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

[via llvm-commits]

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@@ -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.
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aankit-ca wrote:

Can you add a comment here on it's purpose?

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