Subzero: Improve performance of liveness analysis and live range construction.

src/IceTimerTree.cpp

 //===- subzero/src/IceTimerTree.cpp - Pass timer defs ---------------------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines the TimerTree class, which tracks flat and
@@ skipping 63 matching lines @@
 
 // At a state change (e.g. push or pop), updates the flat and
 // cumulative timings for everything on the timer stack.
 void TimerStack::update() {
   ++StateChangeCount;
   // Whenever the stack is about to change, we grab the time delta
   // since the last change and add it to all active cumulative
   // elements and to the flat element for the top of the stack.
   double Current = timestamp();
   double Delta = Current - LastTimestamp;
-  LastTimestamp = Current;
   if (StackTop) {
     TimerIdT Leaf = Nodes[StackTop].Interior;
     if (Leaf >= LeafTimes.size())
       LeafTimes.resize(Leaf + 1);
     LeafTimes[Leaf] += Delta;
   }
   TTindex Prefix = StackTop;
   while (Prefix) {
     Nodes[Prefix].Time += Delta;
     TTindex Next = Nodes[Prefix].Parent;
     assert(Next < Prefix);
     Prefix = Next;
   }
+  // Capture the next timestamp *after* the updates are finished.
+  // This minimizes how much the timer can perturb the reported
+  // timing.  The numbers may not sum to 100%, and the missing amount
+  // is indicative of the overhead of timing.
+  LastTimestamp = timestamp();
 }
 
 void TimerStack::reset() {
   StateChangeCount = 0;
   FirstTimestamp = LastTimestamp = timestamp();
   LeafTimes.assign(LeafTimes.size(), 0);
   for (TimerTreeNode &Node : Nodes) {
     Node.Time = 0;
   }
 }
@@ skipping 45 matching lines @@
   dumpHelper(Str, FlatMap, TotalTime);
   Str << "Number of timer updates: " << StateChangeCount << "\n";
 }
 
 double TimerStack::timestamp() {
   // TODO: Implement in terms of std::chrono for C++11.
   return llvm::TimeRecord::getCurrentTime(false).getWallTime();
 }
 
 } // end of namespace Ice