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, 24 insertions, 3 deletions

diff --git a/erts/emulator/beam/erl_gc.c b/erts/emulator/beam/erl_gc.c
index 897dbbe82b..807d9d15e4 100644
--- a/erts/emulator/beam/erl_gc.c
+++ b/erts/emulator/beam/erl_gc.c
@@ -178,7 +178,7 @@ Uint erts_test_long_gc_sleep; /* Only used for testing... */
 typedef struct {
     Process *proc;
     Eterm ref;
-    Eterm ref_heap[REF_THING_SIZE];
+    Eterm ref_heap[ERTS_REF_THING_SIZE];
     Uint req_sched;
     erts_smp_atomic32_t refc;
 } ErtsGCInfoReq;
@@ -2354,6 +2354,9 @@ copy_one_frag(Eterm** hpp, ErlOffHeap* off_heap,
 	    switch (val & _HEADER_SUBTAG_MASK) {
 	    case ARITYVAL_SUBTAG:
 		break;
+	    case REF_SUBTAG:
+		if (is_ordinary_ref_thing(fhp - 1))
+		    goto the_default;
 	    case REFC_BINARY_SUBTAG:
 	    case FUN_SUBTAG:
 	    case EXTERNAL_PID_SUBTAG:
@@ -2363,6 +2366,7 @@ copy_one_frag(Eterm** hpp, ErlOffHeap* off_heap,
 		cpy_sz = thing_arityval(val);
 		goto cpy_words;
 	    default:
+	    the_default:
 		cpy_sz = header_arity(val);
 
 	    cpy_words:
@@ -2862,6 +2866,15 @@ sweep_off_heap(Process *p, int fullsweep)
 		    }
 		    break;
 		}
+	    case REF_SUBTAG:
+		{
+		    ErtsMagicBinary *bptr;
+		    ASSERT(is_magic_ref_thing(ptr));
+		    bptr = ((ErtsMRefThing *) ptr)->mb;
+		    if (erts_refc_dectest(&bptr->refc, 0) == 0)
+			erts_bin_free((Binary *) bptr);		    
+		    break;
+		}
 	    default:
 		ASSERT(is_external_header(ptr->thing_word));
 		erts_deref_node_entry(((ExternalThing*)ptr)->node);
@@ -2883,7 +2896,8 @@ sweep_off_heap(Process *p, int fullsweep)
 	}
 	else {
 	    ASSERT(is_fun_header(ptr->thing_word) ||
-		   is_external_header(ptr->thing_word));
+		   is_external_header(ptr->thing_word)
+		   || is_magic_ref_thing(ptr));
 	    prev = &ptr->next;
 	    ptr = ptr->next;
 	}
@@ -2978,6 +2992,9 @@ offset_heap(Eterm* hp, Uint sz, Sint offs, char* area, Uint area_size)
 	      }
 	      tari = thing_arityval(val);
 	      switch (thing_subtag(val)) {
+	      case REF_SUBTAG:
+		  if (is_ordinary_ref_thing(hp))
+		      break;
 	      case REFC_BINARY_SUBTAG:
 	      case FUN_SUBTAG:
 	      case EXTERNAL_PID_SUBTAG:
@@ -3175,7 +3192,7 @@ reply_gc_info(void *vgcirp)
 	if (hpp)
 	    ref_copy = STORE_NC(hpp, ohp, gcirp->ref);
 	else
-	    *szp += REF_THING_SIZE;
+	    *szp += ERTS_REF_THING_SIZE;
 
 	msg = erts_bld_tuple(hpp, szp, 3,
 			     make_small(esdp->no),
@@ -3526,6 +3543,10 @@ erts_check_off_heap2(Process *p, Eterm *htop)
 	case EXTERNAL_REF_SUBTAG:
 	    refc = erts_smp_refc_read(&u.ext->node->refc, 1);
 	    break;
+	case REF_SUBTAG:
+	    ASSERT(is_magic_ref_thing(u.hdr));
+	    refc = erts_refc_read(&u.mref->mb->refc, 1);
+	    break;
 	default:
 	    ASSERT(!"erts_check_off_heap2: Invalid thing_word");
 	}