUnit : PointerIntPair<SUnit *, 2> {
+    TaggedSUnit(SUnit *SU, InstrTag Tag)
+        : PointerIntPair<SUnit *, 2>(SU, unsigned(Tag)) {}
+
+    InstrTag getTag() const { return InstrTag(getInt()); }
+  };
+
+  /// Holds loads and stores with memory related information.
+  struct LoadStoreChunk {
+    SmallVector<SUnitWithMemInfo, 4> Loads;
+    SmallVector<SUnitWithMemInfo, 4> Stores;
+
+    void append(SUnit *SU);
+  };
+
+  SwingSchedulerDAG *DAG;
+  BatchAAResults *BAA;
+  std::vector<SUnit> &SUnits;
+
+  /// The size of SUnits, for convenience.
+  const unsigned N;
+
+  /// Loop-carried Edges.
+  std::vector<BitVector> LoopCarried;
+
+  /// Instructions related to chain dependencies. They are one of the
+  /// following:
+  ///
+  ///  1. Barrier event.
+  ///  2. Load, but neither a barrier event, invariant load, nor may load trap
+  ///     value.
+  ///  3. Store, but not a barrier event.
+  ///  4. None of them, but may raise floating-point exceptions.
+  ///
+  /// This is used when analyzing loop-carried dependencies that access global
+  /// barrier instructions.
+  std::vector<TaggedSUnit> TaggedSUnits;
+
+  const TargetInstrInfo *TII = nullptr;
+  const TargetRegisterInfo *TRI = nullptr;
+
+public:
+  LoopCarriedOrderDepsTracker(SwingSchedulerDAG *SSD, BatchAAResults *BAA,
+                              const TargetInstrInfo *TII,
+                              const TargetRegisterInfo *TRI);
+
+  /// The main function to compute loop-carried order-dependencies.
+  void computeDependencies();
+
+  const BitVector &getLoopCarried(unsigned Idx) const {
----------------
aankit-ca wrote:

Rename to getLoopCarriedEdges;

https://github.com/llvm/llvm-project/pull/137663