Explain requirements on fields accessed from generated code.

runtime/vm/scavenger.h

 // Copyright (c) 2012, 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_SCAVENGER_H_
 #define RUNTIME_VM_SCAVENGER_H_
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/dart.h"
@@ skipping 236 matching lines @@
     return end_ < to_->end();
   }
 
   void UpdateMaxHeapCapacity();
   void UpdateMaxHeapUsage();
 
   void ProcessWeakReferences();
 
   intptr_t NewSizeInWords(intptr_t old_size_in_words) const;
 
-  // Current allocation top and end. These values are being accessed directly
-  // 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 top_;
   uword end_;
 
   SemiSpace* to_;
 
   Heap* heap_;
 
   // A pointer to the first unscanned object.  Scanning completes when
   // this value meets the allocation top.
   uword resolved_top_;
@@ skipping 22 matching lines @@
 
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;
 
   DISALLOW_COPY_AND_ASSIGN(Scavenger);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_SCAVENGER_H_