Reuse empty StoreBufferBlocks.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index a93e869d2e1a9029c520dcb66c0dfa12f08f481d..0bf07a909a9b35771bf2030dfb67905a094c793e 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -14,7 +14,6 @@
 #include "vm/megamorphic_cache_table.h"
 #include "vm/metrics.h"
 #include "vm/random.h"
-#include "vm/store_buffer.h"
 #include "vm/tags.h"
 #include "vm/thread.h"
 #include "vm/os_thread.h"
@@ -76,6 +75,7 @@ class ServiceIdZone;
 class Simulator;
 class StackResource;
 class StackZone;
+class StoreBuffer;
 class StubCode;
 class TypeArguments;
 class TypeParameter;
@@ -141,7 +141,7 @@ class Isolate : public BaseIsolate {
                                   bool visit_prologue_weak_persistent_handles);
   void VisitPrologueWeakPersistentHandles(HandleVisitor* visitor);
 
-  StoreBuffer* store_buffer() { return &store_buffer_; }
+  StoreBuffer* store_buffer() { return store_buffer_; }
 
   ClassTable* class_table() { return &class_table_; }
   static intptr_t class_table_offset() {
@@ -722,7 +722,7 @@ class Isolate : public BaseIsolate {
 
   Thread* mutator_thread_;
   uword vm_tag_;
-  StoreBuffer store_buffer_;
+  StoreBuffer* store_buffer_;

[Ivan Posva, 2015/06/16 21:48:19]

I don't see the reason why this was necessary. store_buffer.h does not need
anything really from within isolate.h, so it is not likely going to be recursive
includes.

[koda, 2015/06/18 17:40:22]

Not necessary, but the reasoning is in the CL description:

"Make Isolate::store_buffer_ a pointer, now that it's less frequently accessed,
to keep the offsets of other fields small"

Other fields of Isolate will likely still be accessed from generated code, even
after the thread refactoring, and keeping the offsets small enough to use the
best encoding seems like a good general rule.

Also, minimizing dependencies helps keep the VM build time reasonable.

[Ivan Posva, 2015/06/18 18:28:31]

My concern was about dependent loads in generated code, which is not going to be
an issue in this case.

   ClassTable class_table_;
   MegamorphicCacheTable megamorphic_cache_table_;
   Dart_MessageNotifyCallback message_notify_callback_;