Explain requirements on fields accessed from generated code.

runtime/vm/isolate.h

 // 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.
 
 #ifndef RUNTIME_VM_ISOLATE_H_
 #define RUNTIME_VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 678 matching lines @@
                         bool is_mutator,
                         bool bypass_safepoint = false);
 
   // DEPRECATED: Use Thread's methods instead. During migration, these default
   // to using the mutator thread (which must also be the current thread).
   Zone* current_zone() const {
     ASSERT(Thread::Current() == mutator_thread());
     return mutator_thread()->zone();
   }
 
-  // Accessed from generated code:
+  // Accessed from generated code.
+  // ** This block of fields must come first! **
+  // For AOT cross-compilation, we rely on these members having the same offsets
+  // in SIMARM(IA32) and ARM, and the same offsets in SIMARM64(X64) and ARM64.
+  // We use only word-sized fields to avoid differences in struct packing on the
+  // different architectures. See also CheckOffsets in dart.cc.
   StoreBuffer* store_buffer_;
   Heap* heap_;
   uword user_tag_;
   RawUserTag* current_tag_;
   RawUserTag* default_tag_;
   RawCode* ic_miss_code_;
   ObjectStore* object_store_;
   ClassTable class_table_;
   bool single_step_;
 
@@ skipping 277 matching lines @@
   intptr_t* spawn_count_;
 
   Dart_IsolateFlags isolate_flags_;
   bool paused_;
   bool errors_are_fatal_;
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_ISOLATE_H_