GH-107596: Specialize str[int]

Misc/NEWS.d/next/Core and Builtins/2023-08-03-11-13-09.gh-issue-107596.T3yPGI.rst

@@ -0,0 +1 @@
+Specialize subscripting :class:`str` objects by :class:`int` indexes.

Python/bytecodes.c

@@ -509,6 +509,7 @@ dummy_func(
             BINARY_SUBSCR_DICT,
             BINARY_SUBSCR_GETITEM,
             BINARY_SUBSCR_LIST_INT,
+            BINARY_SUBSCR_STR_INT,
             BINARY_SUBSCR_TUPLE_INT,
         };
 
@@ -574,6 +575,21 @@ dummy_func(
             Py_DECREF(list);
         }
 
+        inst(BINARY_SUBSCR_STR_INT, (unused/1, str, sub -- res)) {
+            DEOPT_IF(!PyLong_CheckExact(sub), BINARY_SUBSCR);
+            DEOPT_IF(!PyUnicode_CheckExact(str), BINARY_SUBSCR);
+            DEOPT_IF(!_PyLong_IsNonNegativeCompact((PyLongObject *)sub), BINARY_SUBSCR);
+            Py_ssize_t index = ((PyLongObject*)sub)->long_value.ob_digit[0];
+            DEOPT_IF(index >= PyUnicode_GET_LENGTH(str), BINARY_SUBSCR);
+            // Specialize for reading an ASCII character from any string:
+            Py_UCS4 ch = PyUnicode_READ_CHAR(str, index);
+            DEOPT_IF(ch >= Py_ARRAY_LENGTH(_Py_SINGLETON(strings).ascii), BINARY_SUBSCR);
+            STAT_INC(BINARY_SUBSCR, hit);
+            res = (PyObject*)&_Py_SINGLETON(strings).ascii[ch];
+            _Py_DECREF_SPECIALIZED(sub, (destructor)PyObject_Free);
+            Py_DECREF(str);
+        }
+
         inst(BINARY_SUBSCR_TUPLE_INT, (unused/1, tuple, sub -- res)) {
             DEOPT_IF(!PyLong_CheckExact(sub), BINARY_SUBSCR);
             DEOPT_IF(!PyTuple_CheckExact(tuple), BINARY_SUBSCR);