Heap maps: add VM service message.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
===================================================================
--- runtime/vm/pages.cc	(revision 33596)
+++ runtime/vm/pages.cc	(working copy)
@@ -428,6 +428,36 @@
 }
 
 
+class HeapMapAsJSONVisitor : public ObjectVisitor {
+ public:
+  explicit HeapMapAsJSONVisitor(JSONArray* array)
+      : ObjectVisitor(NULL), array_(array) {}
+  virtual void VisitObject(RawObject* obj) {
+    array_->AddValue(obj->Size() / kObjectAlignment);
+    // Freelist is most interesting, so map it to a fixed unused value.
+    ASSERT(0 == kIllegalCid);
+    array_->AddValue(obj->IsFreeListElement() ? 0 : obj->GetClassId());

[Cutch, 2014/03/12 19:02:02]

I'm concerned about mapping kFreeListElement to kIllegalCid.

What if kept it as kFreeListElement include the index in the HeapMap message
(see below).

[koda, 2014/03/12 19:08:01]

Good idea. Done.

+  }
+ private:
+  JSONArray* array_;
+};
+
+
+void PageSpace::PrintHeapMapToJSONStream(JSONStream* stream) {
+  JSONObject heap_map(stream);
+  heap_map.AddProperty("type", "HeapMap");
+  heap_map.AddProperty("id", "heapmap");

[Cutch, 2014/03/12 19:02:02]

heap_map.AddProperty("free_class_id", kFreeListElement);

[koda, 2014/03/12 19:08:01]

Done.

+  {
+    JSONArray all_pages(&heap_map, "pages");

[Cutch, 2014/03/12 19:02:02]

Maybe add a comment here saying the format of "pages" is:

[page0, page1, ..., pageN]

Format of a page is:

[size, class id, ..., size, class id]

[koda, 2014/03/12 19:08:01]

Done.

+    for (HeapPage* page = pages_; page != NULL; page = page->next()) {
+      JSONArray page_map(&all_pages);
+      HeapMapAsJSONVisitor printer(&page_map);
+      page->VisitObjects(&printer);
+    }
+  }
+}
+
+
 bool PageSpace::ShouldCollectCode() {
   // Try to collect code if enough time has passed since the last attempt.
   const int64_t start = OS::GetCurrentTimeMicros();