reuse the tuple xxhash algorithm for entries in frozendict_hash by unrolling it for tuples of len 2

Author: lambda-abstraction

Objects/dictobject.c

@@ -129,7 +129,7 @@ As a consequence of this, split keys have a maximum size of 16.
 #include "pycore_pyerrors.h"      // _PyErr_GetRaisedException()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_setobject.h"     // _PySet_NextEntry()
-#include "pycore_tuple.h"         // _PyTuple_Recycle()
+#include "pycore_tuple.h"         // _PyTuple_Recycle(), _PyTuple_HASH_XXPRIME1, _PyTuple_HASH_XXPRIME2, _PyTuple_HASH_XXPRIME5, _PyTuple_HASH_XXROTATE
 #include "pycore_unicodeobject.h" // _PyUnicode_InternImmortal()
 
 #include "stringlib/eq.h"                // unicode_eq()
@@ -8222,24 +8222,27 @@ frozendict_repr(PyObject *self)
     return res;
 }
 
+// Code unrolled from tuple_hash() for 2 values
 static inline Py_uhash_t
-_combine_hashes(Py_uhash_t h1, Py_uhash_t h2)
-{
-#if SIZEOF_PY_HASH_T == 8
-    // 2^sizeof(Py_hash_t) / phi
-    const Py_uhash_t GOLDEN_C = 0x9e3779b97f4a7c15ULL;
-    h1 += GOLDEN_C;
-    h1 ^= h2;
-    h1 ^= (h1 << 13) + (h1 >> 3);
-    h1 ^= h1 >> 33;
-#else
-    const Py_uhash_t GOLDEN_C = 0x9e3779b9UL;
-    h1 += GOLDEN_C;
-    h1 ^= h2;
-    h1 ^= (h1 << 6) + (h1 >> 2);
-    h1 ^= h1 >> 16;
-#endif
-    return h1;
+_tuple2_xxhash(Py_uhash_t h1, Py_uhash_t h2)
+{
+    Py_uhash_t acc = _PyTuple_HASH_XXPRIME_5;
+
+    acc += h1 * _PyTuple_HASH_XXPRIME_2;
+    acc = _PyTuple_HASH_XXROTATE(acc);
+    acc *= _PyTuple_HASH_XXPRIME_1;
+
+    acc += h2 * _PyTuple_HASH_XXPRIME_2;
+    acc = _PyTuple_HASH_XXROTATE(acc);
+    acc *= _PyTuple_HASH_XXPRIME_1;
+
+    acc += 2 ^ (_PyTuple_HASH_XXPRIME_5 ^ 3527539UL);
+
+    if (acc == (Py_uhash_t)-1) {
+        acc = 1546275796;
+    }
+
+    return acc;
 }
 
 // Code copied from frozenset_hash()
@@ -8253,7 +8256,7 @@ frozendict_hash(PyObject *op)
     }
 
     PyDictObject *mp = _PyAnyDict_CAST(op);
-    Py_uhash_t hash = 0xfd1c74; // start at a different value from frozenset to avoid collision with empty frozenset
+    Py_uhash_t hash = 0xfd1c74; // start at a different value from frozenset to avoid collision with frozenset(frozendict.items())
 
     PyObject *key, *value;  // borrowed refs
     Py_ssize_t pos = 0;
@@ -8266,7 +8269,7 @@ frozendict_hash(PyObject *op)
         if (value_hash == -1) {
             return -1;
         }
-        hash ^= _combine_hashes(key_hash, value_hash);
+        hash ^= _tuple2_xxhash(key_hash, value_hash);
     }
 
     /* Factor in the number of active entries */