Optimize the empty set "literal"

[serhiy-storchaka]

BPO	33200
Nosy	@brettcannon, @mdickinson, @ncoghlan, @pitrou, @benjaminp, @serhiy-storchaka, @1st1, @MojoVampire
PRs	bpo-33200: Optimize bytecode for the empty set "literal". #6330

^{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 2018-04-09.16:35:29.582>
created_at = <Date 2018-04-01.10:29:26.674>
labels = ['interpreter-core', '3.8', 'performance']
title = 'Optimize the empty set "literal"'
updated_at = <Date 2018-04-09.16:35:29.581>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2018-04-09.16:35:29.581>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2018-04-09.16:35:29.582>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2018-04-01.10:29:26.674>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 33200
keywords = ['patch']
message_count = 6.0
messages = ['314769', '314771', '314772', '314790', '314849', '315138']
nosy_count = 8.0
nosy_names = ['brett.cannon', 'mark.dickinson', 'ncoghlan', 'pitrou', 'benjamin.peterson', 'serhiy.storchaka', 'yselivanov', 'josh.r']
pr_nums = ['6330']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue33200'
versions = ['Python 3.8']

[serhiy-storchaka]

The following PR optimizes bytecode for the empty set "literal": {*()}.

Currently it is compiled to

LOAD_CONST               0 (())
BUILD_SET_UNPACK         1

It will optimized to

BUILD_SET                0

$ ./python -m perf timeit --duplicate 1000 '{*()}'

Unpatched: Mean +- std dev: 68.6 ns +- 1.1 ns
Patched: Mean +- std dev: 31.8 ns +- 2.8 ns

[mdickinson]

Does anyone seriously write "{*()}" instead of "set()"?

[serhiy-storchaka]

Don't know. :-)

Consider this as a 1st April egg.

[pitrou]

Does anyone seriously write "{*()}" instead of "set()"?

It doesn't seem to be laughing to me.

[MojoVampire]

I may have immediately latched onto this, dubbing it the "one-eyed monkey operator", the moment the generalized unpacking released.

I always hated the lack of an empty set literal, and enjoyed having this exist just to fill that asymmetry with the other built-in collection types (that said, I never use it in production code, nor teach it in classes I run).

[serhiy-storchaka]

Closed due to the lack of practical applications. Can be reopened if this idiom become popular.