Improved phi reachability computation a bit.

src/hydrogen.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1657 matching lines @@
     connected_set->Add(i);
     connected_phis.Add(connected_set);
   }
 
   // (2) Do a fixed point iteration to find the set of connected phis.  A
   // phi is connected to another phi if its value is used either directly or
   // indirectly through a transitive closure of the def-use relation.
   bool change = true;
   while (change) {
     change = false;
-    for (int i = 0; i < phi_count; ++i) {
+    // We normally have far more "forward edges" than "backward edges",
+    // so we terminate faster when we walk backwards.
+    for (int i = phi_count - 1; i >= 0; --i) {
       HPhi* phi = phi_list->at(i);
       for (HUseIterator it(phi->uses()); !it.Done(); it.Advance()) {
         HValue* use = it.value();
         if (use->IsPhi()) {
           int id = HPhi::cast(use)->phi_id();
           if (connected_phis[i]->UnionIsChanged(*connected_phis[id]))
             change = true;
         }
       }
     }
@@ skipping 5127 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify();
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal