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Aggressive reordering of dict comparisons. In case of collision, it stands 

to reason that me_key is much more likely to match the key we're looking
for than to match dummy, and if the key is absent me_key is much more
likely to be NULL than dummy:  most dicts don't even have a dummy entry.
Running instrumented dict code over the test suite and some apps confirmed
that matching dummy was 200-300x less frequent than matching key in
practice.  So this reorders the tests to try the common case first.
It can lose if a large dict with many collisions is mostly deleted, not
resized, and then frequently searched, but that's hardly a case we
should be favoring.
This commit is contained in:
Tim Peters 2001-05-13 06:43:53 +00:00
parent 2f228e75e4
commit 342c65e19a
1 changed files with 21 additions and 30 deletions
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51
Objects/dictobject.c
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@ -219,26 +219,21 @@ lookdict(dictobject *mp, PyObject *key, register long hash)
incr = (hash ^ ((unsigned long)hash >> 3)) & mask;
if (!incr)
incr = mask;
/* In the loop, me_key == dummy is by far (factor of 100s) the
least likely outcome, so test for that last. */
for (;;) {
ep = &ep0[(i+incr)&mask];
if (ep->me_key == NULL) {
if (restore_error)
PyErr_Restore(err_type, err_value, err_tb);
if (freeslot != NULL)
return freeslot;
else
return ep;
return freeslot == NULL ? ep : freeslot;
}
if (ep->me_key == dummy) {
if (freeslot == NULL)
freeslot = ep;
}
else if (ep->me_key == key) {
if (ep->me_key == key) {
if (restore_error)
PyErr_Restore(err_type, err_value, err_tb);
return ep;
}
else if (ep->me_hash == hash) {
else if (ep->me_hash == hash && ep->me_key != dummy) {
if (!checked_error) {
checked_error = 1;
if (PyErr_Occurred()) {
@ -257,11 +252,12 @@ lookdict(dictobject *mp, PyObject *key, register long hash)
else if (cmp < 0)
PyErr_Clear();
}
else if (ep->me_key == dummy && freeslot == NULL)
freeslot = ep;
/* Cycle through GF(2^n)-{0} */
incr = incr << 1;
incr <<= 1;
if (incr > mask)
incr ^= mp->ma_poly; /* This will implicitly clear
the highest bit */
incr ^= mp->ma_poly; /* clears the highest bit */
}
}
@ -314,28 +310,23 @@ lookdict_string(dictobject *mp, PyObject *key, register long hash)
incr = (hash ^ ((unsigned long)hash >> 3)) & mask;
if (!incr)
incr = mask;
/* In the loop, me_key == dummy is by far (factor of 100s) the
least likely outcome, so test for that last. */
for (;;) {
ep = &ep0[(i+incr)&mask];
if (ep->me_key == NULL) {
if (freeslot != NULL)
return freeslot;
else
return ep;
}
if (ep->me_key == dummy) {
if (freeslot == NULL)
freeslot = ep;
}
else if (ep->me_key == key
|| (ep->me_hash == hash
&& compare(ep->me_key, key) == 0)) {
if (ep->me_key == NULL)
return freeslot == NULL ? ep : freeslot;
if (ep->me_key == key
|| (ep->me_hash == hash
&& ep->me_key != dummy
&& compare(ep->me_key, key) == 0))
return ep;
}
else if (ep->me_key == dummy && freeslot == NULL)
freeslot = ep;
/* Cycle through GF(2^n)-{0} */
incr = incr << 1;
incr <<= 1;
if (incr > mask)
incr ^= mp->ma_poly; /* This will implicitly clear
the highest bit */
incr ^= mp->ma_poly; /* clears the highest bit */
}
}