Resolve some disagreements between SIMARM and XARM on offsets used in compiled code.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 0afc840e6a34e8c2b52512e7833a2ad668e7e107..2ec47d38ae438c7acc011f8e1f02cf421032a4d2 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -191,9 +191,6 @@ class Isolate : public BaseIsolate {
 
   ObjectStore* object_store() const { return object_store_; }
   void set_object_store(ObjectStore* value) { object_store_ = value; }
-  static intptr_t object_store_offset() {
-    return OFFSET_OF(Isolate, object_store_);
-  }
 
   // DEPRECATED: Use Thread's methods instead. During migration, these default
   // to using the mutator thread (which must also be the current thread).
@@ -819,11 +816,18 @@ class Isolate : public BaseIsolate {
 
   template<class T> T* AllocateReusableHandle();
 
-  uword vm_tag_;
+  // Accessed from generated code:
+  uword stack_limit_;

[Florian Schneider, 2015/09/30 09:38:13]

Add a COMPILE_ASSERT at for the first member, and maybe ALIGNx for the others to
make sure the offsets are consistent across compilers.

[rmacnak, 2015/10/01 22:00:49]

The compiler won't allow use of OFFSET_OF in a COMPILE_ASSERT.

runtime/vm/globals.h:54:69: error: a cast to a type other than an integral or
enumeration type cannot appear in a constant-expression
   (reinterpret_cast<intptr_t>(&(reinterpret_cast<type*>(kOffsetOfPtr)->field))

[Florian Schneider, 2015/10/02 13:41:09]

gcc has the offsetof macro which can appear as compile-time constant - Not sure
about MSVC, but the problem you're fixing here affects gcc, so we could use it
here under an #ifdef.

   StoreBuffer* store_buffer_;
   Heap* heap_;
-  ThreadRegistry* thread_registry_;
+  uword vm_tag_;
+  uword user_tag_;
+  RawUserTag* current_tag_;
+  RawUserTag* default_tag_;
   ClassTable class_table_;
+  bool single_step_;
+
+  ThreadRegistry* thread_registry_;
   Dart_MessageNotifyCallback message_notify_callback_;
   char* name_;
   char* debugger_name_;
@@ -840,7 +844,6 @@ class Isolate : public BaseIsolate {
   Dart_LibraryTagHandler library_tag_handler_;
   ApiState* api_state_;
   Debugger* debugger_;
-  bool single_step_;
   bool resume_request_;
   int64_t last_resume_timestamp_;
   bool has_compiled_;
@@ -850,7 +853,6 @@ class Isolate : public BaseIsolate {
   TimerList timer_list_;
   intptr_t deopt_id_;
   Mutex* mutex_;  // protects stack_limit_ and saved_stack_limit_.
-  uword stack_limit_;
   uword saved_stack_limit_;
   uword stack_base_;
   uword stack_overflow_flags_;
@@ -886,10 +888,8 @@ class Isolate : public BaseIsolate {
   Mutex profiler_data_mutex_;
 
   VMTagCounters vm_tag_counters_;
-  uword user_tag_;
   RawGrowableObjectArray* tag_table_;
-  RawUserTag* current_tag_;
-  RawUserTag* default_tag_;
+
 
   RawGrowableObjectArray* collected_closures_;
   RawGrowableObjectArray* deoptimized_code_array_;