From b079018e38272604ffacfece9b97924a9e39df5c Mon Sep 17 00:00:00 2001
From: Rickard Green <rickard@erlang.org>
Date: Mon, 23 Jan 2017 17:10:18 +0100
Subject: 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.
---
 erts/emulator/beam/copy.c | 60 +++++++++++++++++++++++++++++++++++++----------
 1 file changed, 48 insertions(+), 12 deletions(-)

(limited to 'erts/emulator/beam/copy.c')

diff --git a/erts/emulator/beam/copy.c b/erts/emulator/beam/copy.c
index d20102cb2c..85db23393c 100644
--- a/erts/emulator/beam/copy.c
+++ b/erts/emulator/beam/copy.c
@@ -858,20 +858,24 @@ Eterm copy_struct_x(Eterm obj, Uint sz, Eterm** hpp, ErlOffHeap* off_heap, Uint
 	    case EXTERNAL_PORT_SUBTAG:
 	    case EXTERNAL_REF_SUBTAG:
 		{
-		  ExternalThing *etp = (ExternalThing *) htop;
-
+		  ExternalThing *etp = (ExternalThing *) objp;
+		  erts_smp_refc_inc(&etp->node->refc, 2);
+		}
+	    L_off_heap_node_container_common:
+		{
+		  struct erl_off_heap_header *ohhp;
+		  ohhp = (struct erl_off_heap_header *) htop;
 		  i  = thing_arityval(hdr) + 1;
+		  *argp = make_boxed(htop);
 		  tp = htop;
 
 		  while (i--)  {
 		    *htop++ = *objp++;
 		  }
 
-		  etp->next = off_heap->first;
-		  off_heap->first = (struct erl_off_heap_header*)etp;
-		  erts_smp_refc_inc(&etp->node->refc, 2);
+		  ohhp->next = off_heap->first;
+		  off_heap->first = ohhp;
 
-		  *argp = make_external(tp);
 		}
 		break;
 	    case MAP_SUBTAG:
@@ -899,6 +903,13 @@ Eterm copy_struct_x(Eterm obj, Uint sz, Eterm** hpp, ErlOffHeap* off_heap, Uint
 	    case BIN_MATCHSTATE_SUBTAG:
 		erts_exit(ERTS_ABORT_EXIT,
 			 "copy_struct: matchstate term not allowed");
+	    case REF_SUBTAG:
+		if (is_magic_ref_thing(objp)) {
+		    ErtsMRefThing *mreft = (ErtsMRefThing *) objp;
+		    erts_refc_inc(&mreft->mb->refc, 2);
+		    goto L_off_heap_node_container_common;
+		}
+		/* Fall through... */
 	    default:
 		i = thing_arityval(hdr)+1;
 		hbot -= i;
@@ -1649,20 +1660,33 @@ Uint copy_shared_perform(Eterm obj, Uint size, erts_shcopy_t *info,
 	    }
 	    case EXTERNAL_PID_SUBTAG:
 	    case EXTERNAL_PORT_SUBTAG:
-	    case EXTERNAL_REF_SUBTAG: {
-		ExternalThing *etp = (ExternalThing *) hp;
+	    case EXTERNAL_REF_SUBTAG:
+	    {
+		ExternalThing *etp = (ExternalThing *) ptr;
+		erts_smp_refc_inc(&etp->node->refc, 2);
+	    }
+	  off_heap_node_container_common:
+	    {
+		struct erl_off_heap_header *ohhp;
+		ohhp = (struct erl_off_heap_header *) hp;
 		sz = thing_arityval(hdr);
-		*resp = make_external(hp);
+		*resp = make_boxed(hp);
 		*hp++ = hdr;
 		ptr++;
 		while (sz-- > 0) {
 		    *hp++ = *ptr++;
 		}
-		etp->next = off_heap->first;
-		off_heap->first = (struct erl_off_heap_header*) etp;
-		erts_smp_refc_inc(&etp->node->refc, 2);
+		ohhp->next = off_heap->first;
+		off_heap->first = ohhp;
 		goto cleanup_next;
 	    }
+	    case REF_SUBTAG:
+		if (is_magic_ref_thing(ptr)) {
+		    ErtsMRefThing *mreft = (ErtsMRefThing *) ptr;
+		    erts_refc_inc(&mreft->mb->refc, 2);
+		    goto off_heap_node_container_common;
+		}
+		/* Fall through... */
 	    default:
 		sz = thing_arityval(hdr);
 		*resp = make_boxed(hp);
@@ -1859,6 +1883,15 @@ Eterm copy_shallow(Eterm* ptr, Uint sz, Eterm** hpp, ErlOffHeap* off_heap)
 		    off_heap->first = ohh;
 		}
 		break;
+	    case REF_SUBTAG: {
+		ErtsRefThing *rtp = (ErtsRefThing *) (tp - 1);
+		if (is_magic_ref_thing(rtp)) {
+		    ErtsMRefThing *mreft = (ErtsMRefThing *) rtp;
+		    erts_refc_inc(&mreft->mb->refc, 2);
+		    goto off_heap_common;
+		}
+		/* Fall through... */
+	    }
 	    default:
 		{
 		    int tari = header_arity(val);
@@ -1959,6 +1992,9 @@ move_one_frag(Eterm** hpp, ErlHeapFragment* frag, ErlOffHeap* off_heap, int lite
 	    ASSERT(ptr + header_arity(val) < end);
 	    MOVE_BOXED(ptr, val, hp, &dummy_ref);
 	    switch (val & _HEADER_SUBTAG_MASK) {
+	    case REF_SUBTAG:
+		if (is_ordinary_ref_thing(hdr))
+		    break;
 	    case REFC_BINARY_SUBTAG:
 	    case FUN_SUBTAG:
 	    case EXTERNAL_PID_SUBTAG:
-- 
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