Explain requirements on fields accessed from generated code.

runtime/vm/thread.h

 // Copyright (c) 2015, 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_THREAD_H_
 #define RUNTIME_VM_THREAD_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 655 matching lines @@
   void InitVMConstants();
 
 #ifndef PRODUCT
   void PrintJSON(JSONStream* stream) const;
 #endif
 
  private:
   template <class T>
   T* AllocateReusableHandle();
 
-  // 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.
   uword stack_limit_;
   uword stack_overflow_flags_;
   Isolate* isolate_;
   Heap* heap_;
   uword top_exit_frame_info_;
   StoreBufferBlock* store_buffer_block_;
   uword vm_tag_;
   TaskKind task_kind_;
 // State that is cached in the TLS for fast access in generated code.
 #define DECLARE_MEMBERS(type_name, member_name, expr, default_init_value)      \
@@ skipping 113 matching lines @@
 // Disable thread interrupts.
 class DisableThreadInterruptsScope : public StackResource {
  public:
   explicit DisableThreadInterruptsScope(Thread* thread);
   ~DisableThreadInterruptsScope();
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_THREAD_H_