Implement magic references

Magic references are *intentionally* indistinguishable from ordinary
references for the Erlang software. Magic references do not change
the language, and are intended as a pure runtime internal optimization.

An ordinary reference is typically used as a key in some table. A
magic reference has a direct pointer to a reference counted magic
binary. This makes it possible to implement various things without
having to do lookups in a table, but instead access the data directly.
Besides very fast lookups this can also improve scalability by
removing a potentially contended table. A couple of examples of
planned future usage of magic references are ETS table identifiers,
and BIF timer identifiers.

Besides future optimizations using magic references it should also
be possible to replace the exposed magic binary cludge with magic
references. That is, magic binaries that are exposed as empty
binaries to the Erlang software.

1 files changed, 38 insertions, 2 deletions

diff --git a/erts/emulator/beam/erl_term.c b/erts/emulator/beam/erl_term.c
index 7d857ad326..6b25728af7 100644
--- a/erts/emulator/beam/erl_term.c
+++ b/erts/emulator/beam/erl_term.c
@@ -59,6 +59,42 @@ erts_set_literal_tag(Eterm *term, Eterm *hp_start, Eterm hsz)
 #endif
 }
 
+void
+erts_term_init(void)
+{
+#ifdef ERTS_ORDINARY_REF_MARKER
+    /* Ordinary and magic references of same size... */
+
+    ErtsRefThing ref_thing;
+
+    ERTS_CT_ASSERT(ERTS_ORDINARY_REF_MARKER == ~((Uint32)0));
+    ref_thing.m.header = ERTS_REF_THING_HEADER;
+    ref_thing.m.mb = (ErtsMagicBinary *) ~((UWord) 3);
+    ref_thing.m.next = (struct erl_off_heap_header *) ~((UWord) 3);
+    if (ref_thing.o.marker == ERTS_ORDINARY_REF_MARKER)
+        ERTS_INTERNAL_ERROR("Cannot differentiate between magic and ordinary references");
+
+    ERTS_CT_ASSERT(offsetof(ErtsORefThing,marker) != 0);
+    ERTS_CT_ASSERT(sizeof(ErtsORefThing) == sizeof(ErtsMRefThing));
+#  ifdef ERTS_MAGIC_REF_THING_HEADER
+#    error Magic ref thing header should not have been defined...
+#  endif
+
+#else
+    /* Ordinary and magic references of different sizes... */
+
+#  ifndef ERTS_MAGIC_REF_THING_HEADER
+#    error Magic ref thing header should have been defined...
+#  endif
+    ERTS_CT_ASSERT(sizeof(ErtsORefThing) != sizeof(ErtsMRefThing));
+
+#endif
+
+    ERTS_CT_ASSERT(ERTS_REF_THING_SIZE*sizeof(Eterm) == sizeof(ErtsORefThing));
+    ERTS_CT_ASSERT(ERTS_MAGIC_REF_THING_SIZE*sizeof(Eterm) == sizeof(ErtsMRefThing));
+
+}
+
 /*
  * XXX: define NUMBER_CODE() here when new representation is used
  */
@@ -95,8 +131,8 @@ ET_DEFINE_CHECKED(Eterm*,tuple_val,Wterm,is_tuple);
 ET_DEFINE_CHECKED(struct erl_node_*,internal_pid_node,Eterm,is_internal_pid);
 ET_DEFINE_CHECKED(struct erl_node_*,internal_port_node,Eterm,is_internal_port);
 ET_DEFINE_CHECKED(Eterm*,internal_ref_val,Wterm,is_internal_ref);
-ET_DEFINE_CHECKED(Uint,internal_ref_data_words,Wterm,is_internal_ref);
-ET_DEFINE_CHECKED(Uint32*,internal_ref_data,Wterm,is_internal_ref);
+ET_DEFINE_CHECKED(Uint32*,internal_magic_ref_numbers,Wterm,is_internal_magic_ref);
+ET_DEFINE_CHECKED(Uint32*,internal_ordinary_ref_numbers,Wterm,is_internal_ordinary_ref);
 ET_DEFINE_CHECKED(struct erl_node_*,internal_ref_node,Eterm,is_internal_ref);
 ET_DEFINE_CHECKED(Eterm*,external_val,Wterm,is_external);
 ET_DEFINE_CHECKED(Uint,external_data_words,Wterm,is_external);