Merge branch 'john/erts/explain-deferred-fun-field-removal'

* john/erts/explain-deferred-fun-field-removal:
  erts: Explain why we can't get rid of old_index and old_uniq

Author: jhogberg

erts/emulator/beam/erl_fun.h

@@ -43,7 +43,17 @@ typedef struct erl_fun_entry {
     int arity;                      /* The arity of the fun. */
     int index;                      /* New style index. */
 
-    /* These have been prepared for removal in OTP 29. */
+    /* These are not used to identify the function anymore, and could in theory
+     * be removed at any time if we didn't care about backwards compatibility.
+     *
+     * However, we have leaked their presence through `phash2`. If we would
+     * replace these with placeholder values or whatever, the hash of a
+     * fun would depend on what node version it runs on. What's worse, if we
+     * were to have the _compiler_ emit the placeholder value for a few
+     * versions before removal, we would still run afoul of this mess if a
+     * really old fun was stored in ETF.
+     *
+     * We'll need decide what to do with this in OTP 30+ */
     int old_uniq;                   /* Unique number (old_style) */
     int old_index;                  /* Old style index */
 } ErlFunEntry;