diff options
Diffstat (limited to 'erts/emulator/beam/utils.c')
| -rw-r--r-- | erts/emulator/beam/utils.c | 20 |
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); |