Add comment to compilation-cache.cc explaining choice of 5 generations.

src/compilation-cache.cc

 // Copyright 2008 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 25 matching lines @@
 // The number of sub caches covering the different types to cache.
 static const int kSubCacheCount = 4;
 
 // The number of generations for each sub cache.
 #if defined(ANDROID)
 static const int kScriptGenerations = 1;
 static const int kEvalGlobalGenerations = 1;
 static const int kEvalContextualGenerations = 1;
 static const int kRegExpGenerations = 1;
 #else
+// The number of ScriptGenerations is carefully chosen based on histograms.
+// See issue 458: http://code.google.com/p/v8/issues/detail?id=458
 static const int kScriptGenerations = 5;
 static const int kEvalGlobalGenerations = 2;
 static const int kEvalContextualGenerations = 2;
 static const int kRegExpGenerations = 2;
 #endif
 
 // Initial size of each compilation cache table allocated.
 static const int kInitialCacheSize = 64;
 
 // The compilation cache consists of several generational sub-caches which uses
@@ skipping 423 matching lines @@
 }
 
 
 void CompilationCache::Disable() {
   enabled = false;
   Clear();
 }
 
 
 } }  // namespace v8::internal