Index: llvm/lib/Transforms/Scalar/LoopRerollPass.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopRerollPass.cpp
+++ llvm/lib/Transforms/Scalar/LoopRerollPass.cpp
@@ -879,6 +879,8 @@
 bool LoopReroll::DAGRootTracker::validateRootSet(DAGRootSet &DRS) {
   if (DRS.Roots.empty())
     return false;
+  if (hasUsesOutsideLoop(DRS.BaseInst, L))
+    return false;
 
   // Consider a DAGRootSet with N-1 roots (so N different values including
   //   BaseInst).
Index: llvm/test/Transforms/LoopReroll/external_use.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopReroll/external_use.ll
@@ -0,0 +1,60 @@
+; RUN: opt < %s -loop-reroll -S | FileCheck %s
+
+; Check whether rerolling is rejected if values of the base and root
+; instruction are used outside the loop block.
+
+; Only the base/root instructions except a loop increment instruction
+define void @test1() {
+entry:
+  br label %loop1
+
+loop1:
+;CHECK-LABEL: loop1:
+;CHECK-NEXT:   %indvar = phi i64 [ 0, %entry ], [ %indvar.next, %loop1 ]
+;CHECK-NEXT:   %indvar.1 = add nsw i64 %indvar, 1
+
+  %indvar = phi i64 [ 0, %entry ], [ %indvar.next, %loop1 ]
+  %indvar.1 = add nsw i64 %indvar, 1
+  %indvar.next = add nsw i64 %indvar, 2
+  %cmp = icmp slt i64 %indvar.next, 200
+  br i1 %cmp, label %loop1, label %exit
+
+exit:
+  %var1 = phi i64 [ %indvar.1, %loop1 ]
+  %var2 = phi i64 [ %indvar, %loop1 ]
+  ret void
+}
+
+; Both the base/root instructions and reduction instructions
+define void @test2() {
+entry:
+  br label %loop2
+
+loop2:
+;CHECK-LABEL: loop2:
+;CHECK-NEXT:   %indvar = phi i32  [ 0, %entry ], [ %indvar.next, %loop2 ]
+;CHECK-NEXT:   %redvar = phi i32 [ 0, %entry ], [ %add.2, %loop2 ]
+;CHECK-NEXT:   %indvar.1 = add nuw nsw i32 %indvar, 1
+;CHECK-NEXT:   %indvar.2 = add nuw nsw i32 %indvar, 2
+
+  %indvar = phi i32 [ 0, %entry ], [ %indvar.next, %loop2 ]
+  %redvar = phi i32 [ 0, %entry ], [ %add.2, %loop2 ]
+  %indvar.1 = add nuw nsw i32 %indvar, 1
+  %indvar.2 = add nuw nsw i32 %indvar, 2
+  %mul.0 = mul nsw i32 %indvar, %indvar
+  %mul.1 = mul nsw i32 %indvar.1, %indvar.1
+  %mul.2 = mul nsw i32 %indvar.2, %indvar.2
+  %add.0 = add nsw i32 %redvar, %mul.0
+  %add.1 = add nsw i32 %add.0, %mul.1
+  %add.2 = add nsw i32 %add.1, %mul.2
+  %indvar.next = add nuw nsw i32 %indvar, 3
+  %cmp = icmp slt i32 %indvar.next, 300
+  br i1 %cmp, label %loop2, label %exit
+
+exit:
+  %a = phi i32 [ %indvar, %loop2 ]
+  %b = phi i32 [ %indvar.1, %loop2 ]
+  %c = phi i32 [ %indvar.2, %loop2 ]
+  %x = phi i32 [ %add.2, %loop2 ]
+  ret void
+}