Index: llvm/include/llvm/CodeGen/MachineScheduler.h
===================================================================
--- llvm/include/llvm/CodeGen/MachineScheduler.h
+++ llvm/include/llvm/CodeGen/MachineScheduler.h
@@ -756,7 +756,8 @@
 
   unsigned getOtherResourceCount(unsigned &OtherCritIdx);
 
-  void releaseNode(SUnit *SU, unsigned ReadyCycle);
+  template <bool InPQueue>
+  void releaseNode(SUnit *SU, unsigned ReadyCycle, unsigned Idx = 0);
 
   void bumpCycle(unsigned NextCycle);
 
@@ -954,7 +955,7 @@
     if (SU->isScheduled)
       return;
 
-    Top.releaseNode(SU, SU->TopReadyCycle);
+    Top.releaseNode<false>(SU, SU->TopReadyCycle);
     TopCand.SU = nullptr;
   }
 
@@ -962,7 +963,7 @@
     if (SU->isScheduled)
       return;
 
-    Bot.releaseNode(SU, SU->BotReadyCycle);
+    Bot.releaseNode<false>(SU, SU->BotReadyCycle);
     BotCand.SU = nullptr;
   }
 
@@ -1042,7 +1043,7 @@
   void releaseTopNode(SUnit *SU) override {
     if (SU->isScheduled)
       return;
-    Top.releaseNode(SU, SU->TopReadyCycle);
+    Top.releaseNode<false>(SU, SU->TopReadyCycle);
   }
 
   // Only called for roots.
Index: llvm/lib/CodeGen/MachineScheduler.cpp
===================================================================
--- llvm/lib/CodeGen/MachineScheduler.cpp
+++ llvm/lib/CodeGen/MachineScheduler.cpp
@@ -2083,7 +2083,8 @@
   return OtherCritCount;
 }
 
-void SchedBoundary::releaseNode(SUnit *SU, unsigned ReadyCycle) {
+template <bool InPQueue>
+void SchedBoundary::releaseNode(SUnit *SU, unsigned ReadyCycle, unsigned Idx) {
   assert(SU->getInstr() && "Scheduled SUnit must have instr");
 
 #ifndef NDEBUG
@@ -2094,17 +2095,30 @@
     MaxObservedStall = std::max(ReadyCycle - CurrCycle, MaxObservedStall);
 #endif
 
-  if (ReadyCycle < MinReadyCycle)
-    MinReadyCycle = ReadyCycle;
-
   // Check for interlocks first. For the purpose of other heuristics, an
   // instruction that cannot issue appears as if it's not in the ReadyQueue.
   bool IsBuffered = SchedModel->getMicroOpBufferSize() != 0;
-  if ((!IsBuffered && ReadyCycle > CurrCycle) || checkHazard(SU) ||
-      Available.size() >= ReadyListLimit)
-    Pending.push(SU);
-  else
+  bool HazardDetected = ((!IsBuffered && ReadyCycle > CurrCycle) ||
+                         checkHazard(SU));
+
+  // Check on Available's size already performed, if SU is in Pending queue.
+  // See 'SchedBoundary::releasePending'.
+  if (!InPQueue)
+    HazardDetected = HazardDetected || Available.size() >= ReadyListLimit;
+
+  if (!HazardDetected) {
     Available.push(SU);
+
+    if (ReadyCycle < MinReadyCycle)
+      MinReadyCycle = ReadyCycle;
+
+    if (InPQueue)
+      Pending.remove(Pending.begin() + Idx);
+    return;
+  }
+
+  if (!InPQueue)
+    Pending.push(SU);
 }
 
 /// Move the boundary of scheduled code by one cycle.
@@ -2344,26 +2358,14 @@
 
   // Check to see if any of the pending instructions are ready to issue.  If
   // so, add them to the available queue.
-  bool IsBuffered = SchedModel->getMicroOpBufferSize() != 0;
-  for (unsigned i = 0, e = Pending.size(); i != e; ++i) {
-    SUnit *SU = *(Pending.begin()+i);
-    unsigned ReadyCycle = isTop() ? SU->TopReadyCycle : SU->BotReadyCycle;
-
-    if (ReadyCycle < MinReadyCycle)
-      MinReadyCycle = ReadyCycle;
-
-    if (!IsBuffered && ReadyCycle > CurrCycle)
-      continue;
-
-    if (checkHazard(SU))
-      continue;
-
+  for (signed E = Pending.size() - 1; E >= 0; --E) {
     if (Available.size() >= ReadyListLimit)
       break;
 
-    Available.push(SU);
-    Pending.remove(Pending.begin()+i);
-    --i; --e;
+    SUnit *SU = *(Pending.begin() + E);
+    unsigned ReadyCycle = isTop() ? SU->TopReadyCycle : SU->BotReadyCycle;
+
+    releaseNode<true>(SU, ReadyCycle, E);
   }
   CheckPending = false;
 }