gh-158585: Optimize bytes object creation

Add bytes_alloc() helper function. bytes_alloc() and
_PyBytes_FromStringAndSize() have less conditional branches than
previous code, and so should be a little bit faster.

* Rename existing bytes_alloc() to bytes_type_alloc().
* Replace _PyBytes_FromSize(calloc=1) with _PyBytes_FromSizeZero().
* Add set_ob_shash_unsafe(): similar to set_ob_shash() but don't use
  an atomic operation on Free Threading. Use this new function
  on newly allocated bytes objects and in bytes_resize_inplace().
* Replace PyBytes_FromStringAndSize(NULL, size) with
  bytes_alloc(size).
* Use bytes_alloc() in PyBytes_FromString() and _PyBytes_Repeat().
* Add _PyBytes_FromStringAndSize(): similar to
  PyBytes_FromStringAndSize(), but str must not be NULL.
* Replace PyBytes_FromStringAndSize() with
  _PyBytes_FromStringAndSize().
* Test that PyBytes_FromString() and PyBytes_FromStringAndSize()
  return singletons for 0 or 1 bytes.

Author: vstinner

Lib/test/test_capi/test_bytes.py

@@ -61,10 +61,12 @@ def test_fromstringandsize(self):
         self.assertEqual(fromstringandsize(b'abc'), b'abc')
         self.assertEqual(fromstringandsize(b'abc', 2), b'ab')
         self.assertEqual(fromstringandsize(b'abc\0def'), b'abc\0def')
-        self.assertEqual(fromstringandsize(b'a'), b'a')
-        self.assertEqual(fromstringandsize(b'a', 1), b'a')
-        self.assertEqual(fromstringandsize(b'', 0), b'')
-        self.assertEqual(fromstringandsize(NULL, 0), b'')
+
+        # Return singleton for 0 or 1 bytes
+        self.assertIs(fromstringandsize(b'', 0), b'')
+        self.assertIs(fromstringandsize(NULL, 0), b'')
+        self.assertIs(fromstringandsize(b'a', 1), b'a')
+
         self.assertEqual(len(fromstringandsize(NULL, 3)), 3)
         self.assertRaises((MemoryError, OverflowError),
                           fromstringandsize, NULL, PY_SSIZE_T_MAX)
@@ -79,7 +81,10 @@ def test_fromstring(self):
         fromstring = _testlimitedcapi.bytes_fromstring
 
         self.assertEqual(fromstring(b'abc\0def'), b'abc')
-        self.assertEqual(fromstring(b''), b'')
+
+        # Return singleton for 0 or 1 bytes
+        self.assertIs(fromstring(b''), b'')
+        self.assertIs(fromstring(b'a'), b'a')
 
         # CRASHES fromstring(NULL)