Implemented an averaging object histogram in the vm.

runtime/vm/heap_histogram.cc

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "vm/heap_histogram.h"
+
+#include "platform/assert.h"
+#include "vm/flags.h"
+#include "vm/object.h"
+
+namespace dart {
+
+DEFINE_FLAG(bool, print_object_histogram, false,
+            "Print average object histogram at isolate shutdown");
+
+class ObjectHistogramVisitor : public ObjectVisitor {
+ public:
+  explicit ObjectHistogramVisitor(Isolate* isolate) : ObjectVisitor(isolate) { }
+
+  virtual void VisitObject(RawObject* obj) {
+    isolate()->object_histogram()->Add(obj);
+  }
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ObjectHistogramVisitor);
+};
+
+
+void ObjectHistogram::Collect() {
+  major_gc_count_++;
+  ObjectHistogramVisitor object_visitor(Isolate::Current());

[Ivan Posva, 2013/06/12 15:18:09]

Why not use isolate_ instead of Isolate::Current() here?

[bakster, 2013/06/12 15:42:42]

Done.

+  isolate_->heap()->IterateObjects(&object_visitor);
+}
+
+
+ObjectHistogram::ObjectHistogram(Isolate* isolate) {
+  isolate_ = isolate;
+  major_gc_count_ = 0;
+  table_length_ = 512;
+  table_ = reinterpret_cast<Element*>(
+    calloc(table_length_, sizeof(Element)));  // NOLINT 
+  for (int index = 0; index < table_length_; index++) {
+    table_[index].class_id = index;
+  }
+}
+
+
+ObjectHistogram::~ObjectHistogram() {
+  free(table_);
+}
+
+
+void ObjectHistogram::RegisterClass(const Class& cls) {
+  int class_id = cls.id();
+  if (class_id < table_length_) return;
+  // Resize the table.
+  int new_table_length = table_length_ * 2;
+  Element* new_table = reinterpret_cast<Element*>(
+          realloc(table_, new_table_length * sizeof(Element)));  // NOLINT
+  for (int i = table_length_; i < new_table_length; i++) {
+    new_table[i].class_id = i;
+    new_table[i].count = 0;
+    new_table[i].size = 0;
+  }
+  table_ = new_table;
+  table_length_ = new_table_length;
+  ASSERT(class_id < table_length_);
+}
+
+
+void ObjectHistogram::Add(RawObject* obj) {
+  intptr_t class_id = obj->GetClassId();
+  if (class_id == kFreeListElement) return;
+  ASSERT(class_id < table_length_);
+  table_[class_id].Add(obj->Size());
+}
+
+
+int ObjectHistogram::compare(const Element** a, const Element** b) {
+  return (*b)->size - (*a)->size;
+}
+
+
+void ObjectHistogram::Print() {
+  HandleScope handle_scope(isolate_);

[Ivan Posva, 2013/06/12 15:18:09]

Why is there a HandleScope here?

[bakster, 2013/06/12 15:42:42]

Moved as suggested.

+  OS::Print("Printing Object Histogram\n");
+  OS::Print("____bytes___count_description____________\n");
+  // First count the number of non empty entries.
+  int length = 0;
+  for (int index = 0; index < table_length_; index++) {
+    if (table_[index].count > 0) length++;
+  }
+  // Then add them to a new array and sort.
+  Element** array = reinterpret_cast<Element**>(
+    calloc(length, sizeof(Element*)));  // NOLINT
+  int pos = 0;
+  for (int index = 0; index < table_length_; index++) {
+    if (table_[index].count > 0) array[pos++] = &table_[index];

[Ivan Posva, 2013/06/12 15:18:09]

You do not strictly need to have the class id in the elements, as the address of
the element in the sorted list encodes the class id when scaled correctly.

[bakster, 2013/06/12 15:42:42]

Correct, but that code is error prone in my mind.
Keeping class_id_ is the better solution. 

+  }
+  typedef int (*CmpFunc)(const void*, const void*);
+  qsort(array, length, sizeof(Element*),  // NOLINT
+        reinterpret_cast<CmpFunc>(compare));
+
+  // Finally print the sorted array.
+  Class& cls = Class::Handle();
+  String& str = String::Handle();
+  Library& lib = Library::Handle();
+  for (pos = 0; pos < length; pos++) {
+    Element* e = array[pos];
+    if (e->count > 0) {
+      cls = isolate_->class_table()->At(e->class_id);
+      str = cls.Name();
+      lib = cls.library();
+      OS::Print("%9d %7d ",
+                e->size / major_gc_count_,
+                e->count / major_gc_count_);
+      if (e->class_id < kNumPredefinedCids) {

[Ivan Posva, 2013/06/12 15:18:09]

To print VM names in back quotes this should really be kInstanceCid.

[bakster, 2013/06/12 15:42:42]

Done.

+        OS::Print("`%s`", str.ToCString());  // VM names.
+      } else {
+        OS::Print("%s", str.ToCString());
+      }
+      if (lib.IsNull()) {
+        OS::Print("\n");
+      } else {
+        str = lib.url();
+        OS::Print(", library \'%s\'\n", str.ToCString());
+      }
+    }
+  }
+  // Deallocate the array for sorting.
+  free(array);
+}
+
+}  // namespace dart