Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -224,13 +224,21 @@
          BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
     auto *PredBB = IncomingBB;
     auto *SuccBB = PhiBB;
+    SmallPtrSet<BasicBlock *, 16> Visited;
     while (true) {
       BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
       if (PredBr && PredBr->isConditional())
         return {PredBB, SuccBB};
+      Visited.insert(PredBB);
       auto *SinglePredBB = PredBB->getSinglePredecessor();
       if (!SinglePredBB)
         return {nullptr, nullptr};
+
+      // Stop searching when SinglePredBB has been visited. It means we see
+      // an unreachable loop.
+      if (Visited.count(SinglePredBB))
+        return {nullptr, nullptr};
+
       SuccBB = PredBB;
       PredBB = SinglePredBB;
     }
@@ -253,7 +261,9 @@
       return;
 
     BasicBlock *PredBB = PredOutEdge.first;
-    BranchInst *PredBr = cast<BranchInst>(PredBB->getTerminator());
+    BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
+    if (!PredBr)
+      return;
 
     uint64_t PredTrueWeight, PredFalseWeight;
     // FIXME: We currently only set the profile data when it is missing.
Index: llvm/test/Transforms/JumpThreading/unreachable-loops.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/unreachable-loops.ll
@@ -0,0 +1,63 @@
+; RUN: opt -jump-threading -S < %s | FileCheck %s
+; RUN: opt -passes=jump-threading -S < %s | FileCheck %s
+; Check the unreachable loop won't cause infinite loop
+; in jump-threading when it tries to update the predecessors'
+; profile metadata from a phi node.
+
+define void @unreachable_single_bb_loop() {
+; CHECK-LABEL: @unreachable_single_bb_loop()
+bb:
+  %tmp = call i32 @a()
+  %tmp1 = icmp eq i32 %tmp, 1
+  br i1 %tmp1, label %bb5, label %bb8
+
+; unreachable single bb loop.
+bb2:                                              ; preds = %bb2
+  %tmp4 = icmp ne i32 %tmp, 1
+  switch i1 %tmp4, label %bb2 [
+    i1 0, label %bb5
+    i1 1, label %bb8
+  ]
+
+bb5:                                              ; preds = %bb2, %bb
+  %tmp6 = phi i1 [ %tmp1, %bb ], [ false, %bb2 ]
+  br i1 %tmp6, label %bb8, label %bb7, !prof !0
+
+bb7:                                              ; preds = %bb5
+  br label %bb8
+
+bb8:                                              ; preds = %bb8, %bb7, %bb5, %bb2
+  ret void
+}
+
+define void @unreachable_multi_bbs_loop() {
+; CHECK-LABEL: @unreachable_multi_bbs_loop()
+bb:
+  %tmp = call i32 @a()
+  %tmp1 = icmp eq i32 %tmp, 1
+  br i1 %tmp1, label %bb5, label %bb8
+
+; unreachable two bbs loop.
+bb3:                                              ; preds = %bb2
+  br label %bb2
+
+bb2:                                              ; preds = %bb3
+  %tmp4 = icmp ne i32 %tmp, 1
+  switch i1 %tmp4, label %bb3 [
+    i1 0, label %bb5
+    i1 1, label %bb8
+  ]
+
+bb5:                                              ; preds = %bb2, %bb
+  %tmp6 = phi i1 [ %tmp1, %bb ], [ false, %bb2 ]
+  br i1 %tmp6, label %bb8, label %bb7, !prof !0
+
+bb7:                                              ; preds = %bb5
+  br label %bb8
+
+bb8:                                              ; preds = %bb8, %bb7, %bb5, %bb2
+  ret void
+}
+declare i32 @a()
+
+!0 = !{!"branch_weights", i32 2146410443, i32 1073205}