Explain requirements on fields accessed from generated code.

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index 76a72adaa6f7a2e6904edbc344d189ce90db277b..b680a4f1bf6f6800fe7d11e3c28682abed975194 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -673,7 +673,12 @@ class Thread : public BaseThread {
   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_;