[PATCH] D23510: MachineScheduler: Restore TopCand after comparing top and bottom candidates

[Tom Stellard via llvm-commits]

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In the GenericScheduler, before comparing top and bottom candidates, we set
TopCand.Reason to NoCand, so that we can determine wether or not it was selected
over BotCand.  However, in the case where the bottom candidate is selected,
TopCand needs to have Reason restored to its original value, otherwise
the scheduler will continue selecting from the bottom, until a new
instruction is added to the Top queue.

https://reviews.llvm.org/D23510

Files:
  lib/CodeGen/MachineScheduler.cpp

Index: lib/CodeGen/MachineScheduler.cpp
===================================================================
--- lib/CodeGen/MachineScheduler.cpp
+++ lib/CodeGen/MachineScheduler.cpp
@@ -3000,12 +3000,15 @@
   assert(BotCand.isValid());
   assert(TopCand.isValid());
   SchedCandidate Cand = BotCand;
+  GenericSchedulerBase::CandReason TopReason = TopCand.Reason;
   TopCand.Reason = NoCand;
   tryCandidate(Cand, TopCand, nullptr);
   if (TopCand.Reason != NoCand) {
     Cand.setBest(TopCand);
     DEBUG(traceCandidate(Cand));
-  }
+  } else
+    // Restore the original Reason for TopCand if it isn't selected.
+    TopCand.Reason = TopReason;
 
   IsTopNode = Cand.AtTop;
   tracePick(Cand);


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