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, 85 insertions, 2 deletions

diff --git a/erts/emulator/beam/erl_bif_info.c b/erts/emulator/beam/erl_bif_info.c
index 37dcb5e7a6..be113b7c88 100644
--- a/erts/emulator/beam/erl_bif_info.c
+++ b/erts/emulator/beam/erl_bif_info.c
@@ -187,6 +187,32 @@ bld_bin_list(Uint **hpp, Uint *szp, ErlOffHeap* oh)
     return res;
 }
 
+static Eterm
+bld_magic_ref_bin_list(Uint **hpp, Uint *szp, ErlOffHeap* oh)
+{
+    struct erl_off_heap_header* ohh;
+    Eterm res = NIL;
+    Eterm tuple;
+
+    for (ohh = oh->first; ohh; ohh = ohh->next) {
+	if (is_ref_thing_header((*((Eterm *) ohh)))) {
+            ErtsMRefThing *mrtp = (ErtsMRefThing *) ohh;
+	    Eterm val = erts_bld_uword(hpp, szp, (UWord) mrtp->mb);
+	    Eterm orig_size = erts_bld_uint(hpp, szp, mrtp->mb->orig_size);
+    
+	    if (szp)
+		*szp += 4+2;
+	    if (hpp) {
+		Uint refc = (Uint) erts_refc_read(&mrtp->mb->refc, 1);
+		tuple = TUPLE3(*hpp, val, orig_size, make_small(refc));
+		res = CONS(*hpp + 4, tuple, res);
+		*hpp += 4+2;
+	    }
+	}
+    }
+    return res;
+}
+
 
 /*
   make_monitor_list:
@@ -613,7 +639,8 @@ static Eterm pi_args[] = {
     am_current_location,
     am_current_stacktrace,
     am_message_queue_data,
-    am_garbage_collection_info
+    am_garbage_collection_info,
+    am_magic_ref
 };
 
 #define ERTS_PI_ARGS ((int) (sizeof(pi_args)/sizeof(Eterm)))
@@ -664,6 +691,7 @@ pi_arg2ix(Eterm arg)
     case am_current_stacktrace:			return 31;
     case am_message_queue_data:			return 32;
     case am_garbage_collection_info:		return 33;
+    case am_magic_ref:                          return 34;
     default:					return -1;
     }
 }
@@ -1599,6 +1627,14 @@ process_info_aux(Process *BIF_P,
 	hp = HAlloc(BIF_P, 3);
 	break;
 
+    case am_magic_ref: {
+	Uint sz = 3;
+	(void) bld_magic_ref_bin_list(NULL, &sz, &MSO(rp));
+	hp = HAlloc(BIF_P, sz);
+	res = bld_magic_ref_bin_list(&hp, NULL, &MSO(rp));
+	break;
+    }
+
     default:
 	return THE_NON_VALUE; /* will produce badarg */
 
@@ -3531,6 +3567,11 @@ BIF_RETTYPE error_logger_warning_map_0(BIF_ALIST_0)
 
 static erts_smp_atomic_t available_internal_state;
 
+static void empty_magic_ref_destructor(Binary *bin)
+{
+
+}
+
 BIF_RETTYPE erts_debug_get_internal_state_1(BIF_ALIST_1)
 {
     /*
@@ -3896,6 +3937,34 @@ BIF_RETTYPE erts_debug_get_internal_state_1(BIF_ALIST_1)
 		}
 		return make_atom(ix);
 	    }
+	    else if (ERTS_IS_ATOM_STR("magic_ref", tp[1])) {
+                Binary *bin;
+                UWord bin_addr, refc;
+                Eterm bin_addr_term, refc_term, test_type;
+                Uint sz;
+                Eterm *hp;
+                if (!is_internal_magic_ref(tp[2])) {
+                    if (is_internal_ordinary_ref(tp[2])) {
+                        ErtsORefThing *rtp;
+                        rtp = (ErtsORefThing *) internal_ref_val(tp[2]);
+                        if (erts_is_ref_numbers_magic(rtp->num))
+                            BIF_RET(am_true);
+                    }
+                    BIF_RET(am_false);
+                }
+                bin = erts_magic_ref2bin(tp[2]);
+                refc = erts_refc_read(&bin->refc, 1);
+                bin_addr = (UWord) bin;
+                sz = 4;
+                erts_bld_uword(NULL, &sz, bin_addr);
+                erts_bld_uword(NULL, &sz, refc);
+                hp = HAlloc(BIF_P, sz);
+                bin_addr_term = erts_bld_uword(&hp, NULL, bin_addr);
+                refc_term = erts_bld_uword(&hp, NULL, refc);
+                test_type = (ERTS_MAGIC_BIN_DESTRUCTOR(bin) == empty_magic_ref_destructor
+                             ? am_true : am_false);
+                BIF_RET(TUPLE3(hp, bin_addr_term, refc_term, test_type));
+	    }
 
 	    break;
 	}
@@ -3984,7 +4053,6 @@ static void broken_halt_test(Eterm bif_arg_2)
     erts_exit(ERTS_DUMP_EXIT, "%T", bif_arg_2);
 }
 
-
 BIF_RETTYPE erts_debug_set_internal_state_2(BIF_ALIST_2)
 {
     /*
@@ -4314,6 +4382,21 @@ BIF_RETTYPE erts_debug_set_internal_state_2(BIF_ALIST_2)
             /* Used by ets_SUITE (stdlib) */
             BIF_RET(erts_ets_restore_meta_state(BIF_P, BIF_ARG_2));
         }
+        else if (ERTS_IS_ATOM_STR("make", BIF_ARG_1)) {
+            if (ERTS_IS_ATOM_STR("magic_ref", BIF_ARG_2)) {
+                Binary *bin = erts_create_magic_binary(0, empty_magic_ref_destructor);
+                UWord bin_addr = (UWord) bin;
+                Eterm bin_addr_term, magic_ref, res;
+                Eterm *hp;
+                Uint sz = ERTS_MAGIC_REF_THING_SIZE + 3;
+                erts_bld_uword(NULL, &sz, bin_addr);
+                hp = HAlloc(BIF_P, sz);
+                bin_addr_term = erts_bld_uword(&hp, NULL, bin_addr);
+                magic_ref = erts_mk_magic_ref(&hp, &BIF_P->off_heap, bin);
+                res = TUPLE2(hp, magic_ref, bin_addr_term);
+                BIF_RET(res);
+            }
+        }
     }
 
     BIF_ERROR(BIF_P, BADARG);