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Unnecessary JUMP_FORWARD(0) (NOP) in if statements without else or elif #66554

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AttilaFazekas mannequin opened this issue Sep 7, 2014 · 3 comments
Closed

Unnecessary JUMP_FORWARD(0) (NOP) in if statements without else or elif #66554

AttilaFazekas mannequin opened this issue Sep 7, 2014 · 3 comments
Labels
interpreter-core (Objects, Python, Grammar, and Parser dirs) performance Performance or resource usage

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@AttilaFazekas
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AttilaFazekas mannequin commented Sep 7, 2014

BPO 22358
Nosy @jcea, @serhiy-storchaka
Superseder
  • bpo-11471: If without else generates redundant jump
    		•
    Files
  • python_nop_ifelse.patch: python_nop_ifelse.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 = None
closed_at = <Date 2014-09-08.07:28:36.778>
created_at = <Date 2014-09-07.23:15:51.266>
labels = ['interpreter-core', 'performance']
title = 'Unnecessary JUMP_FORWARD(0) (NOP) in if statements without else or elif'
updated_at = <Date 2014-09-08.07:28:36.776>
user = 'https://bugs.python.org/AttilaFazekas'

    bugs.python.org fields:

    activity = <Date 2014-09-08.07:28:36.776>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2014-09-08.07:28:36.778>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2014-09-07.23:15:51.266>
creator = 'Attila.Fazekas'
dependencies = []
files = ['36569']
hgrepos = []
issue_num = 22358
keywords = ['patch']
message_count = 3.0
messages = ['226547', '226554', '226560']
nosy_count = 3.0
nosy_names = ['jcea', 'serhiy.storchaka', 'Attila.Fazekas']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '11471'
type = 'performance'
url = 'https://bugs.python.org/issue22358'
versions = ['Python 3.5']
    @AttilaFazekas
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    AttilaFazekas mannequin commented Sep 7, 2014

    The following example function compiles to bytecode which contains,
    an unnecessary JUMP_FORWARD 0 instruction:

    def func():
    if a: pass

    Actual:
    dis.dis(func)
    2 0 LOAD_GLOBAL 0 (a)
    3 POP_JUMP_IF_FALSE 9
    6 JUMP_FORWARD 0 (to 9)
    >> 9 LOAD_CONST 0 (None)
    12 RETURN_VALUE

    Expected:
    dis.dis(func)
    2 0 LOAD_GLOBAL 0 (a)
    3 POP_JUMP_IF_FALSE 6
    >> 6 LOAD_CONST 0 (None)
    9 RETURN_VALUE

    The above JUMP_FORWARD instruction increases the code size and also has a negative performance effect.
    I do not see any reason to have the extra NOP in the byte code in this case.

    The attached patch removes this NOP generation from the code compilation part, so it will take effect by default.

    I had a little trouble when the code compiled from ast,
    because the If.orelse had a different content. (NULL vs. zero sizes asdl_seq)

    • The generated Assembly code updated in dis unit test.
    • The compilation test updated to test a real 'if' by using a variable in the condition. (The True and False is not a variable anymore)

    @AttilaFazekas AttilaFazekas mannequin added interpreter-core (Objects, Python, Grammar, and Parser dirs) performance Performance or resource usage labels Sep 7, 2014
    @jcea
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    jcea commented Sep 8, 2014

    What about the peephole optimizer?.

    @serhiy-storchaka
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    This is a duplicate of bpo-11471.

    Explicit check for NULL is not needed because the asdl_seq_LEN() macro checks its argument for NULL.

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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