Revert "GH-128914: Remove conditional stack effects from `bytecodes.c…

…` and the code generators (GH-128918)" (GH-129202)

The commit introduced a ~2.5-3% regression in the free threading build.

This reverts commit ab61d3f.

Doc/library/dis.rst

@@ -75,8 +75,7 @@ the following command can be used to display the disassembly of
    >>> dis.dis(myfunc)
      2           RESUME                   0
    <BLANKLINE>
-     3           LOAD_GLOBAL              0 (len)
-                 PUSH_NULL
+     3           LOAD_GLOBAL              1 (len + NULL)
                  LOAD_FAST                0 (alist)
                  CALL                     1
                  RETURN_VALUE
@@ -208,7 +207,6 @@ Example:
     ...
     RESUME
     LOAD_GLOBAL
-    PUSH_NULL
     LOAD_FAST
     CALL
     RETURN_VALUE
@@ -1217,28 +1215,21 @@ iterations of the loop.
 
 .. opcode:: LOAD_ATTR (namei)
 
-   Replaces ``STACK[-1]`` with ``getattr(STACK[-1], co_names[namei>>1])``.
+   If the low bit of ``namei`` is not set, this replaces ``STACK[-1]`` with
+   ``getattr(STACK[-1], co_names[namei>>1])``.
 
-   .. versionchanged:: 3.12
-      If the low bit of ``namei`` is set, then a ``NULL`` or ``self`` is
-      pushed to the stack before the attribute or unbound method respectively.
-
-   .. versionchanged:: 3.14
-      Reverted change from 3.12. The low bit of ``namei`` has no special meaning.
-
-
-.. opcode:: LOAD_METHOD (namei)
-
-   Attempt to load a method named ``co_names[namei>>1]`` from the ``STACK[-1]`` object.
-   ``STACK[-1]`` is popped.
+   If the low bit of ``namei`` is set, this will attempt to load a method named
+   ``co_names[namei>>1]`` from the ``STACK[-1]`` object. ``STACK[-1]`` is popped.
    This bytecode distinguishes two cases: if ``STACK[-1]`` has a method with the
    correct name, the bytecode pushes the unbound method and ``STACK[-1]``.
    ``STACK[-1]`` will be used as the first argument (``self``) by :opcode:`CALL`
    or :opcode:`CALL_KW` when calling the unbound method.
    Otherwise, ``NULL`` and the object returned by
    the attribute lookup are pushed.
 
-   .. versionadded:: 3.14
+   .. versionchanged:: 3.12
+      If the low bit of ``namei`` is set, then a ``NULL`` or ``self`` is
+      pushed to the stack before the attribute or unbound method respectively.
 
 
 .. opcode:: LOAD_SUPER_ATTR (namei)
@@ -1935,6 +1926,12 @@ but are replaced by real opcodes or removed before bytecode is generated.
       This opcode is now a pseudo-instruction.
 
 
+.. opcode:: LOAD_METHOD
+
+   Optimized unbound method lookup. Emitted as a ``LOAD_ATTR`` opcode
+   with a flag set in the arg.
+
+
 .. _opcode_collections:
 
 Opcode collections

Include/internal/pycore_code.h

@@ -334,8 +334,6 @@ extern void _Py_Specialize_LoadSuperAttr(_PyStackRef global_super, _PyStackRef c
                                          _Py_CODEUNIT *instr, int load_method);
 extern void _Py_Specialize_LoadAttr(_PyStackRef owner, _Py_CODEUNIT *instr,
                                     PyObject *name);
-extern void _Py_Specialize_LoadMethod(_PyStackRef owner, _Py_CODEUNIT *instr,
-                                      PyObject *name);
 extern void _Py_Specialize_StoreAttr(_PyStackRef owner, _Py_CODEUNIT *instr,
                                      PyObject *name);
 extern void _Py_Specialize_LoadGlobal(PyObject *globals, PyObject *builtins,

Include/internal/pycore_magic_number.h

@@ -267,7 +267,6 @@ Known values:
     Python 3.14a4 3612 (Add POP_ITER and INSTRUMENTED_POP_ITER)
     Python 3.14a4 3613 (Add LOAD_CONST_MORTAL instruction)
     Python 3.14a5 3614 (Add BINARY_OP_EXTEND)
-    Python 3.14a5 3615 (Remove conditional stack effects)
 
     Python 3.15 will start with 3650
 
@@ -280,7 +279,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3615
+#define PYC_MAGIC_NUMBER 3614
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \