Add historgram for number of GC needed to collect a detached context.

src/isolate.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
 
 #include "src/v8.h"
@@ skipping 2577 matching lines @@
   int length = detached_contexts->length();
   if (length == 0) return;
   int new_length = 0;
   for (int i = 0; i < length; i += 2) {
     int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
     WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
     if (!cell->cleared()) {
       detached_contexts->set(new_length, Smi::FromInt(mark_sweeps + 1));
       detached_contexts->set(new_length + 1, cell);
       new_length += 2;
+    } else {
+      counters()->detached_context_age_in_gc()->AddSample(mark_sweeps);
     }
   }
-  PrintF("%d detached contexts are collected out of %d\n", length - new_length,
-         length);
-  for (int i = 0; i < new_length; i += 2) {
-    int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
-    WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
-    if (mark_sweeps > 3) {
-      PrintF("detached context 0x%p\n survived %d GCs (leak?)\n",
-             static_cast<void*>(cell->value()), mark_sweeps);
+  if (FLAG_trace_detached_contexts) {
+    PrintF("%d detached contexts are collected out of %d\n",
+           length - new_length, length);
+    for (int i = 0; i < new_length; i += 2) {
+      int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
+      WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
+      if (mark_sweeps > 3) {
+        PrintF("detached context 0x%p\n survived %d GCs (leak?)\n",
+               static_cast<void*>(cell->value()), mark_sweeps);
+      }
     }
   }
-  if (length == new_length) {
+  if (new_length == 0) {
     heap()->set_detached_contexts(heap()->empty_fixed_array());
-  } else {
+  } else if (new_length < length) {
     heap()->RightTrimFixedArray<Heap::FROM_GC>(*detached_contexts,
                                                length - new_length);
   }
 }
 
 
 bool StackLimitCheck::JsHasOverflowed() const {
   StackGuard* stack_guard = isolate_->stack_guard();
 #ifdef USE_SIMULATOR
   // The simulator uses a separate JS stack.
@@ skipping 10 matching lines @@
   if (prev_ && prev_->Intercept(flag)) return true;
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 } }  // namespace v8::internal