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authorSverker Eriksson <[email protected]>2015-03-12 15:19:35 +0100
committerBjörn-Egil Dahlberg <[email protected]>2015-03-12 19:15:35 +0100
commit1b13e90161bdb1409b04a273a6b71f02fbb9d5f1 (patch)
tree32d58f5e0a9e0289ff540ae34bc7343e92671be2 /erts
parent2aeb8cfd42be8ab1b7eee4a7f144122223dd7261 (diff)
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erts: Cleanup comments for make_internal_hash
Diffstat (limited to 'erts')
-rw-r--r--erts/emulator/beam/utils.c20
1 files changed, 14 insertions, 6 deletions
diff --git a/erts/emulator/beam/utils.c b/erts/emulator/beam/utils.c
index d098ee4c72..23ae885b6d 100644
--- a/erts/emulator/beam/utils.c
+++ b/erts/emulator/beam/utils.c
@@ -1570,14 +1570,22 @@ make_hash2(Eterm term)
}
/* Term hash function for internal use.
- * Is not "portable" in any way betweem different VM instances.
+ *
+ * Limitation #1: Is not "portable" in any way between different VM instances.
+ *
+ * Limitation #2: The hash value is only valid as long as the term exists
+ * somewhere in the VM. Why? Because external pids, ports and refs are hashed
+ * by mixing the node *pointer* value. If a node disappears and later reappears
+ * with a new ErlNode struct, externals from that node will hash different than
+ * before.
*
* One IMPORTANT property must hold (for hamt).
* EVERY BIT of the term that is significant for equality (see EQ)
- * must be used as input for the hash. Two different terms must always have a
- * chance of hashing different when salted: h([Salt|A]) vs h([Salt|B]).
+ * MUST BE USED AS INPUT FOR THE HASH. Two different terms must always have a
+ * chance of hashing different when salted: hash([Salt|A]) vs hash([Salt|B]).
*
* This is why we can not use cached hash values for atoms for example.
+ *
*/
#define CONST_HASH(AConst) \
@@ -1854,7 +1862,7 @@ make_internal_hash(Eterm term)
ExternalThing* thing = external_thing_ptr(term);
ASSERT(external_thing_ref_no_of_numbers(thing) == 3);
- /*SVERK Is it really ok to hash node POINTER? */
+ /* See limitation #2 */
#ifdef ARCH_64
POINTER_HASH(thing->node, HCONST_7);
UINT32_HASH(external_thing_ref_numbers(thing)[0], HCONST_7);
@@ -1868,7 +1876,7 @@ make_internal_hash(Eterm term)
}
case EXTERNAL_PID_SUBTAG: {
ExternalThing* thing = external_thing_ptr(term);
- /*SVERK Is it really ok to hash node POINTER? */
+ /* See limitation #2 */
#ifdef ARCH_64
POINTER_HASH(thing->node, HCONST_5);
UINT32_HASH(thing->data.ui[0], HCONST_5);
@@ -1879,7 +1887,7 @@ make_internal_hash(Eterm term)
}
case EXTERNAL_PORT_SUBTAG: {
ExternalThing* thing = external_thing_ptr(term);
- /*SVERK Is it really ok to hash node POINTER? */
+ /* See limitation #2 */
#ifdef ARCH_64
POINTER_HASH(thing->node, HCONST_6);
UINT32_HASH(thing->data.ui[0], HCONST_6);