Just calculate pretenuring ratio when needed.

src/objects-inl.h

 // Copyright 2012 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 1533 matching lines @@
   ASSERT(FLAG_allocation_site_pretenuring);
   int value = memento_create_count();
   set_memento_create_count(value + 1);
 }
 
 
 inline bool AllocationSite::DigestPretenuringFeedback() {
   bool decision_changed = false;
   int create_count = memento_create_count();
   int found_count = memento_found_count();
-  double ratio = static_cast<double>(found_count) / create_count;
+  double ratio = 0.0;
   PretenureFlag current_mode = GetPretenureMode();
 
   if (create_count >= kPretenureMinimumCreated) {
+    ratio = static_cast<double>(found_count) / create_count;

[mvstanton, 2014/02/06 11:23:33]

Per our chat, something like:

bool minimum_met = create_count >= kPretenureMinimumCreated;
double ratio = (minimum_met | FLAG_trace_pretenuring_statistics)
  ? static_cast<double>(found_count) / create_count
  : 0;

and later...
if (minimum_met) { ...decision code.

[Hannes Payer (out of office), 2014/02/06 11:26:53]

Done.

     PretenureDecision result = ratio >= kPretenureRatio
         ? kTenure
         : kDontTenure;
     set_pretenure_decision(result);
     if (current_mode != GetPretenureMode()) {
       decision_changed = true;
       dependent_code()->MarkCodeForDeoptimization(
           GetIsolate(),
           DependentCode::kAllocationSiteTenuringChangedGroup);
     }
   }
 
   if (FLAG_trace_pretenuring_statistics) {
+    ratio = static_cast<double>(found_count) / create_count;
     PrintF(
         "AllocationSite(%p): (created, found, ratio) (%d, %d, %f) %s => %s\n",
          static_cast<void*>(this), create_count, found_count, ratio,
          current_mode == TENURED ? "tenured" : "not tenured",
          GetPretenureMode() == TENURED ? "tenured" : "not tenured");
   }
 
   // Clear feedback calculation fields until the next gc.
   set_memento_found_count(0);
   set_memento_create_count(0);
@@ skipping 5206 matching lines @@
 #undef READ_UINT32_FIELD
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_