Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -158,6 +158,8 @@
   TLI = &getAnalysis<TargetLibraryInfo>();
   LVI = &getAnalysis<LazyValueInfo>();
 
+  removeUnreachableBlocks(F);
+
   FindLoopHeaders(F);
 
   bool Changed, EverChanged = false;
Index: test/Transforms/JumpThreading/pr15851_hang.ll
===================================================================
--- /dev/null
+++ test/Transforms/JumpThreading/pr15851_hang.ll
@@ -0,0 +1,22 @@
+; RUN: timeout 10s opt -S -jump-threading < %s | FileCheck %s
+
+; CHECK-LABEL: @f(
+; CHECK-LABEL: entry
+; CHECK: ret void
+; CHECK-NOT: for.cond1
+; CHECK-NOT: for.body
+
+define void @f() {
+entry:
+  ret void
+
+for.cond1:
+  %i.025 = phi i32 [ %inc, %for.body ], [ %inc, %for.body ], [ 1, %for.cond1 ]
+  %cmp = icmp slt i32 %i.025, 2
+  br i1 %cmp, label %for.body, label %for.cond1
+
+for.body:
+  %inc = add nsw i32 %i.025, 0
+  %a = icmp ugt i32 %inc, 2
+  br i1 %a, label %for.cond1, label %for.cond1
+}
Index: test/Transforms/JumpThreading/select.ll
===================================================================
--- test/Transforms/JumpThreading/select.ll
+++ test/Transforms/JumpThreading/select.ll
@@ -127,7 +127,7 @@
 ; CHECK: test_switch_default
 ; CHECK: entry:
 ; CHECK: load
-; CHECK: switch
+; CHECK: icmp
 ; CHECK: [[THREADED:[A-Za-z.0-9]+]]:
 ; CHECK: store
 ; CHECK: br