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bpo-40602: Add _Py_HashPointerRaw() function #20056

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Merged
vstinner merged 1 commit into from
May 12, 2020
Merged

bpo-40602: Add _Py_HashPointerRaw() function #20056

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bpo-40602: Add _Py_HashPointerRaw() function 
Add a new _Py_HashPointerRaw() function which avoids replacing -1
with -2 to micro-optimize hash table using pointer keys: using
_Py_hashtable_hash_ptr() hash function.
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@vstinner
vstinner committed May 12, 2020
commit 299db3233096fbbfa3aa4dea5b3c5845007164d8
2 changes: 2 additions & 0 deletions Include/pyhash.h
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Expand Up @@ -9,6 +9,8 @@ extern "C" {
#ifndef Py_LIMITED_API
PyAPI_FUNC(Py_hash_t) _Py_HashDouble(double);
PyAPI_FUNC(Py_hash_t) _Py_HashPointer(const void*);
// Similar to _Py_HashPointer(), but don't replace -1 with -2
PyAPI_FUNC(Py_hash_t) _Py_HashPointerRaw(const void*);
PyAPI_FUNC(Py_hash_t) _Py_HashBytes(const void*, Py_ssize_t);
#endif

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2 changes: 1 addition & 1 deletion Python/hashtable.c
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Expand Up @@ -109,7 +109,7 @@ _Py_hashtable_hash_ptr(struct _Py_hashtable_t *ht, const void *pkey)
{
void *key;
_Py_HASHTABLE_READ_KEY(ht, pkey, key);
return (Py_uhash_t)_Py_HashPointer(key);
return (Py_uhash_t)_Py_HashPointerRaw(key);
}


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14 changes: 10 additions & 4 deletions Python/pyhash.c
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Expand Up @@ -129,16 +129,22 @@ _Py_HashDouble(double v)
}

Py_hash_t
_Py_HashPointer(const void *p)
_Py_HashPointerRaw(const void *p)
{
Py_hash_t x;
size_t y = (size_t)p;
/* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid
excessive hash collisions for dicts and sets */
y = (y >> 4) | (y << (8 * SIZEOF_VOID_P - 4));
x = (Py_hash_t)y;
if (x == -1)
return (Py_hash_t)y;
}

Py_hash_t
_Py_HashPointer(const void *p)
{
Py_hash_t x = _Py_HashPointerRaw(p);
if (x == -1) {
x = -2;
}
return x;
}

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