File 2248-erts-Optimize-make_internal_hash-for-maps.patch of Package erlang
From 3684673aab9006d114db4298ab882805bf9bf657 Mon Sep 17 00:00:00 2001
From: Sverker Eriksson <sverker@erlang.org>
Date: Thu, 27 Apr 2017 19:27:36 +0200
Subject: [PATCH] erts: Optimize make_internal_hash for maps
No need for the xor-trick, to be order independent, as both
flatmaps and hashmaps have a constant key-value order internally.
---
erts/emulator/beam/utils.c | 43 ++++++-------------------------------------
1 file changed, 6 insertions(+), 37 deletions(-)
diff --git a/erts/emulator/beam/utils.c b/erts/emulator/beam/utils.c
index 9263798a2..cdf766b20 100644
--- a/erts/emulator/beam/utils.c
+++ b/erts/emulator/beam/utils.c
@@ -1581,9 +1581,6 @@ Uint32
make_internal_hash(Eterm term)
{
Uint32 hash;
- Uint32 hash_xor_pairs;
-
- ERTS_UNDEF(hash_xor_pairs, 0);
/* Optimization. Simple cases before declaration of estack. */
if (primary_tag(term) == TAG_PRIMARY_IMMED1) {
@@ -1664,6 +1661,11 @@ make_internal_hash(Eterm term)
Eterm* ptr = boxed_val(term) + 1;
Uint size;
int i;
+
+ /*
+ * We rely on key-value iteration order being constant
+ * for identical maps (in this VM instance).
+ */
switch (hdr & _HEADER_MAP_SUBTAG_MASK) {
case HAMT_SUBTAG_HEAD_FLATMAP:
{
@@ -1675,26 +1677,10 @@ make_internal_hash(Eterm term)
if (size == 0)
goto pop_next;
- /* We want a hash function that is *independent* of
- * the order in which keys and values are encountered.
- * We therefore calculate context independent hashes for all .
- * key-value pairs and then xor them together.
- *
- * We *do* need to use an initial seed for each pair, i.e. the
- * hash value, so the hash value is reset for each pair with 'hash'.
- * If we don't, no additional entropy is given to the system and the
- * hash collision resolution in maps:from_list/1 would fail.
- */
- ESTACK_PUSH(s, hash_xor_pairs);
- ESTACK_PUSH(s, hash);
- ESTACK_PUSH(s, HASH_MAP_TAIL);
for (i = size - 1; i >= 0; i--) {
- ESTACK_PUSH(s, hash); /* initial seed for all pairs */
- ESTACK_PUSH(s, HASH_MAP_PAIR);
ESTACK_PUSH(s, vs[i]);
ESTACK_PUSH(s, ks[i]);
}
- hash_xor_pairs = 0;
goto pop_next;
}
case HAMT_SUBTAG_HEAD_ARRAY:
@@ -1703,10 +1689,6 @@ make_internal_hash(Eterm term)
UINT32_HASH(size, HCONST_16);
if (size == 0)
goto pop_next;
- ESTACK_PUSH(s, hash_xor_pairs);
- ESTACK_PUSH(s, hash);
- ESTACK_PUSH(s, HASH_MAP_TAIL);
- hash_xor_pairs = 0;
}
switch (hdr & _HEADER_MAP_SUBTAG_MASK) {
case HAMT_SUBTAG_HEAD_ARRAY:
@@ -1722,8 +1704,6 @@ make_internal_hash(Eterm term)
while (i) {
if (is_list(*ptr)) {
Eterm* cons = list_val(*ptr);
- ESTACK_PUSH(s, hash); /* initial seed for all pairs */
- ESTACK_PUSH(s, HASH_MAP_PAIR);
ESTACK_PUSH(s, CDR(cons));
ESTACK_PUSH(s, CAR(cons));
}
@@ -1739,7 +1719,7 @@ make_internal_hash(Eterm term)
case EXPORT_SUBTAG:
{
Export* ep = *((Export **) (export_val(term) + 1));
- /* Assumes Export entries never moves */
+ /* Assumes Export entries never move */
POINTER_HASH(ep, HCONST_14);
goto pop_next;
}
@@ -1919,17 +1899,6 @@ make_internal_hash(Eterm term)
term = ESTACK_POP(s);
switch (term) {
- case HASH_MAP_TAIL: {
- hash = (Uint32) ESTACK_POP(s);
- UINT32_HASH(hash_xor_pairs, HCONST_19);
- hash_xor_pairs = (Uint32) ESTACK_POP(s);
- goto pop_next;
- }
- case HASH_MAP_PAIR:
- hash_xor_pairs ^= hash;
- hash = (Uint32) ESTACK_POP(s); /* initial seed for all pairs */
- goto pop_next;
-
case HASH_CDR:
CONST_HASH(HCONST_18); /* Hash CDR i cons cell */
goto pop_next;
--
2.12.2
