Dead code in wordcode

[serhiy-storchaka]

BPO	28517
Nosy	@rhettinger, @gpshead, @serhiy-storchaka, @matrixise, @serprex
PRs	[Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	peephole_remove_unreachable_code.patch

^{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/serhiy-storchaka'
closed_at = <Date 2016-10-25.07:20:39.328>
created_at = <Date 2016-10-24.05:16:39.388>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'Dead code in wordcode'
updated_at = <Date 2018-11-08.21:09:05.632>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2018-11-08.21:09:05.632>
actor = 'gregory.p.smith'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-10-25.07:20:39.328>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2016-10-24.05:16:39.388>
creator = 'serhiy.storchaka'
dependencies = []
files = ['45202']
hgrepos = []
issue_num = 28517
keywords = ['patch']
message_count = 8.0
messages = ['279297', '279298', '279320', '279348', '279359', '279366', '279367', '329486']
nosy_count = 6.0
nosy_names = ['rhettinger', 'gregory.p.smith', 'python-dev', 'serhiy.storchaka', 'matrixise', 'Demur Rumed']
pr_nums = ['552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28517'
versions = ['Python 3.6', 'Python 3.7']

[serhiy-storchaka]

>>> def func(test):
...     if test == 1:
...         return 1
...     elif test == 2:
...         return 2
...     return 3
... 
>>> import dis
>>> dis.dis(func)

Python 3.5:

2 0 LOAD_FAST 0 (test)
3 LOAD_CONST 1 (1)
6 COMPARE_OP 2 (==)
9 POP_JUMP_IF_FALSE 16

3 12 LOAD_CONST 1 (1)
15 RETURN_VALUE

4 >> 16 LOAD_FAST 0 (test)
19 LOAD_CONST 2 (2)
22 COMPARE_OP 2 (==)
25 POP_JUMP_IF_FALSE 32

5 28 LOAD_CONST 2 (2)
31 RETURN_VALUE

6 >> 32 LOAD_CONST 3 (3)
35 RETURN_VALUE

Python 3.6:

2 0 LOAD_FAST 0 (test)
2 LOAD_CONST 1 (1)
4 COMPARE_OP 2 (==)
6 POP_JUMP_IF_FALSE 14

3 8 LOAD_CONST 1 (1)
10 RETURN_VALUE
12 JUMP_FORWARD 12 (to 26)

4 >> 14 LOAD_FAST 0 (test)
16 LOAD_CONST 2 (2)
18 COMPARE_OP 2 (==)
20 POP_JUMP_IF_FALSE 26

5 22 LOAD_CONST 2 (2)
24 RETURN_VALUE

6 >> 26 LOAD_CONST 3 (3)
28 RETURN_VALUE

Note JUMP_FORWARD after RETURN_VALUE in 3.6 listing.

[serhiy-storchaka]

Proposed patch fixes removing unreachable code after RETURN_VALUE in peephole optimizer.

[matrixise]

Hi Serhiy,

Could you help me, because I don't understand your patch ? because a RETURN_VALUE will go to the end of the block, and in this case, I don't understand why there is a JUMP_FORWARD at 12 in Python 3.6.

[rhettinger]

+0 The situation this addresses isn't common and the patch will rarely produce a perceptable benefit (just making the disassembly look a little nicer). That said, change looks simple enough and doesn't add any overhead.

[python-dev]

New changeset 5784cc37b5f4 by Serhiy Storchaka in branch '3.6':
Issue bpo-28517: Fixed of-by-one error in the peephole optimizer that caused
https://hg.python.org/cpython/rev/5784cc37b5f4

New changeset 8d571fab4d66 by Serhiy Storchaka in branch 'default':
Issue bpo-28517: Fixed of-by-one error in the peephole optimizer that caused
https://hg.python.org/cpython/rev/8d571fab4d66

[serhiy-storchaka]

The main problem was that this bug caused unnecessary breakage of tests in Victor's bytecode [1].

Stéphane, the number of opcodes to the end of the block was counted incorrectly. Just off-by-one error. One unreachable opcode always was left.

[1] https://github.com/haypo/bytecode

[matrixise]

ah ok, thanks for the explanation.

[gpshead]

the off by one error fix here introduced a new off by one error. PR coming, follow https://bugs.python.org/issue35193 for that.