Preserve the LOAD_CLOSURE opcode as an alias for LOAD_FAST.

Author: ericsnowcurrently

Doc/library/dis.rst

@@ -1043,20 +1043,7 @@ All of the following opcodes use their arguments.
 
 .. opcode:: LOAD_FAST (var_num)
 
-   Pushes a reference to a local variable or closure cell onto the stack.
-   The corresponding variable name is
-   ``(co_varnames + co_cellvars + co_freevars)[var_num]``.
-
-   For closures, note that ``LOAD_FAST`` loads the cell object contained
-   in the corresponding slot of the cell and free variable storage,
-   at index ``var_num - len(co_varnames)``.
-   In contrast, ``LOAD_DEREF`` gets the object the cell references.
-
-   Use of ``LOAD_FAST`` for closures is primarily to share cells objects
-   from an outer closure when creating an inner one with ``MAKE_FUNCTION``.
-
-   .. versionchanged:: 3.10
-      Closure cells are handled here now instead of ``LOAD_CLOSURE`` (removed).
+   Pushes a reference to the local ``co_varnames[var_num]`` onto the stack.
 
 
 .. opcode:: STORE_FAST (var_num)
@@ -1069,6 +1056,19 @@ All of the following opcodes use their arguments.
    Deletes local ``co_varnames[var_num]``.
 
 
+.. opcode:: LOAD_CLOSURE (i)
+
+   Pushes a reference to the cell contained in slot ``i - len(co_varnames)``
+   of the cell and free variable storage.  The name of the variable is
+   ``(co_varnames + co_cellvars + co_freevars)[i]``.
+
+   Note that ``LOAD_CLOSURE`` is effectively an alias for ``LOAD_FAST``.
+   It exists to keep bytecode a little more readable.
+
+   .. versionchanged:: 3.10
+      ``i`` is offset by the length of ``co_varnames``.
+
+
 .. opcode:: LOAD_DEREF (i)
 
    Loads the cell contained in slot ``i - len(co_varnames)`` of the cell

Include/opcode.h

@@ -181,6 +181,8 @@ def jabs_op(name, op):
 def_op('MAKE_FUNCTION', 132)    # Flags
 def_op('BUILD_SLICE', 133)      # Number of items
 
+def_op('LOAD_CLOSURE', 135)
+hasfree.append(135)
 def_op('LOAD_DEREF', 136)
 hasfree.append(136)
 def_op('STORE_DEREF', 137)

Lib/opcode.py

@@ -427,7 +427,7 @@ def foo(x):
     return foo
 
 dis_nested_0 = """\
-%3d           0 LOAD_FAST                2 (y)
+%3d           0 LOAD_CLOSURE             2 (y)
               2 BUILD_TUPLE              1
               4 LOAD_CONST               1 (<code object foo at 0x..., file "%s", line %d>)
               6 LOAD_CONST               2 ('_h.<locals>.foo')
@@ -444,7 +444,7 @@ def foo(x):
 
 dis_nested_1 = """%s
 Disassembly of <code object foo at 0x..., file "%s", line %d>:
-%3d           0 LOAD_FAST                1 (x)
+%3d           0 LOAD_CLOSURE             1 (x)
               2 BUILD_TUPLE              1
               4 LOAD_CONST               1 (<code object <listcomp> at 0x..., file "%s", line %d>)
               6 LOAD_CONST               2 ('_h.<locals>.foo.<locals>.<listcomp>')
@@ -962,8 +962,8 @@ def jumpy():
 Instruction = dis.Instruction
 expected_opinfo_outer = [
   Instruction(opname='LOAD_CONST', opcode=100, arg=8, argval=(3, 4), argrepr='(3, 4)', offset=0, starts_line=2, is_jump_target=False),
-  Instruction(opname='LOAD_FAST', opcode=124, arg=3, argval='a', argrepr='a', offset=2, starts_line=None, is_jump_target=False),
-  Instruction(opname='LOAD_FAST', opcode=124, arg=4, argval='b', argrepr='b', offset=4, starts_line=None, is_jump_target=False),
+  Instruction(opname='LOAD_CLOSURE', opcode=135, arg=3, argval='a', argrepr='a', offset=2, starts_line=None, is_jump_target=False),
+  Instruction(opname='LOAD_CLOSURE', opcode=135, arg=4, argval='b', argrepr='b', offset=4, starts_line=None, is_jump_target=False),
   Instruction(opname='BUILD_TUPLE', opcode=102, arg=2, argval=2, argrepr='', offset=6, starts_line=None, is_jump_target=False),
   Instruction(opname='LOAD_CONST', opcode=100, arg=3, argval=code_object_f, argrepr=repr(code_object_f), offset=8, starts_line=None, is_jump_target=False),
   Instruction(opname='LOAD_CONST', opcode=100, arg=4, argval='outer.<locals>.f', argrepr="'outer.<locals>.f'", offset=10, starts_line=None, is_jump_target=False),
@@ -985,10 +985,10 @@ def jumpy():
 
