Index: llvm/lib/Transforms/Scalar/LoopFuse.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopFuse.cpp
+++ llvm/lib/Transforms/Scalar/LoopFuse.cpp
@@ -389,7 +389,13 @@
   /// Comparison functor to sort two Control Flow Equivalent fusion candidates
   /// into dominance order.
   /// If LHS dominates RHS and RHS post-dominates LHS, return true;
-  /// IF RHS dominates LHS and LHS post-dominates RHS, return false;
+  /// If RHS dominates LHS and LHS post-dominates RHS, return false;
+  /// If both LHS and RHS are not dominating each other then, non-strictly
+  /// post dominate check will decide the order of candidates. If RHS
+  /// non-strictly post dominates LHS then, return true. If LHS non-strictly
+  /// post dominates RHS then, return false. If both are non-strictly post
+  /// dominate each other then, level in the post dominator tree will decide
+  /// the order of candidates.
   bool operator()(const FusionCandidate &LHS,
                   const FusionCandidate &RHS) const {
     const DominatorTree *DT = &(LHS.DT);
@@ -415,9 +421,29 @@
       return true;
     }
 
-    // If LHS does not dominate RHS and RHS does not dominate LHS then there is
-    // no dominance relationship between the two FusionCandidates. Thus, they
-    // should not be in the same set together.
+    // If two FusionCandidates are in the same level of dominator tree,
+    // they will not dominate each other, but may still be control flow
+    // equivalent. To sort those FusionCandidates, nonStrictlyPostDominate()
+    // function is needed.
+    bool WrongOrder =
+        nonStrictlyPostDominate(LHSEntryBlock, RHSEntryBlock, DT, LHS.PDT);
+    bool RightOrder =
+        nonStrictlyPostDominate(RHSEntryBlock, LHSEntryBlock, DT, LHS.PDT);
+    if (WrongOrder && RightOrder) {
+      // If common predecessor of LHS and RHS post dominates both
+      // FusionCandidates then, Order of FusionCandidate can be
+      // identified by its level in post dominator tree.
+      DomTreeNode *LNode = LHS.PDT->getNode(LHSEntryBlock);
+      DomTreeNode *RNode = LHS.PDT->getNode(RHSEntryBlock);
+      return LNode->getLevel() > RNode->getLevel();
+    } else if (WrongOrder)
+      return false;
+    else if (RightOrder)
+      return true;
+
+    // If LHS does not non-strict Postdominate RHS and RHS does not non-strict
+    // Postdominate LHS then, there is no dominance relationship between the
+    // two FusionCandidates. Thus, they should not be in the same set together.
     llvm_unreachable(
         "No dominance relationship between these fusion candidates!");
   }
Index: llvm/test/Transforms/LoopFusion/undominated_loops.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopFusion/undominated_loops.ll
@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=loop-fusion -S | FileCheck %s
+define void @test_long_1() {
+; CHECK-LABEL: @test_long_1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 true, label [[FOR_BODY6_PREHEADER:%.*]], label [[ENTRY_VECTOR_BODY_CRIT_EDGE:%.*]]
+; CHECK:       entry.vector.body_crit_edge:
+; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
+; CHECK:       vector.body:
+; CHECK-NEXT:    br i1 true, label [[VECTOR_BODY_FOR_COND_CLEANUP5_CRIT_EDGE:%.*]], label [[VECTOR_BODY]]
+; CHECK:       vector.body.for.cond.cleanup5_crit_edge:
+; CHECK-NEXT:    br label [[FOR_COND_CLEANUP5:%.*]]
+; CHECK:       for.body6.preheader:
+; CHECK-NEXT:    br label [[FOR_COND_CLEANUP5]]
+; CHECK:       for.cond.cleanup5:
+; CHECK-NEXT:    br i1 true, label [[FOR_BODY17_PREHEADER:%.*]], label [[FOR_COND_CLEANUP5_VECTOR_BODY23_CRIT_EDGE:%.*]]
+; CHECK:       for.cond.cleanup5.vector.body23_crit_edge:
+; CHECK-NEXT:    br label [[VECTOR_BODY23:%.*]]
+; CHECK:       vector.body23:
+; CHECK-NEXT:    br i1 true, label [[MIDDLE_BLOCK15:%.*]], label [[VECTOR_BODY23]]
+; CHECK:       middle.block15:
+; CHECK-NEXT:    unreachable
+; CHECK:       for.body17.preheader:
+; CHECK-NEXT:    unreachable
+;
+entry:
+  br i1 true, label %for.body6.preheader, label %entry.vector.body_crit_edge
+
+entry.vector.body_crit_edge:                      ; preds = %entry
+  br label %vector.body
+
+vector.body:                                      ; preds = %entry.vector.body_crit_edge, %vector.body
+  br i1 true, label %vector.body.for.cond.cleanup5_crit_edge, label %vector.body
+
+vector.body.for.cond.cleanup5_crit_edge:          ; preds = %vector.body
+  br label %for.cond.cleanup5
+
+for.body6.preheader:                              ; preds = %entry
+  br label %for.cond.cleanup5
+
+for.cond.cleanup5:                                ; preds = %vector.body.for.cond.cleanup5_crit_edge, %for.body6.preheader
+  br i1 true, label %for.body17.preheader, label %for.cond.cleanup5.vector.body23_crit_edge
+
+for.cond.cleanup5.vector.body23_crit_edge:        ; preds = %for.cond.cleanup5
+  br label %vector.body23
+
+vector.body23:                                    ; preds = %for.cond.cleanup5.vector.body23_crit_edge, %vector.body23
+  br i1 true, label %middle.block15, label %vector.body23
+
+middle.block15:                                   ; preds = %vector.body23
+  br label %entry.vector.body_crit_edge
+
+for.body17.preheader:                             ; preds = %for.cond.cleanup5
+  unreachable
+}