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Issue18771: Reduce the cost of hash collisions for set objects.

This commit is contained in:
Raymond Hettinger 2013-08-19 07:36:04 -07:00
parent 319f3a10f9
commit 3c0a4f5def
2 changed files with 88 additions and 22 deletions
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2
Include/setobject.h
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@ -51,9 +51,9 @@ struct _setobject {
*/
setentry *table;
setentry *(*lookup)(PySetObject *so, PyObject *key, Py_hash_t hash);
Py_hash_t hash; /* only used by frozenset objects */
setentry smalltable[PySet_MINSIZE];
Py_hash_t hash; /* only used by frozenset objects */
PyObject *weakreflist; /* List of weak references */
};
#endif /* Py_LIMITED_API */

108
Objects/setobject.c
View File

@ -68,6 +68,11 @@ chaining would be substantial (100% with typical malloc overhead).
The initial probe index is computed as hash mod the table size. Subsequent
probe indices are computed as explained in Objects/dictobject.c.
To improve cache locality, each probe is done in pairs.
After the probe is examined, an adjacent entry is then examined as well.
The likelihood is that an adjacent entry is in the same cache line and
can be examined more cheaply than another probe elsewhere in memory.
All arithmetic on hash should ignore overflow.
Unlike the dictionary implementation, the lookkey functions can return
@ -77,7 +82,7 @@ NULL if the rich comparison returns an error.
static setentry *
set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
{
size_t i; /* Unsigned for defined overflow behavior. */
size_t i, j; /* Unsigned for defined overflow behavior. */
size_t perturb;
setentry *freeslot;
size_t mask = so->mask;
@ -90,7 +95,6 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
entry = &table[i];
if (entry->key == NULL || entry->key == key)
return entry;
if (entry->hash == hash) {
startkey = entry->key;
Py_INCREF(startkey);
@ -107,14 +111,15 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
return set_lookkey(so, key, hash);
}
}
freeslot = (entry->key == dummy) ? entry : NULL;
/* In the loop, key == dummy is by far (factor of 100s)
the least likely outcome, so test for that last. */
for (perturb = hash; ; perturb >>= PERTURB_SHIFT) {
i = i * 5 + perturb + 1;
entry = &table[i & mask];
j = i;
perturb = hash;
while (1) {
j ^= 1;
entry = &table[j];
if (entry->key == NULL) {
if (freeslot != NULL)
entry = freeslot;
@ -134,14 +139,42 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
break;
}
else {
/* The compare did major nasty stuff to the
* set: start over.
*/
return set_lookkey(so, key, hash);
}
}
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
i = i * 5 + perturb + 1;
j = i & mask;
perturb >>= PERTURB_SHIFT;
entry = &table[j];
if (entry->key == NULL) {
if (freeslot != NULL)
entry = freeslot;
break;
}
if (entry->key == key)
break;
if (entry->hash == hash) {
startkey = entry->key;
Py_INCREF(startkey);
cmp = PyObject_RichCompareBool(startkey, key, Py_EQ);
Py_DECREF(startkey);
if (cmp < 0)
return NULL;
if (table == so->table && entry->key == startkey) {
if (cmp > 0)
break;
}
else {
return set_lookkey(so, key, hash);
}
}
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
}
return entry;
}
@ -154,7 +187,7 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
static setentry *
set_lookkey_unicode(PySetObject *so, PyObject *key, Py_hash_t hash)
{
size_t i; /* Unsigned for defined overflow behavior. */
size_t i, j; /* Unsigned for defined overflow behavior. */
size_t perturb;
setentry *freeslot;
size_t mask = so->mask;
@ -169,6 +202,7 @@ set_lookkey_unicode(PySetObject *so, PyObject *key, Py_hash_t hash)
so->lookup = set_lookkey;
return set_lookkey(so, key, hash);
}
i = (size_t)hash & mask;
entry = &table[i];
if (entry->key == NULL || entry->key == key)
@ -181,11 +215,37 @@ set_lookkey_unicode(PySetObject *so, PyObject *key, Py_hash_t hash)
freeslot = NULL;
}
/* In the loop, key == dummy is by far (factor of 100s) the
least likely outcome, so test for that last. */
for (perturb = hash; ; perturb >>= PERTURB_SHIFT) {
entry = &table[i ^ 1];
if (entry->key == NULL)
return freeslot == NULL ? entry : freeslot;
if (entry->key == key
|| (entry->hash == hash
&& entry->key != dummy
&& unicode_eq(entry->key, key)))
return entry;
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
j = i;
perturb = hash;
while (1) {
j ^= 1;
entry = &table[j];
if (entry->key == NULL)
return freeslot == NULL ? entry : freeslot;
if (entry->key == key
|| (entry->hash == hash
&& entry->key != dummy
&& unicode_eq(entry->key, key)))
return entry;
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
i = i * 5 + perturb + 1;
entry = &table[i & mask];
j = i & mask;
perturb >>= PERTURB_SHIFT;
entry = &table[j];
if (entry->key == NULL)
return freeslot == NULL ? entry : freeslot;
if (entry->key == key
@ -244,17 +304,23 @@ is responsible for incref'ing `key`.
static void
set_insert_clean(PySetObject *so, PyObject *key, Py_hash_t hash)
{
size_t i;
size_t perturb;
size_t mask = (size_t)so->mask;
setentry *table = so->table;
setentry *entry;
size_t perturb = hash;
size_t mask = (size_t)so->mask;
size_t i, j;
i = (size_t)hash & mask;
entry = &table[i];
for (perturb = hash; entry->key != NULL; perturb >>= PERTURB_SHIFT) {
i = j = (size_t)hash & mask;
while (1) {
entry = &table[j];
if (entry->key == NULL)
break;
entry = &table[j ^ 1];
if (entry->key == NULL)
break;
i = i * 5 + perturb + 1;
entry = &table[i & mask];
j = i & mask;
perturb >>= PERTURB_SHIFT;
}
so->fill++;
entry->key = key;