Compiler front-end produces a broken CFG

[markshannon]

BPO	42349
Nosy	@markshannon, @1st1, @pablogsal, @isidentical, @sweeneyde
PRs	bpo-42349: Compiler clean up. More yak-shaving for PEP 626. #23267

^{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 = 'https://github.com/markshannon'
closed_at = <Date 2020-11-23.16:49:14.501>
created_at = <Date 2020-11-13.17:23:56.612>
labels = ['type-bug']
title = 'Compiler front-end produces a broken CFG'
updated_at = <Date 2021-06-07.10:46:33.760>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2021-06-07.10:46:33.760>
actor = 'Mark.Shannon'
assignee = 'Mark.Shannon'
closed = True
closed_date = <Date 2020-11-23.16:49:14.501>
closer = 'Mark.Shannon'
components = []
creation = <Date 2020-11-13.17:23:56.612>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 42349
keywords = ['patch']
message_count = 4.0
messages = ['380911', '381279', '395127', '395261']
nosy_count = 5.0
nosy_names = ['Mark.Shannon', 'yselivanov', 'pablogsal', 'BTaskaya', 'Dennis Sweeney']
pr_nums = ['23267']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42349'
versions = []

[markshannon]

The front-end of the bytecode compiler produces a broken CFG.
A number of "basic-block"s have terminators before their end.
This makes the back-end optimizations unsafe as they rely of a well-formed CFG.

The fix is simple. Insert a check that the CFG is well-formed before doing any optimizations, then fix up the front-end.

Once done, we can be more aggressive with optimizations without worrying that things will break for no apparent reason.

[markshannon]

New changeset 266b462 by Mark Shannon in branch 'master':
bpo-42349: Compiler clean up. More yak-shaving for PEP-626. (GH-23267)
266b462

[sweeneyde]

From https://devguide.python.org/compiler/#source-code-to-ast:

Basic blocks themselves are a block of IR that has a single entry point but possibly multiple exit points.

In particular, compile.c's label_exception_targets has the assertion (assert(i == b->b_iused -1);) that jumps only occur as the last instruction of a block.

Does the devguide need updating, or do I have a misunderstanding?

[markshannon]

Basic blocks have only a single exit, at the end.
https://en.wikipedia.org/wiki/Basic_block

If the devguide says otherwise it is wrong.