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bpo-33387: Simplify bytecodes for try-finally, try-except and with blocks. #6641

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Nov 21, 2019
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bpo-33387: Simplify bytecodes for try-finally, try-except and with blocks. #6641

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85 changes: 9 additions & 76 deletions Doc/library/dis.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -706,50 +706,21 @@ iterations of the loop.
popped values are used to restore the exception state.


.. opcode:: POP_FINALLY (preserve_tos)
.. opcode:: RERAISE

Cleans up the value stack and the block stack. If *preserve_tos* is not
``0`` TOS first is popped from the stack and pushed on the stack after
performing other stack operations:
Re-raises the exception currently on top of the stack.

* If TOS is ``NULL`` or an integer (pushed by :opcode:`BEGIN_FINALLY`
or :opcode:`CALL_FINALLY`) it is popped from the stack.
* If TOS is an exception type (pushed when an exception has been raised)
6 values are popped from the stack, the last three popped values are
used to restore the exception state. An exception handler block is
removed from the block stack.
.. versionadded:: 3.8

It is similar to :opcode:`END_FINALLY`, but doesn't change the bytecode
counter nor raise an exception. Used for implementing :keyword:`break`,
:keyword:`continue` and :keyword:`return` in the :keyword:`finally` block.

.. versionadded:: 3.8


.. opcode:: BEGIN_FINALLY

Pushes ``NULL`` onto the stack for using it in :opcode:`END_FINALLY`,
:opcode:`POP_FINALLY`, :opcode:`WITH_CLEANUP_START` and
:opcode:`WITH_CLEANUP_FINISH`. Starts the :keyword:`finally` block.

.. versionadded:: 3.8
.. opcode:: WITH_EXCEPT_START

Calls the function in position 7 on the stack with the top three
items on the stack as arguments.
Used to implement the call ``context_manager.__exit__(*exc_info())`` when an exception
has occurred in a :keyword:`with` statement.

.. opcode:: END_FINALLY

Terminates a :keyword:`finally` clause. The interpreter recalls whether the
exception has to be re-raised or execution has to be continued depending on
the value of TOS.

* If TOS is ``NULL`` (pushed by :opcode:`BEGIN_FINALLY`) continue from
the next instruction. TOS is popped.
* If TOS is an integer (pushed by :opcode:`CALL_FINALLY`), sets the
bytecode counter to TOS. TOS is popped.
* If TOS is an exception type (pushed when an exception has been raised)
6 values are popped from the stack, the first three popped values are
used to re-raise the exception and the last three popped values are used
to restore the exception state. An exception handler block is removed
from the block stack.
.. versionadded:: 3.8


.. opcode:: LOAD_ASSERTION_ERROR
Expand Down Expand Up @@ -780,35 +751,6 @@ iterations of the loop.
.. versionadded:: 3.2


.. opcode:: WITH_CLEANUP_START

Starts cleaning up the stack when a :keyword:`with` statement block exits.

At the top of the stack are either ``NULL`` (pushed by
:opcode:`BEGIN_FINALLY`) or 6 values pushed if an exception has been
raised in the with block. Below is the context manager's
:meth:`~object.__exit__` or :meth:`~object.__aexit__` bound method.

If TOS is ``NULL``, calls ``SECOND(None, None, None)``,
removes the function from the stack, leaving TOS, and pushes ``None``
to the stack. Otherwise calls ``SEVENTH(TOP, SECOND, THIRD)``,
shifts the bottom 3 values of the stack down, replaces the empty spot
with ``NULL`` and pushes TOS. Finally pushes the result of the call.


.. opcode:: WITH_CLEANUP_FINISH

Finishes cleaning up the stack when a :keyword:`with` statement block exits.

TOS is result of ``__exit__()`` or ``__aexit__()`` function call pushed
by :opcode:`WITH_CLEANUP_START`. SECOND is ``None`` or an exception type
(pushed when an exception has been raised).