 expected_opinfo_f = [
   Instruction(opname='LOAD_CONST', opcode=100, arg=5, argval=(5, 6), argrepr='(5, 6)', offset=0, starts_line=3, is_jump_target=False),
-  Instruction(opname='LOAD_FAST', opcode=124, arg=5, argval='a', argrepr='a', offset=2, starts_line=None, is_jump_target=False),
-  Instruction(opname='LOAD_FAST', opcode=124, arg=6, argval='b', argrepr='b', offset=4, starts_line=None, is_jump_target=False),
-  Instruction(opname='LOAD_FAST', opcode=124, arg=3, argval='c', argrepr='c', offset=6, starts_line=None, is_jump_target=False),
-  Instruction(opname='LOAD_FAST', opcode=124, arg=4, argval='d', argrepr='d', offset=8, starts_line=None, is_jump_target=False),
+  Instruction(opname='LOAD_CLOSURE', opcode=135, arg=5, argval='a', argrepr='a', offset=2, starts_line=None, is_jump_target=False),
+  Instruction(opname='LOAD_CLOSURE', opcode=135, arg=6, argval='b', argrepr='b', offset=4, starts_line=None, is_jump_target=False),
+  Instruction(opname='LOAD_CLOSURE', opcode=135, arg=3, argval='c', argrepr='c', offset=6, starts_line=None, is_jump_target=False),
+  Instruction(opname='LOAD_CLOSURE', opcode=135, arg=4, argval='d', argrepr='d', offset=8, starts_line=None, is_jump_target=False),
   Instruction(opname='BUILD_TUPLE', opcode=102, arg=4, argval=4, argrepr='', offset=10, starts_line=None, is_jump_target=False),
   Instruction(opname='LOAD_CONST', opcode=100, arg=3, argval=code_object_inner, argrepr=repr(code_object_inner), offset=12, starts_line=None, is_jump_target=False),
   Instruction(opname='LOAD_CONST', opcode=100, arg=4, argval='outer.<locals>.f.<locals>.inner', argrepr="'outer.<locals>.f.<locals>.inner'", offset=14, starts_line=None, is_jump_target=False),

Lib/test/test_dis.py

@@ -1,4 +1,4 @@
 Compute cell offsets relative to locals in compiler. Allows the interpreter
 to treats locals and cells a single array, which is slightly more efficient.
-
-Delete the LOAD_CLOSURE bytecode.
+Also make the LOAD_CLOSURE opcode an alias for LOAD_FAST.  Preserving
+LOAD_CLOSURE helps keep bytecode a bit more readable.

Misc/NEWS.d/next/Core and Builtins/2021-04-02-15-02-16.bpo-43693.l3Ureu.rst

@@ -1814,6 +1814,8 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             DISPATCH();
         }
 
+        /* We keep LOAD_CLOSURE so that the bytecode stays more readable. */
+        case TARGET(LOAD_CLOSURE):
         case TARGET(LOAD_FAST): {
             PyObject *value = GETLOCAL(oparg);
             if (value == NULL) {

Python/ceval.c

@@ -75,9 +75,6 @@
 #define SETUP_WITH 253
 #define POP_BLOCK 252
 
-/* Artificial instruction, will be converted to LOAD_FAST */
-#define LOAD_CLOSURE 251
-
 #define IS_TOP_LEVEL_AWAIT(c) ( \
         (c->c_flags->cf_flags & PyCF_ALLOW_TOP_LEVEL_AWAIT) \
         && (c->u->u_ste->ste_type == ModuleBlock))
@@ -1193,6 +1190,8 @@ stack_effect(int opcode, int oparg, int jump)
                 return -1;
 
         /* Closures */
+        case LOAD_CLOSURE:
+            return 1;
         case LOAD_DEREF:
         case LOAD_CLASSDEREF:
             return 1;
@@ -7285,8 +7284,6 @@ offset_derefs(basicblock *entryblock, int nlocals)
             struct instr *inst = &b->b_instr[i];
             switch(inst->i_opcode) {
                 case LOAD_CLOSURE:
-                    inst->i_opcode = LOAD_FAST;
-                    /* fall through */
                 case LOAD_DEREF:
                 case STORE_DEREF:
                 case DELETE_DEREF: