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/erl_monitors.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

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

diff --git a/erts/emulator/beam/erl_monitors.c b/erts/emulator/beam/erl_monitors.c
index c207dea10f..bdfc7f30d9 100644
--- a/erts/emulator/beam/erl_monitors.c
+++ b/erts/emulator/beam/erl_monitors.c
@@ -45,6 +45,7 @@
 #include "bif.h"
 #include "big.h"
 #include "erl_monitors.h"
+#include "erl_bif_unique.h"
 
 #define STACK_NEED 50
 #define MAX_MONITORS 0xFFFFFFFFUL
@@ -79,7 +80,24 @@ static ERTS_INLINE int cmp_mon_ref(Eterm ref1, Eterm ref2)
     b2 = boxed_val(ref2);
     if (is_ref_thing_header(*b1)) {
 	if (is_ref_thing_header(*b2)) {
-	    return memcmp(b1+1,b2+1,ERTS_REF_WORDS*sizeof(Uint));
+	    Uint32 *num1, *num2;
+	    if (is_ordinary_ref_thing(b1)) {
+		ErtsORefThing *rtp = (ErtsORefThing *) b1;
+		num1 = rtp->num;
+	    }
+	    else {
+		ErtsMRefThing *mrtp = (ErtsMRefThing *) b1;
+		num1 = mrtp->mb->refn;
+	    }
+	    if (is_ordinary_ref_thing(b2)) {
+		ErtsORefThing *rtp = (ErtsORefThing *) b2;
+		num2 = rtp->num;
+	    }
+	    else {
+		ErtsMRefThing *mrtp = (ErtsMRefThing *) b2;
+		num2 = mrtp->mb->refn;
+	    }
+	    return erts_internal_ref_number_cmp(num1, num2);
 	}
 	return -1;
     }
@@ -97,7 +115,8 @@ do {								\
 	Uint i__;						\
 	Uint len__;						\
 	ASSERT((Hp));						\
-	ASSERT(is_internal_ref((From)) || is_external((From)));	\
+	ASSERT(is_internal_ordinary_ref((From))                 \
+               || is_external((From)));                         \
 	(To) = make_boxed((Hp));				\
 	len__ = thing_arityval(*boxed_val((From))) + 1;		\
 	for(i__ = 0; i__ < len__; i__++)			\
@@ -339,6 +358,8 @@ void erts_add_monitor(ErtsMonitor **root, Uint type, Eterm ref, Eterm pid,
     int state = 0;
     ErtsMonitor **this = root;
     Sint c;
+
+    ASSERT(is_internal_ordinary_ref(ref) || is_external_ref(ref));
   
     dstack[0] = DIR_END;
     for (;;) {
-- 
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