gh-136599: Improve long_hash

Lib/test/test_long.py

@@ -1693,5 +1693,11 @@ class MyInt(int):
         # GH-117195 -- This shouldn't crash
         object.__sizeof__(1)
 
+    def test_long_hash(self):
+        # gh-136599
+        assert hash(10) == 10
+        assert hash(-1) == -2
+        assert hash(-2**61) != -1
+
 if __name__ == "__main__":
     unittest.main()

Objects/longobject.c

@@ -3676,7 +3676,17 @@ long_hash(PyObject *obj)
     }
     i = _PyLong_DigitCount(v);
     sign = _PyLong_NonCompactSign(v);
-    x = 0;
+
+    // unroll first two digits
+    assert(i>=2);
+    --i;
+    x = v->long_value.ob_digit[i];
+    assert(x < _PyHASH_MODULUS);
+    --i;
+    x = ((x << PyLong_SHIFT));
+    x += v->long_value.ob_digit[i];
+    assert(x < _PyHASH_MODULUS);
+
     while (--i >= 0) {
         /* Here x is a quantity in the range [0, _PyHASH_MODULUS); we
            want to compute x * 2**PyLong_SHIFT + v->long_value.ob_digit[i] modulo