Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -337,6 +337,8 @@
     dbgs() << "LVI for function '" << F.getName() << "':\n";
     LVI->printLVI(F, DTU.getDomTree(), dbgs());
   }
+  if (Changed)
+    EliminateUnreachableBlocks(F, &DTU);
   return Changed;
 }
 
Index: llvm/test/Transforms/JumpThreading/SimplifyUnreachableBlock.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/SimplifyUnreachableBlock.ll
@@ -0,0 +1,109 @@
+; RUN: opt < %s -jump-threading -S | FileCheck %s
+
+; JumpThreading removes the jump to bb27, then bb27 is removed, which makes
+; bb28, bb31, bb32 unreachable from entry.
+; But in bb11 we have
+;   %12 = phi i16 [ undef, %10 ], [ %21, %32 ]
+; The second incoming block %32 is just made unreachable. This causes problem
+; in later loop optimizations, because %21 is defined in loop body but used
+; in preheader which violates LCSSA. Removing the second incoming edge of
+; phinode %12 solves the problem.
+
+@c = internal unnamed_addr global i1 false, align 4
+@b = internal unnamed_addr global i1 false, align 4
+@h = internal unnamed_addr global i16* null, align 8
+@f = internal unnamed_addr global i1 false, align 8
+@d = internal unnamed_addr global i1 false, align 4
+
+define void @j() unnamed_addr {
+  %1 = alloca i16, align 4
+  %2 = load i1, i1* @c, align 4
+  br i1 %2, label %5, label %3
+
+3:                                                ; preds = %0
+  store i1 true, i1* @b, align 4
+  store i1 true, i1* @c, align 4
+  %4 = bitcast i16* %1 to i8*
+  call void @llvm.lifetime.start.p0i8(i64 2, i8* nonnull %4)
+  br label %10
+
+5:                                                ; preds = %0
+  %6 = load i1, i1* @b, align 4
+  %7 = bitcast i16* %1 to i8*
+  call void @llvm.lifetime.start.p0i8(i64 2, i8* nonnull %7)
+  br i1 %6, label %10, label %8
+
+8:                                                ; preds = %5
+  store i16* %1, i16** @h, align 8
+  br label %9
+
+9:                                                ; preds = %9, %8
+  br label %9
+
+10:                                               ; preds = %5, %3
+  store i1 true, i1* @d, align 4
+  br label %11
+
+11:                                               ; preds = %32, %10
+  %12 = phi i16 [ undef, %10 ], [ %21, %32 ]
+  %13 = load i1, i1* @f, align 8
+  %14 = zext i1 %13 to i16
+  br label %17
+
+15:                                               ; preds = %22
+  %16 = icmp slt i16 %23, 77
+  br i1 %16, label %25, label %27
+
+17:                                               ; preds = %25, %11
+  %18 = phi i16 [ 0, %11 ], [ %26, %25 ]
+  %19 = phi i16 [ %12, %11 ], [ %21, %25 ]
+  br label %20
+
+; CHECK: or
+20:                                               ; preds = %17
+  %21 = or i16 %19, %14
+  store i16 %21, i16* %1, align 4
+  br label %22
+
+22:                                               ; preds = %20
+  %23 = add i16 %18, 1
+  %24 = icmp slt i16 %23, 7
+  br i1 %24, label %25, label %15
+
+25:                                               ; preds = %22, %15
+  %26 = phi i16 [ %23, %22 ], [ 0, %15 ]
+  br label %17, !llvm.loop !17
+
+; This bb will be deleted because in bb15 the jump to here is removed.
+27:                                               ; preds = %15
+  br label %28
+
+; This bb will become unreachable because bb27 is deleted, but it
+; still has predecessor %32 so it won't be trivally deleted.
+; CHECK-NOT: load i1, i1* @d
+28:                                               ; preds = %27, %32
+  %29 = phi i16 [ %33, %32 ], [ %23, %27 ]
+  %30 = load i1, i1* @d, align 4
+  br i1 %30, label %32, label %31
+
+; This bb will become unreachable because bb27 is deleted.
+; CHECK-NOT: store i1 true, i1* @f
+31:                                               ; preds = %28
+  store i1 true, i1* @f, align 8
+  br label %32
+
+; This bb will become unreachable because bb27 is deleted.
+; CHECK-NOT: store i1 false, i1* @d
+32:                                               ; preds = %31, %28
+  %33 = and i16 %29, 11
+  store i1 false, i1* @d, align 4
+  %34 = icmp eq i16 %33, 0
+  br i1 %34, label %11, label %28, !llvm.loop !19
+}
+
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg %0, i8* nocapture %1)
+
+!17 = distinct !{!17, !18}
+!18 = !{!"llvm.loop.mustprogress"}
+!19 = distinct !{!19, !18}