Simplify bytecodes for try-finally, try-except and with blocks.

[markshannon]

BPO	33387
Nosy	@gpshead, @ncoghlan, @vstinner, @masklinn, @markshannon, @serhiy-storchaka, @csabella, @miss-islington, @iritkatriel
PRs	bpo-33387: Simplify bytecodes for try-finally, try-except and with blocks. #6641 bpo-33387: Correct release version to 3.9 for new bytecodes #17318 bpo-33387: Fix compiler warning in frame_block_unwind() #18099 bpo-33387: Correct name of bytecode in change note. #22723 [3.9] bpo-33387: Correct name of bytecode in change note. (GH-22723) #22765 bpo-33387: update documentation for exception handling opcode changes #24334

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-01-26.10:27:14.384>
created_at = <Date 2018-04-29.18:28:42.198>
labels = ['interpreter-core', '3.9']
title = 'Simplify bytecodes for try-finally, try-except and with blocks.'
updated_at = <Date 2021-01-26.10:27:14.384>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2021-01-26.10:27:14.384>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-26.10:27:14.384>
closer = 'iritkatriel'
components = ['Interpreter Core']
creation = <Date 2018-04-29.18:28:42.198>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 33387
keywords = ['patch']
message_count = 14.0
messages = ['315905', '339817', '357147', '357148', '357155', '357172', '357246', '357279', '360393', '378715', '378720', '385664', '385679', '385703']
nosy_count = 9.0
nosy_names = ['gregory.p.smith', 'ncoghlan', 'vstinner', 'xmorel', 'Mark.Shannon', 'serhiy.storchaka', 'cheryl.sabella', 'miss-islington', 'iritkatriel']
pr_nums = ['6641', '17318', '18099', '22723', '22765', '24334']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33387'
versions = ['Python 3.9']

[markshannon]

The six complex bytecodes currently used for implementing 'with' and 'try' statements can be replaced with just two simpler bytecodes.
The six bytecodes are WITH_CLEANUP_START, WITH_CLEANUP_FINISH,
BEGIN_FINALLY, END_FINALLY, CALL_FINALLY and POP_FINALLY.
They can be replaced with RERAISE and WITH_EXCEPT_FINISH.

See https://bugs.python.org/issue32949 for more details of the new bytecodes and how they are used in the 'with' statement.

The try-finally statement can be implemented broadly as
SETUP_FINALLY except
try-body
POP_BLOCK
finally-body
JUMP exit
except:
finally-body
exit:

[csabella]

What would be the best way to resolve the discussion between this and bpo-32949? Would it be good to try to get one of them into 3.8?

[vstinner]

Mark merged his PR 6641 but forgot to mention bpo-33387:

commit fee5526
Author: Mark Shannon <mark@hotpy.org>
Date: Thu Nov 21 09:11:43 2019 +0000

Produce cleaner bytecode for 'with' and 'async with' by generating separate code for normal and exceptional paths. (bpo-6641)

Remove BEGIN_FINALLY, END_FINALLY, CALL_FINALLY and POP_FINALLY bytecodes. Implement finally blocks by code duplication.
Reimplement frame.lineno setter using line numbers rather than bytecode offsets.

[vstinner]

The new doc seems to use Python version 3.8, but the change was merged into master which is the future Python 3.9, no?

.. opcode:: RERAISE

 Re-raises the exception currently on top of the stack. 

 .. versionadded:: 3.8

[markshannon]

Thanks for noticing.
#17318

[markshannon]

New changeset 82f897b by Mark Shannon in branch 'master':
Correct release version to 3.9 for RERAISE and WITH_EXCEPT_START bytecodes. (bpo-17318)
82f897b

[gpshead]

sweet! =)

[vstinner]

Can this issue be closed now?

[vstinner]

New changeset 629023c by Victor Stinner in branch 'master':
bpo-33387: Fix compiler warning in frame_block_unwind() (GH-18099)
629023c

[masklinn]

The 3.9 changelog mentions WITH_EXCEPT_FINISH but that seems to have ultimately been called WITH_EXCEPT_START, maybe it shoudl be updated also?

[markshannon]

Yes, thanks for pointing that out.

[iritkatriel]

I'm reopening this to delete an obsolete comment left behind.

[iritkatriel]

There's another place that needs to be updated: https://docs.python.org/3/library/dis.html#opcode-SETUP_WITH

need to replace WITH_CLEANUP_START by WITH_EXCEPT_START

[markshannon]

New changeset dea5bf9 by Irit Katriel in branch 'master':
bpo-33387: update documentation for exception handling opcode changes (GH-24334)
dea5bf9