Pops two values from the stack. If SECOND is not None and TOS is true
unwinds the EXCEPT_HANDLER block which was created when the exception
was caught and pushes ``NULL`` to the stack.


All of the following opcodes use their arguments.

.. opcode:: STORE_NAME (namei)
Expand Down Expand Up @@ -1060,15 +1002,6 @@ All of the following opcodes use their arguments.
stack. *delta* points to the finally block or the first except block.


.. opcode:: CALL_FINALLY (delta)

Pushes the address of the next instruction onto the stack and increments
bytecode counter by *delta*. Used for calling the finally block as a
"subroutine".

.. versionadded:: 3.8


.. opcode:: LOAD_FAST (var_num)

Pushes a reference to the local ``co_varnames[var_num]`` onto the stack.
Expand Down
8 changes: 2 additions & 6 deletions Include/opcode.h
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4 changes: 3 additions & 1 deletion Lib/importlib/_bootstrap_external.py
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Expand Up @@ -272,6 +272,8 @@ def _write_atomic(path, data, mode=0o666):
# only args in ast.arguments #37593)
# Python 3.8b4 3413 (Fix "break" and "continue" in "finally" #37830)
# Python 3.9a0 3420 (add LOAD_ASSERTION_ERROR #34880)
# Python 3.9a0 3421 (simplified bytecode for with blocks #32949)
# Python 3.9a0 3422 (remove BEGIN_FINALLY, END_FINALLY, CALL_FINALLY, POP_FINALLY bytecodes #33387)
#
# MAGIC must change whenever the bytecode emitted by the compiler may no
# longer be understood by older implementations of the eval loop (usually
Expand All @@ -280,7 +282,7 @@ def _write_atomic(path, data, mode=0o666):
# Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
# in PC/launcher.c must also be updated.

MAGIC_NUMBER = (3420).to_bytes(2, 'little') + b'\r\n'
MAGIC_NUMBER = (3422).to_bytes(2, 'little') + b'\r\n'
_RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little') # For import.c

_PYCACHE = '__pycache__'
Expand Down
11 changes: 5 additions & 6 deletions Lib/opcode.py
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Expand Up @@ -84,10 +84,12 @@ def jabs_op(name, op):
def_op('INPLACE_FLOOR_DIVIDE', 28)
def_op('INPLACE_TRUE_DIVIDE', 29)

def_op('RERAISE', 48)
def_op('WITH_EXCEPT_START', 49)
def_op('GET_AITER', 50)
def_op('GET_ANEXT', 51)
def_op('BEFORE_ASYNC_WITH', 52)
def_op('BEGIN_FINALLY', 53)

def_op('END_ASYNC_FOR', 54)
def_op('INPLACE_ADD', 55)
def_op('INPLACE_SUBTRACT', 56)
Expand Down Expand Up @@ -115,14 +117,13 @@ def jabs_op(name, op):
def_op('INPLACE_AND', 77)
def_op('INPLACE_XOR', 78)
def_op('INPLACE_OR', 79)
def_op('WITH_CLEANUP_START', 81)
def_op('WITH_CLEANUP_FINISH', 82)

def_op('RETURN_VALUE', 83)
def_op('IMPORT_STAR', 84)
def_op('SETUP_ANNOTATIONS', 85)
def_op('YIELD_VALUE', 86)
def_op('POP_BLOCK', 87)
def_op('END_FINALLY', 88)

def_op('POP_EXCEPT', 89)

HAVE_ARGUMENT = 90 # Opcodes from here have an argument:
Expand Down Expand Up @@ -210,7 +211,5 @@ def jabs_op(name, op):

name_op('LOAD_METHOD', 160)
def_op('CALL_METHOD', 161)
jrel_op('CALL_FINALLY', 162)
def_op('POP_FINALLY', 163)

del def_op, name_op, jrel_op, jabs_op
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