Index: include/llvm/Support/Timer.h
===================================================================
--- include/llvm/Support/Timer.h
+++ include/llvm/Support/Timer.h
@@ -77,9 +77,8 @@
 ///
 class Timer {
   TimeRecord Time;       // The total time captured
-  TimeRecord StartTime;  // The time startTimer() was last called
   std::string Name;      // The name of this time variable.
-  bool Running;          // Is the timer currently running?
+  unsigned RefCount;     // How many times has this timer been started?
   bool Triggered;        // Has the timer ever been triggered?
   TimerGroup *TG;        // The TimerGroup this Timer is in.
 
@@ -119,7 +118,10 @@
   void clear();
 
   /// Return the duration for which this timer has been running.
-  TimeRecord getTotalTime() const { return Time; }
+  TimeRecord getTotalTime() const {
+    assert(RefCount == 0 && "Cannot query active timer");
+    return Time;
+  }
 
 private:
   friend class TimerGroup;
Index: lib/Support/Timer.cpp
===================================================================
--- lib/Support/Timer.cpp
+++ lib/Support/Timer.cpp
@@ -101,8 +101,10 @@
 
 void Timer::init(StringRef N, TimerGroup &tg) {
   assert(!TG && "Timer already initialized");
+  Time = TimeRecord();
   Name.assign(N.begin(), N.end());
-  Running = Triggered = false;
+  Triggered = false;
+  RefCount = 0;
   TG = &tg;
   TG->addTimer(*this);
 }
@@ -136,21 +138,23 @@
 }
 
 void Timer::startTimer() {
-  assert(!Running && "Cannot start a running timer");
-  Running = Triggered = true;
-  StartTime = TimeRecord::getCurrentTime(true);
+  Triggered = true;
+  if (!RefCount)
+    Time -= TimeRecord::getCurrentTime(true);
+  ++RefCount;
 }
 
 void Timer::stopTimer() {
-  assert(Running && "Cannot stop a paused timer");
-  Running = false;
-  Time += TimeRecord::getCurrentTime(false);
-  Time -= StartTime;
+  assert(RefCount > 0 && "Cannot stop a paused timer");
+  --RefCount;
+  if (!RefCount)
+    Time += TimeRecord::getCurrentTime(false);
 }
 
 void Timer::clear() {
-  Running = Triggered = false;
-  Time = StartTime = TimeRecord();
+  RefCount = 0;
+  Triggered = false;
+  Time = TimeRecord();
 }
 
 static void printVal(double Val, double Total, raw_ostream &OS) {
Index: unittests/Support/TimerTest.cpp
===================================================================
--- unittests/Support/TimerTest.cpp
+++ unittests/Support/TimerTest.cpp
@@ -62,4 +62,13 @@
   EXPECT_FALSE(T1.hasTriggered());
 }
 
+TEST(Timer, Nesting) {
+  Timer T1("T1");
+  T1.startTimer();
+  T1.startTimer();
+  T1.stopTimer();
+  T1.stopTimer();
+  EXPECT_TRUE(T1.hasTriggered());
+}
+
 } // end anon namespace