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bpo-43452: Micro-optimizations to PyType_Lookup (GH-24804)

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The common case going through _PyType_Lookup is to have a cache hit. There are some small tweaks that can make this a little cheaper:

* The name field identity is used for a cache hit and is kept alive by the cache. So there's no need to read the hash code o the name - instead, the address can be used as the hash.

*  There's no need to check if the name is cachable on the lookup either, it probably is, and if it is, it'll be in the cache.

*  If we clear the version tag when invalidating a type then we don't actually need to check for a valid version tag bit.
This commit is contained in:
Dino Viehland 2021-03-20 12:12:05 -07:00 • committed by GitHub
parent 2fd16ef406
commit ee48c7d541
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2 changed files with 24 additions and 28 deletions
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1
Misc/NEWS.d/next/Core and Builtins/2021-03-20-01-21-37.bpo-43452.tDVJkc.rst Normal file
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@ -0,0 +1 @@
Added micro-optimizations to ``_PyType_Lookup()`` to improve cache lookup performance in the common case of cache hits.

51
Objects/typeobject.c
View file

@ -32,8 +32,7 @@ class object "PyObject *" "&PyBaseObject_Type"
& ((1 << MCACHE_SIZE_EXP) - 1)) & ((1 << MCACHE_SIZE_EXP) - 1))
#define MCACHE_HASH_METHOD(type, name) \ #define MCACHE_HASH_METHOD(type, name) \
MCACHE_HASH((type)->tp_version_tag, \ MCACHE_HASH((type)->tp_version_tag, ((Py_ssize_t)(name)) >> 3)
((PyASCIIObject *)(name))->hash)
#define MCACHE_CACHEABLE_NAME(name) \ #define MCACHE_CACHEABLE_NAME(name) \
PyUnicode_CheckExact(name) && \ PyUnicode_CheckExact(name) && \
PyUnicode_IS_READY(name) && \ PyUnicode_IS_READY(name) && \
@ -338,6 +337,7 @@ PyType_Modified(PyTypeObject *type)
} }
} }
type->tp_flags &= ~Py_TPFLAGS_VALID_VERSION_TAG; type->tp_flags &= ~Py_TPFLAGS_VALID_VERSION_TAG;
type->tp_version_tag = 0; /* 0 is not a valid version tag */
} }
static void static void
@ -396,6 +396,7 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
Py_XDECREF(type_mro_meth); Py_XDECREF(type_mro_meth);
type->tp_flags &= ~(Py_TPFLAGS_HAVE_VERSION_TAG| type->tp_flags &= ~(Py_TPFLAGS_HAVE_VERSION_TAG|
Py_TPFLAGS_VALID_VERSION_TAG); Py_TPFLAGS_VALID_VERSION_TAG);
type->tp_version_tag = 0; /* 0 is not a valid version tag */
} }
static int static int
@ -3351,18 +3352,15 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
PyObject *res; PyObject *res;
int error; int error;
if (MCACHE_CACHEABLE_NAME(name) && unsigned int h = MCACHE_HASH_METHOD(type, name);
_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG)) { struct type_cache *cache = get_type_cache();
/* fast path */ struct type_cache_entry *entry = &cache->hashtable[h];
unsigned int h = MCACHE_HASH_METHOD(type, name); if (entry->version == type->tp_version_tag &&
struct type_cache *cache = get_type_cache(); entry->name == name) {
struct type_cache_entry *entry = &cache->hashtable[h];
if (entry->version == type->tp_version_tag && entry->name == name) {
#if MCACHE_STATS #if MCACHE_STATS
cache->hits++; cache->hits++;
#endif #endif
return entry->value; return entry->value;
}
} }
/* We may end up clearing live exceptions below, so make sure it's ours. */ /* We may end up clearing live exceptions below, so make sure it's ours. */
@ -3385,24 +3383,21 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
return NULL; return NULL;
} }
if (MCACHE_CACHEABLE_NAME(name)) { if (MCACHE_CACHEABLE_NAME(name) && assign_version_tag(cache, type)) {
struct type_cache *cache = get_type_cache(); h = MCACHE_HASH_METHOD(type, name);
if (assign_version_tag(cache, type)) { struct type_cache_entry *entry = &cache->hashtable[h];
unsigned int h = MCACHE_HASH_METHOD(type, name); entry->version = type->tp_version_tag;
struct type_cache_entry *entry = &cache->hashtable[h]; entry->value = res; /* borrowed */
entry->version = type->tp_version_tag; assert(((PyASCIIObject *)(name))->hash != -1);
entry->value = res; /* borrowed */
assert(((PyASCIIObject *)(name))->hash != -1);
#if MCACHE_STATS #if MCACHE_STATS
if (entry->name != Py_None && entry->name != name) { if (entry->name != Py_None && entry->name != name) {
cache->collisions++; cache->collisions++;
}
else {
cache->misses++;
}
#endif
Py_SETREF(entry->name, Py_NewRef(name));
} }
else {
cache->misses++;
}
#endif
Py_SETREF(entry->name, Py_NewRef(name));
} }
return res; return res;
} }