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, 5 insertions, 0 deletions

diff --git a/erts/emulator/beam/erl_message.c b/erts/emulator/beam/erl_message.c
index 5e160de080..f181c1e3cb 100644
--- a/erts/emulator/beam/erl_message.c
+++ b/erts/emulator/beam/erl_message.c
@@ -176,6 +176,11 @@ erts_cleanup_offheap(ErlOffHeap *offheap)
 		erts_erase_fun_entry(u.fun->fe);		    
 	    }
 	    break;
+	case REF_SUBTAG:
+	    ASSERT(is_magic_ref_thing(u.hdr));
+	    if (erts_refc_dectest(&u.mref->mb->refc, 0) == 0)
+		erts_bin_free((Binary *)u.mref->mb);		    
+	    break;
 	default:
 	    ASSERT(is_external_header(u.hdr->thing_word));
 	    erts_deref_node_entry(u.ext->node);